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IETF RFC 7812
Last modified on Friday, June 17th, 2016
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Internet Engineering Task Force (IETF) A. Atlas
Request for Comments: 7812 C. Bowers
Category: Standards Track Juniper Networks
ISSN: 2070-1721 G. Enyedi
Ericsson
June 2016
An Architecture for IP/LDP Fast Reroute
Using Maximally Redundant Trees (MRT-FRR)
 Abstract
This document defines the architecture for IP and LDP Fast Reroute
using Maximally Redundant Trees (MRT-FRR). MRT-FRR is a technology
that gives link-protection and node-protection with 100% coverage in
any network topology that is still connected after the failure.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 7841.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/RFC 7812.
Copyright Notice
Copyright (c) 2016 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Atlas, et al. Standards Track PAGE 1 
RFC 7812 MRT Unicast FRR Architecture June 2016
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Importance of 100% Coverage . . . . . . . . . . . . . . . 4
1.2. Partial Deployment and Backwards Compatibility . . . . . 5
2. Requirements Language . . . . . . . . . . . . . . . . . . . . 5
3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 5
4. Maximally Redundant Trees (MRT) . . . . . . . . . . . . . . . 7
5. MRT and Fast Reroute . . . . . . . . . . . . . . . . . . . . 9
6. Unicast Forwarding with MRT Fast Reroute . . . . . . . . . . 9
6.1. Introduction to MRT Forwarding Options . . . . . . . . . 10
6.1.1. MRT LDP Labels . . . . . . . . . . . . . . . . . . . 10
6.1.1.1. Topology-Scoped FEC Encoded Using a Single Label
(Option 1A) . . . . . . . . . . . . . . . . . . . 10
6.1.1.2. Topology and FEC Encoded Using a Two-Label Stack
(Option 1B) . . . . . . . . . . . . . . . . . . . 11
6.1.1.3. Compatibility of MRT LDP Label Options 1A and 1B 12
6.1.1.4. Required Support for MRT LDP Label Options . . . 12
6.1.2. MRT IP Tunnels (Options 2A and 2B) . . . . . . . . . 12
6.2. Forwarding LDP Unicast Traffic over MRT Paths . . . . . . 13
6.2.1. Forwarding LDP Traffic Using MRT LDP Label Option 1A 13
6.2.2. Forwarding LDP Traffic Using MRT LDP Label Option 1B 14
6.2.3. Other Considerations for Forwarding LDP Traffic Using
MRT LDP Labels . . . . . . . . . . . . . . . . . . . 14
6.2.4. Required Support for LDP Traffic . . . . . . . . . . 14
6.3. Forwarding IP Unicast Traffic over MRT Paths . . . . . . 14
6.3.1. Tunneling IP Traffic Using MRT LDP Labels . . . . . . 15
6.3.1.1. Tunneling IP Traffic Using MRT LDP Label Option
1A . . . . . . . . . . . . . . . . . . . . . . . 15
6.3.1.2. Tunneling IP Traffic Using MRT LDP Label Option
1B . . . . . . . . . . . . . . . . . . . . . . . 15
6.3.2. Tunneling IP Traffic Using MRT IP Tunnels . . . . . . 16
6.3.3. Required Support for IP Traffic . . . . . . . . . . . 16
7. MRT Island Formation . . . . . . . . . . . . . . . . . . . . 16
7.1. IGP Area or Level . . . . . . . . . . . . . . . . . . . . 17
7.2. Support for a Specific MRT Profile . . . . . . . . . . . 17
7.3. Excluding Additional Routers and Interfaces from the MRT
Island . . . . . . . . . . . . . . . . . . . . . . . . . 18
7.3.1. Existing IGP Exclusion Mechanisms . . . . . . . . . . 18
7.3.2. MRT-Specific Exclusion Mechanism . . . . . . . . . . 19
7.4. Connectivity . . . . . . . . . . . . . . . . . . . . . . 19
7.5. Algorithm for MRT Island Identification . . . . . . . . . 19
8. MRT Profile . . . . . . . . . . . . . . . . . . . . . . . . . 19
8.1. MRT Profile Options . . . . . . . . . . . . . . . . . . . 19
8.2. Router-Specific MRT Parameters . . . . . . . . . . . . . 21
8.3. Default MRT Profile . . . . . . . . . . . . . . . . . . . 21
9. LDP Signaling Extensions and Considerations . . . . . . . . . 22
Atlas, et al. Standards Track PAGE 2
RFC 7812 MRT Unicast FRR Architecture June 2016
10. Inter-area Forwarding Behavior . . . . . . . . . . . . . . . 23
10.1. ABR Forwarding Behavior with MRT LDP Label Option 1A . . 23
10.1.1. Motivation for Creating the Rainbow-FEC . . . . . . 24
10.2. ABR Forwarding Behavior with IP Tunneling (Option 2) . . 24
10.3. ABR Forwarding Behavior with MRT LDP Label Option 1B . . 25
11. Prefixes Multiply Attached to the MRT Island . . . . . . . . 26
11.1. Protecting Multihomed Prefixes Using Tunnel Endpoint
Selection . . . . . . . . . . . . . . . . . . . . . . . 28
11.2. Protecting Multihomed Prefixes Using Named Proxy-Nodes . 29
11.3. MRT Alternates for Destinations outside the MRT Island . 31
12. Network Convergence and Preparing for the Next Failure . . . 32
12.1. Micro-loop Prevention and MRTs . . . . . . . . . . . . . 32
12.2. MRT Recalculation for the Default MRT Profile . . . . . 33
13. Operational Considerations . . . . . . . . . . . . . . . . . 34
13.1. Verifying Forwarding on MRT Paths . . . . . . . . . . . 34
13.2. Traffic Capacity on Backup Paths . . . . . . . . . . . . 34
13.3. MRT IP Tunnel Loopback Address Management . . . . . . . 36
13.4. MRT-FRR in a Network with Degraded Connectivity . . . . 36
13.5. Partial Deployment of MRT-FRR in a Network . . . . . . . 37
14. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 37
15. Security Considerations . . . . . . . . . . . . . . . . . . . 38
16. References . . . . . . . . . . . . . . . . . . . . . . . . . 38
16.1. Normative References . . . . . . . . . . . . . . . . . . 38
16.2. Informative References . . . . . . . . . . . . . . . . . 39
Appendix A. Inter-level Forwarding Behavior for IS-IS . . . . . 41
Appendix B. General Issues with Area Abstraction . . . . . . . . 42
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 43
Contributors . . . . . . . . . . . . . . . . . . . . . . . . . . 43
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 44
1. Introduction
This document describes a solution for IP/LDP fast reroute [RFC 5714].
MRT-FRR creates two alternate forwarding trees that are distinct from
the primary next-hop forwarding used during stable operation. These
two trees are maximally diverse from each other, providing link and
node protection for 100% of paths and failures as long as the failure
does not cut the network into multiple pieces. This document defines
the architecture for IP/LDP fast reroute with MRT.
[RFC 7811] describes how to compute maximally redundant trees using a
specific algorithm: the MRT Lowpoint algorithm. The MRT Lowpoint
algorithm is used by a router that supports the Default MRT Profile,
as specified in this document.
IP/LDP Fast Reroute using Maximally Redundant Trees (MRT-FRR) uses
two maximally diverse forwarding topologies to provide alternates. A
primary next hop should be on only one of the diverse forwarding
Atlas, et al. Standards Track PAGE 3
RFC 7812 MRT Unicast FRR Architecture June 2016
topologies; thus, the other can be used to provide an alternate.
Once traffic has been moved to one of the MRTs by one Point of Local
Repair (PLR), that traffic is not subject to further repair actions
by another PLR, even in the event of multiple simultaneous failures.
Therefore, traffic repaired by MRT-FRR will not loop between
different PLRs responding to different simultaneous failures.
While MRT provides 100% protection for a single link or node failure,
it may not protect traffic in the event of multiple simultaneous
failures, nor does it take into account Shared Risk Link Groups
(SRLGs). Also, while the MRT Lowpoint algorithm is computationally
efficient, it is also new. In order for MRT-FRR to function
properly, all of the other nodes in the network that support MRT must
correctly compute next hops based on the same algorithm and install
the corresponding forwarding state. This is in contrast to other FRR
methods where the calculation of backup paths generally involves
repeated application of the simpler and widely deployed Shortest Path
First (SPF) algorithm, and backup paths themselves reuse the
forwarding state used for shortest path forwarding of normal traffic.
Section 13 provides operational guidance related to verification of
MRT forwarding paths.
In addition to supporting IP and LDP unicast fast reroute, the
diverse forwarding topologies and guarantee of 100% coverage permit
fast-reroute technology to be applied to multicast traffic as
described in [MRT-ARCH]. However, the current document does not
address the multicast applications of MRTs.
1.1. Importance of 100% Coverage
Fast reroute is based upon the single failure assumption: that the
time between single failures is long enough for a network to
reconverge and start forwarding on the new shortest paths. That does
not imply that the network will only experience one failure or
change.
It is straightforward to analyze a particular network topology for
coverage. However, a real network does not always have the same
topology. For instance, maintenance events will take links or nodes
out of use. Simply costing out a link can have a significant effect
on what Loop-Free Alternates (LFAs) are available. Similarly, after
a single failure has happened, the topology is changed and its
associated coverage has changed as well. Finally, many networks have
new routers or links added and removed; each of those changes can
have an effect on the coverage for topology-sensitive methods such as
LFA and Remote LFA. If fast reroute is important for the network
services provided, then a method that guarantees 100% coverage is
important to accommodate natural network topology changes.
Atlas, et al. Standards Track PAGE 4
RFC 7812 MRT Unicast FRR Architecture June 2016
When a network needs to use Ordered FIB [RFC 6976] or Nearside
Tunneling [RFC 5715] as a micro-loop prevention mechanism [RFC 5715],
then the whole IGP area needs to have alternates available. This
allows the micro-loop prevention mechanism, which requires slower
network convergence, to take the necessary time without adversely
impacting traffic. Without complete coverage, traffic to the
unprotected destinations will be dropped for significantly longer
than with current convergence -- where routers individually converge
as fast as possible. See Section 12.1 for more discussion of micro-
loop prevention and MRTs.
1.2. Partial Deployment and Backwards Compatibility
MRT-FRR supports partial deployment. Routers advertise their ability
to support MRT. Inside the MRT-capable connected group of routers
(referred to as an MRT Island), the MRTs are computed. Alternates to
destinations outside the MRT Island are computed and depend upon the
existence of a loop-free neighbor of the MRT Island for that
destination. MRT Islands are discussed in detail in Section 7, and
partial deployment is discussed in more detail in Section 13.5.
2. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC 2119].
3. Terminology
network graph: A graph that reflects the network topology where all
links connect exactly two nodes and broadcast links have been
transformed into the standard pseudonode representation.
cut-link: A link whose removal partitions the network. A cut-link
by definition must be connected between two cut-vertices. If
there are multiple parallel links, then they are referred to as
cut-links in this document if removing the set of parallel links
would partition the network graph.
cut-vertex: A vertex whose removal partitions the network graph.
2-connected: A graph that has no cut-vertices. This is a graph
that requires two nodes to be removed before the network is
partitioned.
2-connected cluster: A maximal set of nodes that are 2-connected.
block: Either a 2-connected cluster, a cut-edge, or a cut-vertex.
Atlas, et al. Standards Track PAGE 5
RFC 7812 MRT Unicast FRR Architecture June 2016
Redundant Trees (RT): A pair of trees where the path from any node
X to the root R along the first tree is node-disjoint with the
path from the same node X to the root along the second tree.
Redundant trees can always be computed in 2-connected graphs.
Maximally Redundant Trees (MRT): A pair of trees where the path
from any node X to the root R along the first tree and the path
from the same node X to the root along the second tree share the
minimum number of nodes and the minimum number of links. Each
such shared node is a cut-vertex. Any shared links are cut-links.
In graphs that are not 2-connected, it is not possible to compute
RTs. However, it is possible to compute MRTs. MRTs are maximally
redundant in the sense that they are as redundant as possible
given the constraints of the network graph.
Directed Acyclic Graph (DAG): A graph where all links are directed
and there are no cycles in it.
Almost Directed Acyclic Graph (ADAG): A graph with one node
designated as the root. The graph has the property that if all
links incoming to the root were removed, then the resulting graph
would be a DAG.
Generalized ADAG (GADAG): A graph that is the combination of the
ADAGs of all blocks.
MRT-Red: MRT-Red is used to describe one of the two MRTs; it is
used to describe the associated forwarding topology and MPLS
Multi-Topology IDentifier (MT-ID). Specifically, MRT-Red is the
decreasing MRT where links in the GADAG are taken in the direction
from a higher topologically ordered node to a lower one.
MRT-Blue: MRT-Blue is used to describe one of the two MRTs; it is
used to described the associated forwarding topology and MPLS
MT-ID. Specifically, MRT-Blue is the increasing MRT where links
in the GADAG are taken in the direction from a lower topologically
ordered node to a higher one.
Rainbow MRT: It is useful to have an MPLS MT-ID that refers to the
multiple MRT forwarding topologies and to the default forwarding
topology. This is referred to as the Rainbow MRT MPLS MT-ID and
is used by LDP to reduce signaling and permit the same label to
always be advertised to all peers for the same (MT-ID, Prefix).
MRT Island: The set of routers that support a particular MRT
profile and the links connecting them that support MRT.
Atlas, et al. Standards Track PAGE 6
RFC 7812 MRT Unicast FRR Architecture June 2016
Island Border Router (IBR): A router in the MRT Island that is
connected to a router not in the MRT Island, both of which are in
a common area or level.
Island Neighbor (IN): A router that is not in the MRT Island but is
adjacent to an IBR and in the same area/level as the IBR.
named proxy-node: A proxy-node can represent a destination prefix
that can be attached to the MRT Island via at least two routers.
It is named if there is a way that traffic can be encapsulated to
reach specifically that proxy node; this could be because there is
an LDP FEC (Forwarding Equivalence Class) for the associated
prefix or because MRT-Red and MRT-Blue IP addresses are advertised
in an undefined fashion for that proxy-node.
4. Maximally Redundant Trees (MRT)
A pair of Maximally Redundant Trees is a pair of directed spanning
trees that provides maximally disjoint paths towards their common
root. Only links or nodes whose failure would partition the network
(i.e., cut-links and cut-vertices) are shared between the trees. The
MRT Lowpoint algorithm is given in [RFC 7811]. This algorithm can be
computed in O(e + n log n); it is less than three SPFs. This
document describes how the MRTs can be used and not how to compute
them.
MRT provides destination-based trees for each destination. Each
router stores its normal primary next hop(s) as well as MRT-Blue next
hop(s) and MRT-Red next hop(s) toward each destination. The
alternate will be selected between the MRT-Blue and MRT-Red.
The most important thing to understand about MRTs is that for each
pair of destination-routed MRTs, there is a path from every node X to
the destination D on the Blue MRT that is as disjoint as possible
from the path on the Red MRT.
For example, in Figure 1, there is a network graph that is
2-connected in (a) and associated MRTs in (b) and (c). One can
consider the paths from B to R; on the Blue MRT, the paths are
B->F->D->E->R or B->C->D->E->R. On the Red MRT, the path is B->A->R.
These are clearly link and node-disjoint. These MRTs are redundant
trees because the paths are disjoint.
Atlas, et al. Standards Track PAGE 7
RFC 7812 MRT Unicast FRR Architecture June 2016
[E]---[D]---| [E]<--[D]<--| [E]-->[D]---|
| | | | ^ | | |
| | | V | | V V
[R] [F] [C] [R] [F] [C] [R] [F] [C]
| | | ^ ^ ^ | |
| | | | | | V |
[A]---[B]---| [A]-->[B]---| [A]<--[B]<--|
(a) (b) (c)
a 2-connected graph Blue MRT towards R Red MRT towards R
Figure 1: A 2-Connected Network
By contrast, in Figure 2, the network in (a) is not 2-connected. If
C, G, or the link C<->G failed, then the network would be
partitioned. It is clearly impossible to have two link-disjoint or
node-disjoint paths from G, J, or H to R. The MRTs given in (b) and
(c) offer paths that are as disjoint as possible. For instance, the
paths from B to R are the same as in Figure 1 and the path from G to
R on the Blue MRT is G->C->D->E->R and on the Red MRT is
G->C->B->A->R.
[E]---[D]---| |---[J]
| | | | |
| | | | |
[R] [F] [C]---[G] |
| | | | |
| | | | |
[A]---[B]---| |---[H]
(a) a graph that is not 2-connected
[E]<--[D]<--| [J] [E]-->[D]---| |---[J]
| ^ | | | | | ^
V | | | V V V |
[R] [F] [C]<--[G] | [R] [F] [C]<--[G] |
^ ^ ^ | ^ | | |
| | | V | V | |
[A]-->[B]---| |---[H] [A]<--[B]<--| [H]
(b) Blue MRT towards R (c) Red MRT towards R
Figure 2: A Network That Is Not 2-Connected
Atlas, et al. Standards Track PAGE 8
RFC 7812 MRT Unicast FRR Architecture June 2016
5. MRT and Fast Reroute
In normal IGP routing, each router has its Shortest Path Tree (SPT)
to all destinations. From the perspective of a particular
destination, D, this looks like a reverse SPT (rSPT). To use MRT, in
addition, each destination D has two MRTs associated with it; by
convention these will be called the MRT-Blue and MRT-Red. MRT-FRR is
realized by using multi-topology forwarding. There is a MRT-Blue
forwarding topology and a MRT-Red forwarding topology.
Any IP/LDP fast-reroute technique beyond LFA requires an additional
dataplane procedure, such as an additional forwarding mechanism. The
well-known options are multi-topology forwarding (used by MRT-FRR),
tunneling (e.g., [RFC 6981] or [RFC 7490]), and per-interface
forwarding (e.g., Loop-Free Failure Insensitive Routing in
[EnyediThesis]).
When there is a link or node failure affecting, but not partitioning,
the network, each node will still have at least one path via one of
the MRTs to reach the destination D. For example, in Figure 2, B
would normally forward traffic to R across the path B->A->R. If the
B<->A link fails, then B could use the MRT-Blue path B->F->D->E->R.
As is always the case with fast-reroute technologies, forwarding does
not change until a local failure is detected. Packets are forwarded
along the shortest path. The appropriate alternate to use is pre-
computed. [RFC 7811] describes exactly how to determine whether the
MRT-Blue next hops or the MRT-Red next hops should be the MRT
alternate next hops for a particular primary next hop to a particular
destination.
MRT alternates are always available to use. It is a local decision
whether to use an MRT alternate, an LFA, or some other type of
alternate.
As described in [RFC 5286], when a worse failure than is anticipated
happens, using LFAs that are not downstream neighbors can cause
looping among alternates. Section 1.1 of [RFC 5286] gives an example
of link-protecting alternates causing a loop on node failure. Even
if a worse failure than anticipated happens, the use of MRT
alternates will not cause looping.
6. Unicast Forwarding with MRT Fast Reroute
There are three possible types of routers involved in forwarding a
packet along an MRT path. At the MRT ingress router, the packet
leaves the shortest path to the destination and follows an MRT path
to the destination. In an FRR application, the MRT ingress router is
Atlas, et al. Standards Track PAGE 9
RFC 7812 MRT Unicast FRR Architecture June 2016
the PLR. An MRT transit router takes a packet that arrives already
associated with the particular MRT, and forwards it on that same MRT.
In some situations (to be discussed later), the packet will need to
leave the MRT path and return to the shortest path. This takes place
at the MRT egress router. The MRT ingress and egress functionality
may depend on the underlying type of packet being forwarded (LDP or
IP). The MRT transit functionality is independent of the type of
packet being forwarded. We first consider several MRT transit
forwarding mechanisms. Then, we look at how these forwarding
mechanisms can be applied to carrying LDP and IP traffic.
6.1. Introduction to MRT Forwarding Options
The following options for MRT forwarding mechanisms are considered.
1. MRT LDP Labels
A. Topology-scoped FEC encoded using a single label
B. Topology and FEC encoded using a two-label stack
2. MRT IP Tunnels
A. MRT IPv4 Tunnels
B. MRT IPv6 Tunnels
6.1.1. MRT LDP Labels
We consider two options for the MRT forwarding mechanisms using MRT
LDP labels.
6.1.1.1. Topology-Scoped FEC Encoded Using a Single Label (Option 1A)
[RFC 7307] provides a mechanism to distribute FEC-label bindings
scoped to a given MPLS topology (represented by MPLS MT-ID). To use
multi-topology LDP to create MRT forwarding topologies, we associate
two MPLS MT-IDs with the MRT-Red and MRT-Blue forwarding topologies,
in addition to the default shortest path forwarding topology with
MT-ID=0.
With this forwarding mechanism, a single label is distributed for
each topology-scoped FEC. For a given FEC in the default topology
(call it default-FEC-A), two additional topology-scoped FECs would be
created, corresponding to the Red and Blue MRT forwarding topologies
(call them red-FEC-A and blue-FEC-A). A router supporting this MRT
transit forwarding mechanism advertises a different FEC-label binding
for each of the three topology-scoped FECs. When a packet is
Atlas, et al. Standards Track PAGE 10
RFC 7812 MRT Unicast FRR Architecture June 2016
received with a label corresponding to red-FEC-A (for example), an
MRT transit router will determine the next hop for the MRT-Red
forwarding topology for that FEC, swap the incoming label with the
outgoing label corresponding to red-FEC-A learned from the MRT-Red
next-hop router, and forward the packet.
This forwarding mechanism has the useful property that the FEC
associated with the packet is maintained in the labels at each hop
along the MRT. We will take advantage of this property when
specifying how to carry LDP traffic on MRT paths using multi-topology
LDP labels.
This approach is very simple for hardware to support. However, it
reduces the label space for other uses, and it increases the memory
needed to store the labels and the communication required by LDP to
distribute FEC-label bindings. In general, this approach will also
increase the time needed to install the FRR entries in the Forwarding
Information Base (FIB) and, hence, the time needed before the next
failure can be protected.
This forwarding option uses the LDP signaling extensions described in
[RFC 7307]. The MRT-specific LDP extensions required to support this
option will be described elsewhere.
6.1.1.2. Topology and FEC Encoded Using a Two-Label Stack (Option 1B)
With this forwarding mechanism, a two-label stack is used to encode
the topology and the FEC of the packet. The top label (topology-id
label) identifies the MRT forwarding topology, while the second label
(FEC label) identifies the FEC. The top label would be a new FEC
type with two values corresponding to MRT Red and Blue topologies.
When an MRT transit router receives a packet with a topology-id
label, the router pops the top label and uses that it to guide the
next-hop selection in combination with the next label in the stack
(the FEC label). The router then swaps the FEC label, using the FEC-
label bindings learned through normal LDP mechanisms. The router
then pushes the topology-id label for the next hop.
As with Option 1A, this forwarding mechanism also has the useful
property that the FEC associated with the packet is maintained in the
labels at each hop along the MRT.
This forwarding mechanism has minimal usage of additional labels,
memory and LDP communication. It does increase the size of packets
and the complexity of the required label operations and lookups.
Atlas, et al. Standards Track PAGE 11
RFC 7812 MRT Unicast FRR Architecture June 2016
This forwarding option is consistent with context-specific label
spaces, as described in [RFC 5331]. However, the precise LDP behavior
required to support this option for MRT has not been specified.
6.1.1.3. Compatibility of MRT LDP Label Options 1A and 1B
MRT transit forwarding based on MRT LDP Label options 1A and 1B can
coexist in the same network, with a packet being forwarded along a
single MRT path using the single label of Option 1A for some hops and
the two-label stack of Option 1B for other hops. However, to
simplify the process of MRT Island formation, we require that all
routers in the MRT Island support at least one common forwarding
mechanism. As an example, the Default MRT Profile requires support
for the MRT LDP Label Option 1A forwarding mechanism. This ensures
that the routers in an MRT island supporting the Default MRT Profile
will be able to establish MRT forwarding paths based on MRT LDP Label
Option 1A. However, an implementation supporting Option 1A may also
support Option 1B. If the scaling or performance characteristics for
the two options differ in this implementation, then it may be
desirable for a pair of adjacent routers to use Option 1B labels
instead of the Option 1A labels. If those routers successfully
negotiate the use of Option 1B labels, they are free to use them.
This can occur without any of the other routers in the MRT Island
being made aware of it.
Note that this document only defines the Default MRT Profile, which
requires support for the MRT LDP Label Option 1A forwarding
mechanism.
6.1.1.4. Required Support for MRT LDP Label Options
If a router supports a profile that includes the MRT LDP Label Option
1A for the MRT transit forwarding mechanism, then it MUST support
Option 1A, which encodes topology-scoped FECs using a single label.
The router MAY also support Option 1B.
If a router supports a profile that includes the MRT LDP Label Option
1B for the MRT transit forwarding mechanism, then it MUST support
Option 1B, which encodes the topology and FEC using a two-label
stack. The router MAY also support Option 1A.
6.1.2. MRT IP Tunnels (Options 2A and 2B)
IP tunneling can also be used as an MRT transit forwarding mechanism.
Each router supporting this MRT transit forwarding mechanism
announces two additional loopback addresses and their associated MRT
color. Those addresses are used as destination addresses for MRT-
blue and MRT-red IP tunnels, respectively. The special loopback
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RFC 7812 MRT Unicast FRR Architecture June 2016
addresses allow the transit nodes to identify the traffic as being
forwarded along either the MRT-blue or MRT-red topology to reach the
tunnel destination. For example, an MRT ingress router can cause a
packet to be tunneled along the MRT-red path to router X by
encapsulating the packet using the MRT-red loopback address
advertised by router X. Upon receiving the packet, router X would
remove the encapsulation header and forward the packet based on the
original destination address.
Either IPv4 (Option 2A) or IPv6 (Option 2B) can be used as the
tunneling mechanism.
Note that the two forwarding mechanisms using LDP Label options do
not require additional loopbacks per router, as is required by the IP
tunneling mechanism. This is because LDP labels are used on a hop-
by-hop basis to identify MRT-blue and MRT-red forwarding topologies.
6.2. Forwarding LDP Unicast Traffic over MRT Paths
In the previous section, we examined several options for providing
MRT transit forwarding functionality, which is independent of the
type of traffic being carried. We now look at the MRT ingress
functionality, which will depend on the type of traffic being carried
(IP or LDP). We start by considering LDP traffic.
We also simplify the initial discussion by assuming that the network
consists of a single IGP area, and that all routers in the network
participate in MRT. Other deployment scenarios that require MRT
egress functionality are considered later in this document.
In principle, it is possible to carry LDP traffic in MRT IP tunnels.
However, for LDP traffic, it is desirable to avoid tunneling.
Tunneling LDP traffic to a remote node requires knowledge of remote
FEC-label bindings so that the LDP traffic can continue to be
forwarded properly when it leaves the tunnel. This requires targeted
LDP sessions, which can add management complexity. As described
below, the two MRT forwarding mechanisms that use LDP labels do not
require targeted LDP sessions.
6.2.1. Forwarding LDP Traffic Using MRT LDP Label Option 1A
The MRT LDP Label Option 1A forwarding mechanism uses topology-scoped
FECs encoded using a single label as described in Section 6.1.1.1.
When a PLR receives an LDP packet that needs to be forwarded on the
MRT-Red (for example), it does a label swap operation, replacing the
usual LDP label for the FEC with the MRT-Red label for that FEC
received from the next-hop router in the MRT-Red computed by the PLR.
When the next-hop router in the MRT-Red receives the packet with the
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RFC 7812 MRT Unicast FRR Architecture June 2016
MRT-Red label for the FEC, the MRT transit forwarding functionality
continues as described in Section 6.1.1.1. In this way, the original
FEC associated with the packet is maintained at each hop along the
MRT.
6.2.2. Forwarding LDP Traffic Using MRT LDP Label Option 1B
The MRT LDP Label Option 1B forwarding mechanism encodes the topology
and the FEC using a two-label stack as described in Section 6.1.1.2.
When a PLR receives an LDP packet that needs to be forwarded on the
MRT-Red, it first does a normal LDP label swap operation, replacing
the incoming normal LDP label associated with a given FEC with the
outgoing normal LDP label for that FEC learned from the next hop on
the MRT-Red. In addition, the PLR pushes the topology-id label
associated with the MRT-Red, and forward the packet to the
appropriate next hop on the MRT-Red. When the next-hop router in the
MRT-Red receives the packet with the MRT-Red label for the FEC, the
MRT transit forwarding functionality continues as described in
Section 6.1.1.2. As with Option 1A, the original FEC associated with
the packet is maintained at each hop along the MRT.
6.2.3. Other Considerations for Forwarding LDP Traffic Using MRT LDP
Labels
Note that forwarding LDP traffic using MRT LDP Labels can be done
without the use of targeted LDP sessions when an MRT path to the
destination FEC is used. The alternates selected in [RFC 7811] use
the MRT path to the destination FEC, so targeted LDP sessions are not
needed. If instead one found it desirable to have the PLR use an MRT
to reach the primary next-next-hop for the FEC, and then continue
forwarding the LDP packet along the shortest path from the primary
next-next-hop, this would require tunneling to the primary next-next-
hop and a targeted LDP session for the PLR to learn the FEC-label
binding for primary next-next-hop to correctly forward the packet.
6.2.4. Required Support for LDP Traffic
For greatest hardware compatibility, routers implementing MRT fast
reroute of LDP traffic MUST support Option 1A of encoding the MT-ID
in the labels (See Section 9).
6.3. Forwarding IP Unicast Traffic over MRT Paths
For IPv4 traffic, there is no currently practical alternative except
tunneling to gain the bits needed to indicate the MRT-Blue or MRT-Red
forwarding topology. For IPv6 traffic, in principle, one could
define bits in the IPv6 options header to indicate the MRT-Blue or
MRT-Red forwarding topology. However, in this document, we have
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RFC 7812 MRT Unicast FRR Architecture June 2016
chosen not to define a solution that would work for IPv6 traffic but
not for IPv4 traffic.
The choice of tunnel egress is flexible since any router closer to
the destination than the next hop can work. This architecture
assumes that the original destination in the area is selected (see
Section 11 for handling of multihomed prefixes); another possible
choice is the next-next-hop towards the destination. As discussed in
the previous section, for LDP traffic, using the MRT to the original
destination simplifies MRT-FRR by avoiding the need for targeted LDP
sessions to the next-next-hop. For IP, that consideration doesn't
apply.
Some situations require tunneling IP traffic along an MRT to a tunnel
endpoint that is not the destination of the IP traffic. These
situations will be discussed in detail later. We note here that an
IP packet with a destination in a different IGP area/level from the
PLR should be tunneled on the MRT to the Area Border Router (ABR) or
Level Border Router (LBR) on the shortest path to the destination.
For a destination outside of the PLR's MRT Island, the packet should
be tunneled on the MRT to a non-proxy-node immediately before the
named proxy-node on that particular color MRT.
6.3.1. Tunneling IP Traffic Using MRT LDP Labels
An IP packet can be tunneled along an MRT path by pushing the
appropriate MRT LDP label(s). Tunneling using LDP labels, as opposed
to IP headers, has the advantage that more installed routers can do
line-rate encapsulation and decapsulation using LDP than using IP.
Also, no additional IP addresses would need to be allocated or
signaled.
6.3.1.1. Tunneling IP Traffic Using MRT LDP Label Option 1A
The MRT LDP Label Option 1A forwarding mechanism uses topology-scoped
FECs encoded using a single label as described in Section 6.1.1.1.
When a PLR receives an IP packet that needs to be forwarded on the
MRT-Red to a particular tunnel endpoint, it does a label push
operation. The label pushed is the MRT-Red label for a FEC
originated by the tunnel endpoint, learned from the next hop on the
MRT-Red.
6.3.1.2. Tunneling IP Traffic Using MRT LDP Label Option 1B
The MRT LDP Label Option 1B forwarding mechanism encodes the topology
and the FEC using a two-label stack as described in Section 6.1.1.2.
When a PLR receives an IP packet that needs to be forwarded on the
MRT-Red to a particular tunnel endpoint, the PLR pushes two labels on
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RFC 7812 MRT Unicast FRR Architecture June 2016
the IP packet. The first (inner) label is the normal LDP label
learned from the next hop on the MRT-Red, associated with a FEC
originated by the tunnel endpoint. The second (outer) label is the
topology-id label associated with the MRT-Red.
For completeness, we note here a potential variation that uses a
single label as opposed to two labels. In order to tunnel an IP
packet over an MRT to the destination of the IP packet as opposed to
an arbitrary tunnel endpoint, one could just push a topology-id label
directly onto the packet. An MRT transit router would need to pop
the topology-id label, do an IP route lookup in the context of that
topology-id label, and push the topology-id label.
6.3.2. Tunneling IP Traffic Using MRT IP Tunnels
In order to tunnel over the MRT to a particular tunnel endpoint, the
PLR encapsulates the original IP packet with an additional IP header
using the MRT-Blue or MRT-Red loopback address of the tunnel
endpoint.
6.3.3. Required Support for IP Traffic
For greatest hardware compatibility and ease in removing the MRT-
topology marking at area/level boundaries, routers that support MPLS
and implement IP MRT fast reroute MUST support tunneling of IP
traffic using MRT LDP Label Option 1A (topology-scoped FEC encoded
using a single label).
7. MRT Island Formation
The purpose of communicating support for MRT is to indicate that the
MRT-Blue and MRT-Red forwarding topologies are created for transit
traffic. The MRT architecture allows for different, potentially
incompatible options. In order to create consistent MRT forwarding
topologies, the routers participating in a particular MRT Island need
to use the same set of options. These options are grouped into MRT
profiles. In addition, the routers in an MRT Island all need to use
the same set of nodes and links within the Island when computing the
MRT forwarding topologies. This section describes the information
used by a router to determine the nodes and links to include in a
particular MRT Island. Some information already exists in the IGPs
and can be used by MRT in Island formation, subject to the
interpretation defined here.
Other information needs to be communicated between routers for which
there do not currently exist protocol extensions. This new
information needs to be shared among all routers in an IGP area, so
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RFC 7812 MRT Unicast FRR Architecture June 2016
defining extensions to existing IGPs to carry this information makes
sense. These new protocol extensions will be defined elsewhere.
Deployment scenarios using multi-topology OSPF or IS-IS, or running
both IS-IS and OSPF on the same routers is out of scope for this
specification. As with LFA, MRT-FRR does not support OSPF Virtual
Links.
At a high level, an MRT Island is defined as the set of routers
supporting the same MRT profile, in the same IGP area/level and with
bidirectional links interconnecting those routers. More detailed
descriptions of these criteria are given below.
7.1. IGP Area or Level
All links in an MRT Island are bidirectional and belong to the same
IGP area or level. For IS-IS, a link belonging to both Level-1 and
Level-2 would qualify to be in multiple MRT Islands. A given ABR or
LBR can belong to multiple MRT Islands, corresponding to the areas or
levels in which it participates. Inter-area forwarding behavior is
discussed in Section 10.
7.2. Support for a Specific MRT Profile
All routers in an MRT Island support the same MRT profile. A router
advertises support for a given MRT profile using an 8-bit MRT Profile
ID value. The "MRT Profile Identifier Registry" is defined in this
document. The protocol extensions for advertising the MRT Profile ID
value will be defined in a future specification. A given router can
support multiple MRT profiles and participate in multiple MRT
Islands. The options that make up an MRT Profile, as well as the
Default MRT Profile, are defined in Section 8.
The process of MRT Island formation takes place independently for
each MRT profile advertised by a given router. For example, consider
a network with 40 connected routers in the same area advertising
support for MRT Profile A and MRT Profile B. Two distinct MRT
Islands will be formed corresponding to Profile A and Profile B, with
each island containing all 40 routers. A complete set of maximally
redundant trees will be computed for each island following the rules
defined for each profile. If we add a third MRT Profile to this
example, with Profile C being advertised by a connected subset of 30
routers, there will be a third MRT Island formed corresponding to
those 30 routers, and a third set of maximally redundant trees will
be computed. In this example, 40 routers would compute and install
two sets of MRT transit forwarding entries corresponding to Profiles
A and B, while 30 routers would compute and install three sets of MRT
transit forwarding entries corresponding to Profiles A, B, and C.
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RFC 7812 MRT Unicast FRR Architecture June 2016
7.3. Excluding Additional Routers and Interfaces from the MRT Island
MRT takes into account existing IGP mechanisms for discouraging
traffic from using particular links and routers, and it introduces an
MRT-specific exclusion mechanism for links.
7.3.1. Existing IGP Exclusion Mechanisms
Mechanisms for discouraging traffic from using particular links
already exist in IS-IS and OSPF. In IS-IS, an interface configured
with a metric of 2^24-2 (0xFFFFFE) will only be used as a last
resort. (An interface configured with a metric of 2^24-1 (0xFFFFFF)
will not be advertised into the topology.) In OSPF, an interface
configured with a metric of 2^16-1 (0xFFFF) will only be used as a
last resort. These metrics can be configured manually to enforce
administrative policy or they can be set in an automated manner as
with LDP IGP synchronization [RFC 5443].
Mechanisms also already exist in IS-IS and OSPF to discourage or
prevent transit traffic from using a particular router. In IS-IS,
the overload bit is prevents transit traffic from using a router.
For OSPFv2 and OSPFv3, [RFC 6987] specifies setting all outgoing
interface metrics to 0xFFFF to discourage transit traffic from using
a router. ([RFC 6987] defines the metric value 0xFFFF as
MaxLinkMetric, a fixed architectural value for OSPF.) For OSPFv3,
[RFC 5340] specifies that a router be excluded from the intra-area SPT
computation if the V6-bit or R-bit of the Link State Advertisement
(LSA) options is not set in the Router LSA.
The following rules for MRT Island formation ensure that MRT FRR
protection traffic does not use a link or router that is discouraged
or prevented from carrying traffic by existing IGP mechanisms.
1. A bidirectional link MUST be excluded from an MRT Island if
either the forward or reverse cost on the link is 0xFFFFFE (for
IS-IS) or 0xFFFF for OSPF.
2. A router MUST be excluded from an MRT Island if it is advertised
with the overload bit set (for IS-IS), or it is advertised with
metric values of 0xFFFF on all of its outgoing interfaces (for
OSPFv2 and OSPFv3).
3. A router MUST be excluded from an MRT Island if it is advertised
with either the V6-bit or R-bit of the LSA options not set in the
Router LSA.
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RFC 7812 MRT Unicast FRR Architecture June 2016
7.3.2. MRT-Specific Exclusion Mechanism
This architecture also defines a means of excluding an otherwise
usable link from MRT Islands. The protocol extensions for
advertising that a link is MRT-Ineligible will be defined elsewhere.
A link with either interface advertised as MRT-Ineligible MUST be
excluded from an MRT Island. Note that an interface advertised as
MRT-Ineligible by a router is ineligible with respect to all profiles
advertised by that router.
7.4. Connectivity
All of the routers in an MRT Island MUST be connected by
bidirectional links with other routers in the MRT Island.
Disconnected MRT Islands will operate independently of one another.
7.5. Algorithm for MRT Island Identification
An algorithm that allows a computing router to identify the routers
and links in the local MRT Island satisfying the above rules is given
in Section 5.2 of [RFC 7811].
8. MRT Profile
An MRT Profile is a set of values and options related to MRT
behavior. The complete set of options is designated by the
corresponding 8-bit Profile ID value.
This document specifies the values and options that correspond to the
Default MRT Profile (Profile ID = 0). Future documents may define
other MRT Profiles by specifying the MRT Profile Options below.
8.1. MRT Profile Options
Below is a description of the values and options that define an MRT
Profile.
MRT Algorithm: This identifies the particular algorithm for
computing maximally redundant trees used by the router for this
profile.
MRT-Red MT-ID: This specifies the MPLS MT-ID to be associated with
the MRT-Red forwarding topology. It is allocated from the MPLS
Multi-Topology Identifiers Registry.
MRT-Blue MT-ID: This specifies the MPLS MT-ID to be associated with
the MRT-Blue forwarding topology. It is allocated from the MPLS
Multi-Topology Identifiers Registry.
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RFC 7812 MRT Unicast FRR Architecture June 2016
GADAG Root Selection Policy: This specifies the manner in which the
GADAG root is selected. All routers in the MRT Island need to use
the same GADAG root in the calculations used construct the MRTs.
A valid GADAG Root Selection Policy MUST be such that each router
in the MRT Island chooses the same GADAG root based on information
available to all routers in the MRT Island. GADAG Root Selection
Priority values, advertised as router-specific MRT parameters, MAY
be used in a GADAG Root Selection Policy.
MRT Forwarding Mechanism: This specifies which forwarding mechanism
the router uses to carry transit traffic along MRT paths. A
router that supports a specific MRT forwarding mechanism must
program appropriate next hops into the forwarding plane. The
current options are MRT LDP Label Option 1A, MRT LDP Label Option
1B, IPv4 Tunneling, IPv6 Tunneling, and None. If IPv4 is
supported, then both MRT-Red and MRT-Blue IPv4 loopback addresses
SHOULD be specified. If IPv6 is supported, both MRT-Red and MRT-
Blue IPv6 loopback addresses SHOULD be specified.
Recalculation: Recalculation specifies the process and timing by
which new MRTs are computed after the topology has been modified.
Area/Level Border Behavior: This specifies how traffic traveling on
the MRT-Blue or MRT-Red in one area should be treated when it
passes into another area.
Other Profile-Specific Behavior: Depending upon the use-case for the
profile, there may be additional profile-specific behavior.
When a new MRT Profile is defined, new and unique values should be
allocated from the "MPLS Multi-Topology Identifiers Registry",
corresponding to the MRT-Red and MRT-Blue MT-ID values for the new
MRT Profile.
If a router advertises support for multiple MRT profiles, then it
MUST create the transit forwarding topologies for each of those,
unless the profile specifies the None option for the MRT Forwarding
Mechanism.
The ability of MRT-FRR to support transit forwarding entries for
multiple profiles can be used to facilitate a smooth transition from
an existing deployed MRT Profile to a new MRT Profile. The new
profile can be activated in parallel with the existing profile,
installing the transit forwarding entries for the new profile without
affecting the transit forwarding entries for the existing profile.
Once the new transit forwarding state has been verified, the router
can be configured to use the alternates computed by the new profile
in the event of a failure.
Atlas, et al. Standards Track PAGE 20
RFC 7812 MRT Unicast FRR Architecture June 2016
8.2. Router-Specific MRT Parameters
For some profiles, additional router-specific MRT parameters may need
to be advertised. While the set of options indicated by the MRT
Profile ID must be identical for all routers in an MRT Island, these
router-specific MRT parameters may differ between routers in the same
MRT Island. Several such parameters are described below.
GADAG Root Selection Priority: A GADAG Root Selection Policy MAY
rely on the GADAG Root Selection Priority values advertised by
each router in the MRT Island. A GADAG Root Selection Policy may
use the GADAG Root Selection Priority to allow network operators
to configure a parameter to ensure that the GADAG root is selected
from a particular subset of routers. An example of this use of
the GADAG Root Selection Priority value by the GADAG Root
Selection Policy is given in the Default MRT Profile below.
MRT-Red Loopback Address: This provides the router's loopback
address to reach the router via the MRT-Red forwarding topology.
It can be specified for either IPv4 or IPv6. Note that this
parameter is not needed to support the Default MRT Profile.
MRT-Blue Loopback Address: This provides the router's loopback
address to reach the router via the MRT-Blue forwarding topology.
It can be specified for either IPv4 and IPv6. Note that this
parameter is not needed to support the Default MRT Profile.
Protocol extensions for advertising a router's GADAG Root Selection
Priority value will be defined in other documents. Protocol
extensions for the advertising a router's MRT-Red and MRT-Blue
loopback addresses will be defined elsewhere.
8.3. Default MRT Profile
The following set of options defines the Default MRT Profile. The
Default MRT Profile is indicated by the MRT Profile ID value of 0.
MRT Algorithm: MRT Lowpoint algorithm defined in [RFC 7811].
MRT-Red MPLS MT-ID: This temporary registration has been allocated
from the "MPLS Multi-Topology Identifiers" registry. The
registration request appears in [LDP-MRT].
MRT-Blue MPLS MT-ID: This temporary registration has been allocated
from the "MPLS Multi-Topology Identifiers" registry. The
registration request appears in [LDP-MRT].
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RFC 7812 MRT Unicast FRR Architecture June 2016
GADAG Root Selection Policy: Among the routers in the MRT Island
with the lowest numerical value advertised for GADAG Root
Selection Priority, an implementation MUST pick the router with
the highest Router ID to be the GADAG root. Note that a lower
numerical value for GADAG Root Selection Priority indicates a
higher preference for selection.
Forwarding Mechanisms: MRT LDP Label Option 1A
Recalculation: Recalculation of MRTs SHOULD occur as described in
Section 12.2. This allows the MRT forwarding topologies to
support IP/LDP fast-reroute traffic.
Area/Level Border Behavior: As described in Section 10, ABRs/LBRs
SHOULD ensure that traffic leaving the area also exits the MRT-Red
or MRT-Blue forwarding topology.
9. LDP Signaling Extensions and Considerations
The protocol extensions for LDP will be defined in another document.
A router must indicate that it has the ability to support MRT; having
this explicit allows the use of MRT-specific processing, such as
special handling of FECs sent with the Rainbow MRT MT-ID.
A FEC sent with the Rainbow MRT MT-ID indicates that the FEC applies
to all the MRT-Blue and MRT-Red MT-IDs in supported MRT profiles.
The FEC-label bindings for the default shortest-path-based MT-ID 0
MUST still be sent (even though it could be inferred from the Rainbow
FEC-label bindings) to ensure continuous operation of normal LDP
forwarding. The Rainbow MRT MT-ID is defined to provide an easy way
to handle the special signaling that is needed at ABRs or LBRs. It
avoids the problem of needing to signal different MPLS labels to
different LDP neighbors for the same FEC. Because the Rainbow MRT
MT-ID is used only by ABRs/LBRs or an LDP egress router, it is not
MRT profile specific.
The value of the Rainbow MRT MPLS MT-ID has been temporarily
allocated from the "MPLS Multi-Topology Identifiers" registry. The
registration request appears in [LDP-MRT].
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RFC 7812 MRT Unicast FRR Architecture June 2016
10. Inter-area Forwarding Behavior
An ABR/LBR has two forwarding roles. First, it forwards traffic
within areas. Second, it forwards traffic from one area into
another. These same two roles apply for MRT transit traffic.
Traffic on MRT-Red or MRT-Blue destined inside the area needs to stay
on MRT-Red or MRT-Blue in that area. However, it is desirable for
traffic leaving the area to also exit MRT-Red or MRT-Blue and return
to shortest path forwarding.
For unicast MRT-FRR, the need to stay on an MRT forwarding topology
terminates at the ABR/LBR whose best route is via a different area/
level. It is highly desirable to go back to the default forwarding
topology when leaving an area/level. There are three basic reasons
for this. First, the default topology uses shortest paths; the
packet will thus take the shortest possible route to the destination.
Second, this allows a single router failure that manifests itself in
multiple areas (as would be the case with an ABR/LBR failure) to be
separately identified and repaired around. Third, the packet can be
fast-rerouted again, if necessary, due to a second distinct failure
in a different area.
In OSPF, an ABR that receives a packet on MRT-Red or MRT-Blue towards
destination Z should continue to forward the packet along MRT-Red or
MRT-Blue only if the best route to Z is in the same OSPF area as the
interface that the packet was received on. Otherwise, the packet
should be removed from MRT-Red or MRT-Blue and forwarded on the
shortest-path default forwarding topology.
The above description applies to OSPF. The same essential behavior
also applies to IS-IS if one substitutes IS-IS level for OSPF area.
However, the analogy with OSPF is not exact. An interface in OSPF
can only be in one area, whereas an interface in IS-IS can be in both
Level-1 and Level-2. Therefore, to avoid confusion and address this
difference, we explicitly describe the behavior for IS-IS in
Appendix A. In the following sections, only the OSPF terminology is
used.
10.1. ABR Forwarding Behavior with MRT LDP Label Option 1A
For LDP forwarding where a single label specifies (MT-ID, FEC), the
ABR is responsible for advertising the proper label to each neighbor.
Assume that an ABR has allocated three labels for a particular
destination: L_primary, L_blue, and L_red. To those routers in the
same area as the best route to the destination, the ABR advertises
the following FEC-label bindings: L_primary for the default topology,
L_blue for the MRT-Blue MT-ID, and L_red for the MRT-Red MT-ID, as
expected. However, to routers in other areas, the ABR advertises the
Atlas, et al. Standards Track PAGE 23
RFC 7812 MRT Unicast FRR Architecture June 2016
following FEC-label bindings: L_primary for the default topology and
L_primary for the Rainbow MRT MT-ID. Associating L_primary with the
Rainbow MRT MT-ID causes the receiving routers to use L_primary for
the MRT-Blue MT-ID and for the MRT-Red MT-ID.
The ABR installs all next hops for the best area: primary next hops
for L_primary, MRT-Blue next hops for L_blue, and MRT-Red next hops
for L_red. Because the ABR advertised (Rainbow MRT MT-ID, FEC) with
L_primary to neighbors not in the best area, packets from those
neighbors will arrive at the ABR with a label L_primary and will be
forwarded into the best area along the default topology. By
controlling what labels are advertised, the ABR can thus enforce that
packets exiting the area do so on the shortest-path default topology.
10.1.1. Motivation for Creating the Rainbow-FEC
The desired forwarding behavior could be achieved in the above
example without using the Rainbow-FEC. This could be done by having
the ABR advertise the following FEC-label bindings to neighbors not
in the best area: L1_primary for the default topology, L1_primary for
the MRT-Blue MT-ID, and L1_primary for the MRT-Red MT-ID. Doing this
would require machinery to spoof the labels used in FEC-label binding
advertisements on a per-neighbor basis. Such label-spoofing
machinery does not currently exist in most LDP implementations and
doesn't have other obvious uses.
Many existing LDP implementations do however have the ability to
filter FEC-label binding advertisements on a per-neighbor basis. The
Rainbow-FEC allows us to reuse the existing per-neighbor FEC
filtering machinery to achieve the desired result. By introducing
the Rainbow FEC, we can use per-neighbor FEC-filtering machinery to
advertise the FEC-label binding for the Rainbow-FEC (and filter those
for MRT-Blue and MRT-Red) to non-best-area neighbors of the ABR.
An ABR may choose to either distribute the Rainbow-FEC or distribute
separate MRT-Blue and MRT-Red advertisements. This is a local
choice. A router that supports the MRT LDP Label Option 1A
forwarding mechanism MUST be able to receive and correctly interpret
the Rainbow-FEC.
10.2. ABR Forwarding Behavior with IP Tunneling (Option 2)
If IP tunneling is used, then the ABR behavior is dependent upon the
outermost IP address. If the outermost IP address is an MRT loopback
address of the ABR, then the packet is decapsulated and forwarded
based upon the inner IP address, which should go on the default SPT
topology. If the outermost IP address is not an MRT loopback address
of the ABR, then the packet is simply forwarded along the associated
Atlas, et al. Standards Track PAGE 24
RFC 7812 MRT Unicast FRR Architecture June 2016
forwarding topology. A PLR sending traffic to a destination outside
its local area/level will pick the MRT and use the associated MRT
loopback address of the selected ABR advertising the lowest cost to
the external destination.
Thus, for these two MRT forwarding mechanisms (MRT LDP Label Option
1A and IP tunneling Option 2), there is no need for additional
computation or per-area forwarding state.
10.3. ABR Forwarding Behavior with MRT LDP Label Option 1B
The other MRT forwarding mechanism described in Section 6 uses two
labels: a topology-id label and a FEC-label. This mechanism would
require that any router whose MRT-Red or MRT-Blue next hop is an ABR
would need to determine whether the ABR would forward the packet out
of the area/level. If so, then that router should pop off the
topology-id label before forwarding the packet to the ABR.
For example, in Figure 3, if node H fails, node E has to put traffic
towards prefix p onto MRT-Red. But since node D knows that ABR1 will
use a best route from another area, it is safe for D to pop the
topology-id label and just forward the packet to ABR1 along the MRT-
Red next hop. ABR1 will use the shortest path in Area 10.
In all cases for IS-IS and most cases for OSPF, the penultimate
router can determine what decision the adjacent ABR will make. The
one case where it can't be determined is when two ASBRs are in
different non-backbone areas attached to the same ABR, then the
ASBR's Area ID may be needed for tie-breaking (prefer the route with
the largest OSPF area ID), and the Area ID isn't announced as part of
the ASBR LSA. In this one case, suboptimal forwarding along the MRT
in the other area would happen. If that becomes a realistic
deployment scenario, protocol extensions could be developed to
address this issue.
Atlas, et al. Standards Track PAGE 25
RFC 7812 MRT Unicast FRR Architecture June 2016
+----[C]---- --[D]--[E] --[D]--[E]
| \ / \ / \
p--[A] Area 10 [ABR1] Area 0 [H]--p +-[ABR1] Area 0 [H]-+
| / \ / | \ / |
+----[B]---- --[F]--[G] | --[F]--[G] |
| |
| other |
+----------[p]-------+
area
(a) Example topology (b) Proxy node view in Area 0 nodes
+----[C]<--- [D]->[E]
V \ \
+-[A] Area 10 [ABR1] Area 0 [H]-+
| ^ / / |
| +----[B]<--- [F]->[G] V
| |
+------------->[p]<--------------+
(c) rSPT towards destination p
->[D]->[E] -<[D]<-[E]
/ \ / \
[ABR1] Area 0 [H]-+ +-[ABR1] [H]
/ | | \
[F]->[G] V V -<[F]<-[G]
| |
| |
[p]<------+ +--------->[p]
(d) MRT-Blue in Area 0 (e) MRT-Red in Area 0
Figure 3: ABR Forwarding Behavior and MRTs
11. Prefixes Multiply Attached to the MRT Island
How a computing router S determines its local MRT Island for each
supported MRT profile is already discussed in Section 7.
There are two types of prefixes or FECs that may be multiply attached
to an MRT Island. The first type are multihomed prefixes that
usually connect at a domain or protocol boundary. The second type
represent routers that do not support the profile for the MRT Island.
Atlas, et al. Standards Track PAGE 26
RFC 7812 MRT Unicast FRR Architecture June 2016
The key difference is whether the traffic, once out of the MRT
Island, might re-enter the MRT Island if a loop-free exit point is
not selected.
FRR using LFA has the useful property that it is able to protect
multihomed prefixes against ABR failure. For instance, if a prefix
from the backbone is available via both ABR A and ABR B, if A fails,
then the traffic should be redirected to B. This can be accomplished
with MRT FRR as well.
If ASBR protection is desired, this has additional complexities if
the ASBRs are in different areas. Similarly, protecting labeled BGP
traffic in the event of an ASBR failure has additional complexities
due to the per-ASBR label spaces involved.
As discussed in [RFC 5286], a multihomed prefix could be:
o An out-of-area prefix announced by more than one ABR,
o An AS-External route announced by two or more ASBRs,
o A prefix with iBGP multipath to different ASBRs,
o etc.
See Appendix B for a discussion of a general issue with multihomed
prefixes connected in two different areas.
There are also two different approaches to protection. The first is
tunnel endpoint selection where the PLR picks a router to tunnel to
where that router is loop-free with respect to the failure-point.
Conceptually, the set of candidate routers to provide LFAs expands to
all routers that can be reached via an MRT alternate, attached to the
prefix.
The second is to use a proxy-node, which can be named via MPLS label
or IP address, and pick the appropriate label or IP address to reach
it on either MRT-Blue or MRT-Red as appropriate to avoid the failure
point. A proxy-node can represent a destination prefix that can be
attached to the MRT Island via at least two routers. It is termed a
named proxy-node if there is a way that traffic can be encapsulated
to reach specifically that proxy-node; this could be because there is
an LDP FEC for the associated prefix or because MRT-Red and MRT-Blue
IP addresses are advertised (in an as-yet undefined fashion) for that
proxy-node. Traffic to a named proxy-node may take a different path
than traffic to the attaching router; traffic is also explicitly
forwarded from the attaching router along a predetermined interface
towards the relevant prefixes.
Atlas, et al. Standards Track PAGE 27
RFC 7812 MRT Unicast FRR Architecture June 2016
For IP traffic, multihomed prefixes can use tunnel endpoint
selection. For IP traffic that is destined to a router outside the
MRT Island, if that router is the egress for a FEC advertised into
the MRT Island, then the named proxy-node approach can be used.
For LDP traffic, there is always a FEC advertised into the MRT
Island. The named proxy-node approach should be used, unless the
computing router S knows the label for the FEC at the selected tunnel
endpoint.
If a FEC is advertised from outside the MRT Island into the MRT
Island and the forwarding mechanism specified in the profile includes
LDP Label Option 1A, then the routers learning that FEC MUST also
advertise labels for (MRT-Red, FEC) and (MRT-Blue, FEC) to neighbors
inside the MRT Island. Any router receiving a FEC corresponding to a
router outside the MRT Island or to a multihomed prefix MUST compute
and install the transit MRT-Blue and MRT-Red next hops for that FEC.
The FEC-label bindings for the topology-scoped FECs ((MT-ID 0, FEC),
(MRT-Red, FEC), and (MRT-Blue, FEC)) MUST also be provided via LDP to
neighbors inside the MRT Island.
11.1. Protecting Multihomed Prefixes Using Tunnel Endpoint Selection
Tunnel endpoint selection is a local matter for a router in the MRT
Island since it pertains to selecting and using an alternate and does
not affect the transit MRT-Red and MRT-Blue forwarding topologies.
Let the computing router be S and the next hop F be the node whose
failure is to be avoided. Let the destination be prefix p. Have A
be the router to which the prefix p is attached for S's shortest path
to p.
The candidates for tunnel endpoint selection are those to which the
destination prefix is attached in the area/level. For a particular
candidate B, it is necessary to determine if B is loop-free to reach
p with respect to S and F for node-protection or at least with
respect to S and the link (S, F) for link-protection. If B will
always prefer to send traffic to p via a different area/level, then
this is definitional. Otherwise, distance-based computations are
necessary and an SPF from B's perspective may be necessary. The
following equations give the checks needed; the rationale is similar
to that given in [RFC 5286]. In the inequalities below, D_opt(X,Y)
means the shortest distance from node X to node Y, and D_opt(X,p)
means the shortest distance from node X to prefix p.
Loop-Free for S: D_opt(B, p) < D_opt(B, S) + D_opt(S, p)
Loop-Free for F: D_opt(B, p) < D_opt(B, F) + D_opt(F, p)
Atlas, et al. Standards Track PAGE 28
RFC 7812 MRT Unicast FRR Architecture June 2016
The latter is equivalent to the following, which avoids the need to
compute the shortest path from F to p.
Loop-Free for F: D_opt(B, p) < D_opt(B, F) + D_opt(S, p) - D_opt(S, F)
Finally, the rules for Endpoint selection are given below. The basic
idea is to repair to the prefix-advertising router selected for the
shortest-path and only to select and tunnel to a different endpoint
if necessary (e.g., A=F or F is a cut-vertex or the link (S,F) is a
cut-link).
1. Does S have a node-protecting alternate to A? If so, select
that. Tunnel the packet to A along that alternate. For example,
if LDP is the forwarding mechanism, then push the label (MRT-Red,
A) or (MRT-Blue, A) onto the packet.
2. If not, then is there a router B that is loop-free to reach p
while avoiding both F and S? If so, select B as the endpoint.
Determine the MRT alternate to reach B while avoiding F. Tunnel
the packet to B along that alternate. For example, with LDP,
push the label (MRT-Red, B) or (MRT-Blue, B) onto the packet.
3. If not, then does S have a link-protecting alternate to A? If
so, select that.
4. If not, then is there a router B that is loop-free to reach p
while avoiding S and the link from S to F? If so, select B as
the endpoint and the MRT alternate for reaching B from S that
avoid the link (S,F).
The tunnel endpoint selected will receive a packet destined to itself
and, being the egress, will pop that MPLS label (or have signaled
Implicit Null) and forward based on what is underneath. This
suffices for IP traffic since the tunnel endpoint can use the IP
header of the original packet to continue forwarding the packet.
However, tunneling of LDP traffic requires targeted LDP sessions for
learning the FEC-label binding at the tunnel endpoint.
11.2. Protecting Multihomed Prefixes Using Named Proxy-Nodes
Instead, the named proxy-node method works with LDP traffic without
the need for targeted LDP sessions. It also has a clear advantage
over tunnel endpoint selection, in that it is possible to explicitly
forward from the MRT Island along an interface to a loop-free island
neighbor when that interface may not be a primary next hop.
Atlas, et al. Standards Track PAGE 29
RFC 7812 MRT Unicast FRR Architecture June 2016
A named proxy-node represents one or more destinations and, for LDP
forwarding, has a FEC associated with it that is signaled into the
MRT Island. Therefore, it is possible to explicitly label packets to
go to (MRT-Red, FEC) or (MRT-Blue, FEC); at the border of the MRT
Island, the label will swap to meaning (MT-ID 0, FEC). It would be
possible to have named proxy-nodes for IP forwarding, but this would
require extensions to signal two IP addresses to be associated with
MRT-Red and MRT-Blue for the proxy-node. A named proxy-node can be
uniquely represented by the two routers in the MRT Island to which it
is connected. The extensions to signal such IP addresses will be
defined elsewhere. The details of what label-bindings must be
originated will be described in another document.
Computing the MRT next hops to a named proxy-node and the MRT
alternate for the computing router S to avoid a particular failure
node F is straightforward. The details of the simple constant-time
functions, Select_Proxy_Node_NHs() and
Select_Alternates_Proxy_Node(), are given in [RFC 7811]. A key point
is that computing these MRT next hops and alternates can be done as
new named proxy-nodes are added or removed without requiring a new
MRT computation or impacting other existing MRT paths. This maps
very well to, for example, how OSPFv2 (see [RFC 2328], Section 16.5)
does incremental updates for new summary-LSAs.
The remaining question is how to attach the named proxy-node to the
MRT Island; all the routers in the MRT Island MUST do this
consistently. No more than two routers in the MRT Island can be
selected; one should only be selected if there are no others that
meet the necessary criteria. The named proxy-node is logically part
of the area/level.
There are two sources for candidate routers in the MRT Island to
connect to the named proxy-node. The first set is made up of those
routers in the MRT Island that are advertising the prefix; the named-
proxy-cost assigned to each prefix-advertising router is the
announced cost to the prefix. The second set is made up of those
routers in the MRT Island that are connected to routers not in the
MRT Island but in the same area/level; such routers will be defined
as Island Border Routers (IBRs). The routers connected to the IBRs
that are not in the MRT Island and are in the same area/level as the
MRT Island are Island Neighbors (INs).
Since packets sent to the named proxy-node along MRT-Red or MRT-Blue
may come from any router inside the MRT Island, it is necessary that
whatever router to which an IBR forwards the packet be loop-free with
respect to the whole MRT Island for the destination. Thus, an IBR is
a candidate router only if it possesses at least one IN whose
shortest path to the prefix does not enter the MRT Island. A method
Atlas, et al. Standards Track PAGE 30
RFC 7812 MRT Unicast FRR Architecture June 2016
for identifying Loop-Free Island Neighbors (LFINs) is given in
[RFC 7811]. The named-proxy-cost assigned to each (IBR, IN) pair is
cost(IBR, IN) + D_opt(IN, prefix).
From the set of prefix-advertising routers and the set of IBRs with
at least one LFIN, the two routers with the lowest named-proxy-cost
are selected. Ties are broken based upon the lowest Router ID. For
ease of discussion, the two selected routers will be referred to as
proxy-node attachment routers.
A proxy-node attachment router has a special forwarding role. When a
packet is received destined to (MRT-Red, prefix) or (MRT-Blue,
prefix), if the proxy-node attachment router is an IBR, it MUST swap
to the shortest path forwarding topology (e.g., swap to the label for
(MT-ID 0, prefix) or remove the outer IP encapsulation) and forward
the packet to the IN whose cost was used in the selection. If the
proxy-node attachment router is not an IBR, then the packet MUST be
removed from the MRT forwarding topology and sent along the
interface(s) that caused the router to advertise the prefix; this
interface might be out of the area/level/AS.
11.3. MRT Alternates for Destinations outside the MRT Island
A natural concern with new functionality is how to have it be useful
when it is not deployed across an entire IGP area. In the case of
MRT FRR, where it provides alternates when appropriate LFAs aren't
available, there are also deployment scenarios where it may make
sense to only enable some routers in an area with MRT FRR. A simple
example of such a scenario would be a ring of six or more routers
that is connected via two routers to the rest of the area.
Destinations inside the local island can obviously use MRT
alternates. Destinations outside the local island can be treated
like a multihomed prefix and either endpoint selection or Named
Proxy-Nodes can be used. Named proxy-nodes MUST be supported when
LDP forwarding is supported and a label-binding for the destination
is sent to an IBR.
Naturally, there are more-complicated options to improve coverage,
such as connecting multiple MRT Islands across tunnels, but the need
for the additional complexity has not been justified.
Atlas, et al. Standards Track PAGE 31
RFC 7812 MRT Unicast FRR Architecture June 2016
12. Network Convergence and Preparing for the Next Failure
After a failure, MRT detours ensure that packets reach their intended
destination while the IGP has not reconverged onto the new topology.
As link-state updates reach the routers, the IGP process calculates
the new shortest paths. Two things need attention: micro-loop
prevention and MRT recalculation.
12.1. Micro-loop Prevention and MRTs
A micro-loop is a transient packet-forwarding loop among two or more
routers that can occur during convergence of IGP forwarding state.
[RFC 5715] discusses several techniques for preventing micro-loops.
This section discusses how MRT-FRR relates to two of the micro-loop
prevention techniques discussed in [RFC 5715]: Nearside and Farside
Tunneling.
In Nearside Tunneling, a router (PLR) adjacent to a failure performs
local repair and informs remote routers of the failure. The remote
routers initially tunnel affected traffic to the nearest PLR, using
tunnels that are unaffected by the failure. Once the forwarding
state for normal shortest path routing has converged, the remote
routers return the traffic to shortest path forwarding. MRT-FRR is
relevant for Nearside Tunneling for the following reason. The
process of tunneling traffic to the PLRs and waiting a sufficient
amount of time for IGP forwarding state convergence with Nearside
Tunneling means that traffic will generally rely on the local repair
at the PLR for longer than it would in the absence of Nearside
Tunneling. Since MRT-FRR provides 100% coverage for single link and
node failure, it may be an attractive option to provide the local
repair paths when Nearside Tunneling is deployed.
MRT-FRR is also relevant for the Farside Tunneling micro-loop
prevention technique. In Farside Tunneling, remote routers tunnel
traffic affected by a failure to a node downstream of the failure
with respect to traffic destination. This node can be viewed as
being on the farside of the failure with respect to the node
initiating the tunnel. Note that the discussion of Farside Tunneling
in [RFC 5715] focuses on the case where the farside node is
immediately adjacent to a failed link or node. However, the farside
node may be any node downstream of the failure with respect to
traffic destination, including the destination itself. The tunneling
mechanism used to reach the farside node must be unaffected by the
failure. The alternative forwarding paths created by MRT-FRR have
the potential to be used to forward traffic from the remote routers
upstream of the failure all the way to the destination. In the event
of failure, either the MRT-Red or MRT-Blue path from the remote
upstream router to the destination is guaranteed to avoid a link
Atlas, et al. Standards Track PAGE 32
RFC 7812 MRT Unicast FRR Architecture June 2016
failure or inferred node failure. The MRT forwarding paths are also
guaranteed to not be subject to micro-loops because they are locked
to the topology before the failure.
We note that the computations in [RFC 7811] address the case of a PLR
adjacent to a failure determining which choice of MRT-Red or MRT-Blue
will avoid a failed link or node. More computation may be required
for an arbitrary remote upstream router to determine whether to
choose MRT-Red or MRT-Blue for a given destination and failure.
12.2. MRT Recalculation for the Default MRT Profile
This section describes how the MRT recalculation SHOULD be performed
for the Default MRT Profile. This is intended to support FRR
applications. Other approaches are possible, but they are not
specified in this document.
When a failure event happens, traffic is put by the PLRs onto the MRT
topologies. After that, each router recomputes its SPT and moves
traffic over to that. Only after all the PLRs have switched to using
their SPTs and traffic has drained from the MRT topologies should
each router install the recomputed MRTs into the FIBs.
At each router, therefore, the sequence is as follows:
1. Receive failure notification
2. Recompute SPT.
3. Install the new SPT in the FIB.
4. If the network was stable before the failure occurred, wait a
configured (or advertised) period for all routers to be using
their SPTs and traffic to drain from the MRTs.
5. Recompute MRTs.
6. Install new MRTs in the FIB.
While the recomputed MRTs are not installed in the FIB, protection
coverage is lowered. Therefore, it is important to recalculate the
MRTs and install them quickly.
New protocol extensions for advertising the time needed to recompute
shortest path routes and install them in the FIB will be defined
elsewhere.
Atlas, et al. Standards Track PAGE 33
RFC 7812 MRT Unicast FRR Architecture June 2016
13. Operational Considerations
The following aspects of MRT-FRR are useful to consider when
deploying the technology in different operational environments and
network topologies.
13.1. Verifying Forwarding on MRT Paths
The forwarding paths created by MRT-FRR are not used by normal (non-
FRR) traffic. They are only used to carry FRR traffic for a short
period of time after a failure has been detected. It is RECOMMENDED
that an operator proactively monitor the MRT forwarding paths in
order to be certain that the paths will be able to carry FRR traffic
when needed. Therefore, an implementation SHOULD provide an operator
with the ability to test MRT paths with Operations, Administration,
and Maintenance (OAM) traffic. For example, when MRT paths are
realized using LDP labels distributed for topology-scoped FECs, an
implementation can use the MPLS ping and traceroute as defined in
[RFC 4379] and extended in [RFC 7307] for topology-scoped FECs.
13.2. Traffic Capacity on Backup Paths
During a fast-reroute event initiated by a PLR in response to a
network failure, the flow of traffic in the network will generally
not be identical to the flow of traffic after the IGP forwarding
state has converged, taking the failure into account. Therefore,
even if a network has been engineered to have enough capacity on the
appropriate links to carry all traffic after the IGP has converged
after the failure, the network may still not have enough capacity on
the appropriate links to carry the flow of traffic during a fast-
reroute event. This can result in more traffic loss during the fast-
reroute event than might otherwise be expected.
Note that there are two somewhat distinct aspects to this phenomenon.
The first is that the path from the PLR to the destination during the
fast-reroute event may be different from the path after the IGP
converges. In this case, any traffic for the destination that
reaches the PLR during the fast-reroute event will follow a different
path from the PLR to the destination than will be followed after IGP
convergence.
The second aspect is that the amount of traffic arriving at the PLR
for affected destinations during the fast-reroute event may be larger
than the amount of traffic arriving at the PLR for affected
destinations after IGP convergence. Immediately after a failure, any
non-PLR routers that were sending traffic to the PLR before the
failure will continue sending traffic to the PLR, and that traffic
will be carried over backup paths from the PLR to the destinations.
Atlas, et al. Standards Track PAGE 34
RFC 7812 MRT Unicast FRR Architecture June 2016
After IGP convergence, upstream non-PLR routers may direct some
traffic away from the PLR.
In order to reduce or eliminate the potential for transient traffic
loss due to inadequate capacity during fast-reroute events, an
operator can model the amount of traffic taking different paths
during a fast-reroute event. If it is determined that there is not
enough capacity to support a given fast-reroute event, the operator
can address the issue either by augmenting capacity on certain links
or modifying the backup paths themselves.
The MRT Lowpoint algorithm produces a pair of diverse paths to each
destination. These paths are generated by following the directed
links on a common GADAG. The decision process for constructing the
GADAG in the MRT Lowpoint algorithm takes into account individual IGP
link metrics. At any given node, links are explored in order from
lowest IGP metric to highest IGP metric. Additionally, the process
for constructing the MRT-Red and Blue trees uses SPF traversals of
the GADAG. Therefore, the IGP link metric values affect the computed
backup paths. However, adjusting the IGP link metrics is not a
generally applicable tool for modifying the MRT backup paths.
Achieving a desired set of MRT backup paths by adjusting IGP metrics
while at the same time maintaining the desired flow of traffic along
the shortest paths is not possible in general.
MRT-FRR allows an operator to exclude a link from the MRT Island, and
thus the GADAG, by advertising it as MRT-Ineligible. Such a link
will not be used on the MRT forwarding path for any destination.
Advertising links as MRT-Ineligible is the main tool provided by MRT-
FRR for keeping backup traffic off of lower bandwidth links during
fast-reroute events.
Note that all of the backup paths produced by the MRT Lowpoint
algorithm are closely tied to the common GADAG computed as part of
that algorithm. Therefore, it is generally not possible to modify a
subset of paths without affecting other paths. This precludes more
fine-grained modification of individual backup paths when using only
paths computed by the MRT Lowpoint algorithm.
However, it may be desirable to allow an operator to use MRT-FRR
alternates together with alternates provided by other FRR
technologies. A policy-based alternate selection process can allow
an operator to select the best alternate from those provided by MRT
and other FRR technologies. As a concrete example, it may be
desirable to implement a policy where a downstream LFA (if it exists
for a given failure mode and destination) is preferred over a given
MRT alternate. This combination gives the operator the ability to
affect where traffic flows during a fast-reroute event, while still
Atlas, et al. Standards Track PAGE 35
RFC 7812 MRT Unicast FRR Architecture June 2016
producing backup paths that use no additional labels for LDP traffic
and will not loop under multiple failures. This and other choices of
alternate selection policy can be evaluated in the context of their
effect on fast-reroute traffic flow and available capacity, as well
as other deployment considerations.
Note that future documents may define MRT profiles in addition to the
default profile defined here. Different MRT profiles will generally
produce alternate paths with different properties. An implementation
may allow an operator to use different MRT profiles instead of or in
addition to the default profile.
13.3. MRT IP Tunnel Loopback Address Management
As described in Section 6.1.2, if an implementation uses IP tunneling
as the mechanism to realize MRT forwarding paths, each node must
advertise an MRT-Red and an MRT-Blue loopback address. These IP
addresses must be unique within the routing domain to the extent that
they do not overlap with each other or with any other routing table
entries. It is expected that operators will use existing tools and
processes for managing infrastructure IP addresses to manage these
additional MRT-related loopback addresses.
13.4. MRT-FRR in a Network with Degraded Connectivity
Ideally, routers in a service provider network using MRT-FRR will be
initially deployed in a 2-connected topology, allowing MRT-FRR to
find completely diverse paths to all destinations. However, a
network can differ from an ideal 2-connected topology for many
possible reasons, including network failures and planned maintenance
events.
MRT-FRR is designed to continue to function properly when network
connectivity is degraded. When a network contains cut-vertices or
cut-links dividing the network into different 2-connected blocks,
MRT-FRR will continue to provide completely diverse paths for
destinations within the same block as the PLR. For a destination in
a different block from the PLR, the redundant paths created by MRT-
FRR will be link and node diverse within each block, and the paths
will only share links and nodes that are cut-links or cut-vertices in
the topology.
If a network becomes partitioned with one set of routers having no
connectivity to another set of routers, MRT-FRR will function
independently in each set of connected routers, providing redundant
paths to destinations in same set of connected routers as a given
PLR.
Atlas, et al. Standards Track PAGE 36
RFC 7812 MRT Unicast FRR Architecture June 2016
13.5. Partial Deployment of MRT-FRR in a Network
A network operator may choose to deploy MRT-FRR only on a subset of
routers in an IGP area. MRT-FRR is designed to accommodate this
partial deployment scenario. Only routers that advertise support for
a given MRT profile will be included in a given MRT Island. For a
PLR within the MRT Island, MRT-FRR will create redundant forwarding
paths to all destinations with the MRT Island using maximally
redundant trees all the way to those destinations. For destinations
outside of the MRT Island, MRT-FRR creates paths to the destination
that use forwarding state created by MRT-FRR within the MRT Island
and shortest path forwarding state outside of the MRT Island. The
paths created by MRT-FRR to non-Island destinations are guaranteed to
be diverse within the MRT Island (if topologically possible).
However, the part of the paths outside of the MRT Island may not be
diverse.
14. IANA Considerations
IANA has created the "MRT Profile Identifier Registry". The range is
0 to 255. The Default MRT Profile defined in this document has value
0. Values 1-200 are allocated by Standards Action. Values 201-220
are for Experimental Use. Values 221-254 are for Private Use. Value
255 is reserved for future registry extension. (The allocation and
use policies are described in [RFC 5226].)
The initial registry is shown below.
Value Description Reference
------- ---------------------------------------- ------------
0 Default MRT Profile RFC 7812
1-200 Unassigned
201-220 Experimental Use
221-254 Private Use
255 Reserved (for future registry extension)
The "MRT Profile Identifier Registry" is a new registry in the IANA
Matrix. Following existing conventions, http://www.iana.org/
protocols displays a new header: "Maximally Redundant Tree (MRT)
Parameters". Under that header, there is an entry for "MRT Profile
Identifier Registry", which links to the registry itself at
http://www.iana.org/assignments/mrt-parameters.
Atlas, et al. Standards Track PAGE 37
RFC 7812 MRT Unicast FRR Architecture June 2016
15. Security Considerations
In general, MRT forwarding paths do not follow shortest paths. The
transit forwarding state corresponding to the MRT paths is created
during normal operations (before a failure occurs). Therefore, a
malicious packet with an appropriate header injected into the network
from a compromised location would be forwarded to a destination along
a non-shortest path. When this technology is deployed, a network
security design should not rely on assumptions about potentially
malicious traffic only following shortest paths.
It should be noted that the creation of non-shortest forwarding paths
is not unique to MRT.
MRT-FRR requires that routers advertise information used in the
formation of MRT backup paths. While this document does not specify
the protocol extensions used to advertise this information, we
discuss security considerations related to the information itself.
Injecting false MRT-related information could be used to direct some
MRT backup paths over compromised transmission links. Combined with
the ability to generate network failures, this could be used to send
traffic over compromised transmission links during a fast-reroute
event. In order to prevent this potential exploit, a receiving
router needs to be able to authenticate MRT-related information that
claims to have been advertised by another router.
16. References
16.1. Normative References
[RFC 2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC 2119, March 1997,
<http://www.rfc-editor.org/info/RFC 2119>.
[RFC 5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 5226,
DOI 10.17487/RFC 5226, May 2008,
<http://www.rfc-editor.org/info/RFC 5226>.
[RFC 7307] Zhao, Q., Raza, K., Zhou, C., Fang, L., Li, L., and D.
King, "LDP Extensions for Multi-Topology", RFC 7307,
DOI 10.17487/RFC 7307, July 2014,
<http://www.rfc-editor.org/info/RFC 7307>.
Atlas, et al. Standards Track PAGE 38
RFC 7812 MRT Unicast FRR Architecture June 2016
[RFC 7811] Enyedi, G., Ed., Csaszar, A., Atlas, A., Ed., Bowers, C.,
and A. Gopalan, "An Algorithm for Computing IP/LDP Fast
Reroute Using Maximally Redundant Trees (MRT-FRR)",
RFC 7811, DOI 10.17487/RFC 7811, June 2016,
<http://www.rfc-editor.org/info/RFC 7811>.
16.2. Informative References
[EnyediThesis]
Enyedi, G., "Novel Algorithms for IP Fast Reroute",
Department of Telecommunications and Media Informatics,
Budapest University of Technology and Economics Ph.D.
Thesis, February 2011,
<https://repozitorium.omikk.bme.hu/bitstream/
handle/10890/1040/ertekezes.pdf>.
[LDP-MRT] Atlas, A., Tiruveedhula, K., Bowers, C., Tantsura, J., and
IJ. Wijnands, "LDP Extensions to Support Maximally
Redundant Trees", Work in Progress, draft-ietf-mpls-ldp-
mrt-03, May 2016.
[MRT-ARCH]
Atlas, A., Kebler, R., Wijnands, IJ., Csaszar, A., and G.
Enyedi, "An Architecture for Multicast Protection Using
Maximally Redundant Trees", Work in Progress, draft-atlas-
rtgwg-mrt-mc-arch-02, July 2013.
[RFC 2328] Moy, J., "OSPF Version 2", STD 54, RFC 2328,
DOI 10.17487/RFC 2328, April 1998,
<http://www.rfc-editor.org/info/RFC 2328>.
[RFC 4379] Kompella, K. and G. Swallow, "Detecting Multi-Protocol
Label Switched (MPLS) Data Plane Failures", RFC 4379,
DOI 10.17487/RFC 4379, February 2006,
<http://www.rfc-editor.org/info/RFC 4379>.
[RFC 5286] Atlas, A., Ed. and A. Zinin, Ed., "Basic Specification for
IP Fast Reroute: Loop-Free Alternates", RFC 5286,
DOI 10.17487/RFC 5286, September 2008,
<http://www.rfc-editor.org/info/RFC 5286>.
[RFC 5331] Aggarwal, R., Rekhter, Y., and E. Rosen, "MPLS Upstream
Label Assignment and Context-Specific Label Space",
RFC 5331, DOI 10.17487/RFC 5331, August 2008,
<http://www.rfc-editor.org/info/RFC 5331>.
Atlas, et al. Standards Track PAGE 39
RFC 7812 MRT Unicast FRR Architecture June 2016
[RFC 5340] Coltun, R., Ferguson, D., Moy, J., and A. Lindem, "OSPF
for IPv6", RFC 5340, DOI 10.17487/RFC 5340, July 2008,
<http://www.rfc-editor.org/info/RFC 5340>.
[RFC 5443] Jork, M., Atlas, A., and L. Fang, "LDP IGP
Synchronization", RFC 5443, DOI 10.17487/RFC 5443, March
2009, <http://www.rfc-editor.org/info/RFC 5443>.
[RFC 5714] Shand, M. and S. Bryant, "IP Fast Reroute Framework",
RFC 5714, DOI 10.17487/RFC 5714, January 2010,
<http://www.rfc-editor.org/info/RFC 5714>.
[RFC 5715] Shand, M. and S. Bryant, "A Framework for Loop-Free
Convergence", RFC 5715, DOI 10.17487/RFC 5715, January
2010, <http://www.rfc-editor.org/info/RFC 5715>.
[RFC 6976] Shand, M., Bryant, S., Previdi, S., Filsfils, C.,
Francois, P., and O. Bonaventure, "Framework for Loop-Free
Convergence Using the Ordered Forwarding Information Base
(oFIB) Approach", RFC 6976, DOI 10.17487/RFC 6976, July
2013, <http://www.rfc-editor.org/info/RFC 6976>.
[RFC 6981] Bryant, S., Previdi, S., and M. Shand, "A Framework for IP
and MPLS Fast Reroute Using Not-Via Addresses", RFC 6981,
DOI 10.17487/RFC 6981, August 2013,
<http://www.rfc-editor.org/info/RFC 6981>.
[RFC 6987] Retana, A., Nguyen, L., Zinin, A., White, R., and D.
McPherson, "OSPF Stub Router Advertisement", RFC 6987,
DOI 10.17487/RFC 6987, September 2013,
<http://www.rfc-editor.org/info/RFC 6987>.
[RFC 7490] Bryant, S., Filsfils, C., Previdi, S., Shand, M., and N.
So, "Remote Loop-Free Alternate (LFA) Fast Reroute (FRR)",
RFC 7490, DOI 10.17487/RFC 7490, April 2015,
<http://www.rfc-editor.org/info/RFC 7490>.
Atlas, et al. Standards Track PAGE 40
RFC 7812 MRT Unicast FRR Architecture June 2016
Appendix A. Inter-level Forwarding Behavior for IS-IS
In the description below, we use the terms "Level-1-only interface",
"Level-2-only interface", and "Level-1-and-Level-2 interface" to mean
an interface that has formed only a Level-1 adjacency, only a Level-2
adjacency, or both Level-1 and Level-2 adjacencies. Note that IS-IS
also defines the concept of areas. A router is configured with an
IS-IS area identifier, and a given router may be configured with
multiple IS-IS area identifiers. For an IS-IS Level-1 adjacency to
form between two routers, at least one IS-IS area identifier must
match. IS-IS Level-2 adjacencies do not require any area identifiers
to match. The behavior described below does not explicitly refer to
IS-IS area identifiers. However, IS-IS area identifiers will
indirectly affect the behavior by affecting the formation of Level-1
adjacencies.
First, consider a packet destined to Z on MRT-Red or MRT-Blue
received on a Level-1-only interface. If the best shortest path
route to Z was learned from a Level-1 advertisement, then the packet
should continue to be forwarded along MRT-Red or MRT-Blue. If,
instead, the best route was learned from a Level-2 advertisement,
then the packet should be removed from MRT-Red or MRT-Blue and
forwarded on the shortest-path default forwarding topology.
Now consider a packet destined to Z on MRT-Red or MRT-Blue received
on a Level-2-only interface. If the best route to Z was learned from
a Level-2 advertisement, then the packet should continue to be
forwarded along MRT-Red or MRT-Blue. If, instead, the best route was
learned from a Level-1 advertisement, then the packet should be
removed from MRT-Red or MRT-Blue and forwarded on the shortest-path
default forwarding topology.
Finally, consider a packet destined to Z on MRT-Red or MRT-Blue
received on a Level-1-and-Level-2 interface. This packet should
continue to be forwarded along MRT-Red or MRT-Blue, regardless of
which level the route was learned from.
An implementation may simplify the decision-making process above by
using the interface of the next hop for the route to Z to determine
the level from which the best route to Z was learned. If the next
hop points out a Level-1-only interface, then the route was learned
from a Level-1 advertisement. If the next hop points out a Level-
2-only interface, then the route was learned from a Level-2
advertisement. A next hop that points out a Level-1-and-Level-2
interface does not provide enough information to determine the source
of the best route. With this simplification, an implementation would
need to continue forwarding along MRT-Red or MRT-Blue when the next-
hop points out a Level-1-and-Level-2 interface. Therefore, a packet
Atlas, et al. Standards Track PAGE 41
RFC 7812 MRT Unicast FRR Architecture June 2016
on MRT-Red or MRT-Blue going from Level-1 to Level-2 (or vice versa)
that traverses a Level-1-and-Level-2 interface in the process will
remain on MRT-Red or MRT-Blue. This simplification may not always
produce the optimal forwarding behavior, but it does not introduce
interoperability problems. The packet will stay on an MRT backup
path longer than necessary, but it will still reach its destination.
Appendix B. General Issues with Area Abstraction
When a multihomed prefix is connected in two different areas, it may
be impractical to protect them without adding the complexity of
explicit tunneling. This is also a problem for LFA and Remote-LFA.
50
|----[ASBR Y]---[B]---[ABR 2]---[C] Backbone Area 0:
| | ABR 1, ABR 2, C, D
| |
| | Area 20: A, ASBR X
| |
p ---[ASBR X]---[A]---[ABR 1]---[D] Area 10: B, ASBR Y
5 p is a Type 1 AS-external
Figure 4: AS External Prefixes in Different Areas
Consider the network in Figure 4 and assume there is a richer
connective topology that isn't shown, where the same prefix is
announced by ASBR X and ASBR Y, which are in different non-backbone
areas. If the link from A to ASBR X fails, then an MRT alternate
could forward the packet to ABR 1 and ABR 1 could forward it to D,
but then D would find the shortest route is back via ABR 1 to Area
20. This problem occurs because the routers, including the ABR, in
one area are not yet aware of the failure in a different area.
The only way to get it from A to ASBR Y is to explicitly tunnel it to
ASBR Y. If the traffic is unlabeled or the appropriate MPLS labels
are known, then explicit tunneling MAY be used as long as the
shortest path of the tunnel avoids the failure point. In that case,
A must determine that it should use an explicit tunnel instead of an
MRT alternate.
Atlas, et al. Standards Track PAGE 42
RFC 7812 MRT Unicast FRR Architecture June 2016
Acknowledgements
The authors would like to thank Mike Shand for his valuable review
and contributions.
The authors would like to thank Joel Halpern, Hannes Gredler, Ted
Qian, Kishore Tiruveedhula, Shraddha Hegde, Santosh Esale, Nitin
Bahadur, Harish Sitaraman, Raveendra Torvi, Anil Kumar SN, Bruno
Decraene, Eric Wu, Janos Farkas, Rob Shakir, Stewart Bryant, and
Alvaro Retana for their suggestions and review.
Contributors
Robert Kebler
Juniper Networks
10 Technology Park Drive
Westford, MA 01886
United States
Email: rkebler@juniper.net
Andras Csaszar
Ericsson
Konyves Kalman krt 11
Budapest 1097
Hungary
Email: Andras.Csaszar@ericsson.com
Jeff Tantsura
Ericsson
300 Holger Way
San Jose, CA 95134
United States
Email: jeff.tantsura@ericsson.com
Russ White
VCE
Email: russw@riw.us
Atlas, et al. Standards Track PAGE 43
RFC 7812 MRT Unicast FRR Architecture June 2016
Authors' Addresses
Alia Atlas
Juniper Networks
10 Technology Park Drive
Westford, MA 01886
United States
Email: akatlas@juniper.net
Chris Bowers
Juniper Networks
1194 N. Mathilda Ave.
Sunnyvale, CA 94089
United States
Email: cbowers@juniper.net
Gabor Sandor Enyedi
Ericsson
Konyves Kalman krt 11.
Budapest 1097
Hungary
Email: Gabor.Sandor.Enyedi@ericsson.com
Atlas, et al. Standards Track PAGE 44
RFC TOTAL SIZE: 110616 bytes
PUBLICATION DATE: Friday, June 17th, 2016
LEGAL RIGHTS: The IETF Trust (see BCP 78)
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