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IETF RFC 5120
M-ISIS: Multi Topology (MT) Routing in Intermediate System to Intermediate Systems (IS-ISs)
Last modified on Friday, February 8th, 2008
 Permanent link to RFC 5120
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Network Working Group T. Przygienda
Request for Comments: 5120 Z2 Sagl
Category: Standards Track N. Shen
Cisco Systems
N. Sheth
Juniper Networks
February 2008
M-ISIS: Multi Topology (MT) Routing in
Intermediate System to Intermediate Systems (IS-ISs)
 Status of This Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Abstract
This document describes an optional mechanism within Intermediate
System to Intermediate Systems (IS-ISs) used today by many ISPs for
IGP routing within their clouds. This document describes how to run,
within a single IS-IS domain, a set of independent IP topologies that
we call Multi-Topologies (MTs). This MT extension can be used for a
variety of purposes, such as an in-band management network "on top"
of the original IGP topology, maintaining separate IGP routing
domains for isolated multicast or IPv6 islands within the backbone,
or forcing a subset of an address space to follow a different
topology.
1. Introduction
Maintaining multiple MTs for IS-IS [ISO10589] [RFC 1195] in a
backwards-compatible manner necessitates several extensions to the
packet encoding and additional Shortest Path First (SPF) procedures.
The problem can be partitioned into the forming of adjacencies and
advertising of prefixes and reachable intermediate systems within
each topology. Having put all the necessary additional information
in place, it must be properly used by MT capable SPF computation.
The following sections describe each of the problems separately. To
simplify the text, "standard" IS-IS topology is defined to be MT ID
#0 (zero).
Przygienda, et al. Standards Track PAGE 1 
RFC 5120 M-ISIS February 2008
1.1. Conventions Used in This Document
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 [RFC 2119].
1.2. Definitions of Terms Used in This Document
CSNP Complete Sequence Number Packet. Used to describe all the
contents of a link state database of IS-IS.
DIS Designated Intermediate System. The intermediate system elected
to advertise the pseudo-node for a broadcast network.
IIH IS-IS Hello. Packets that are used to discover adjacent
intermediate systems.
LSP Link State Packet. Packet generated by an intermediate system
and lists adjacent systems, prefixes, and other information.
PSNP Partial Sequence Number Packet. Used to request information
from an adjacent intermediate system's link state database.
SPF Shortest Path First. An algorithm that takes a database of
nodes within a domain and builds a tree of connectivity along
the shortest paths through the entire network.
2. Maintaining MT Adjacencies
Each adjacency formed MUST be classified as belonging to a set of MTs
on the interface. This is achieved by adding a new TLV into IIH
packets that advertises to which topologies the interface belongs.
If MT #0 is the only MT on the interface, it is optional to advertise
it in the new TLV. Thus, not including such a TLV in the IIH implies
MT ID #0 capability only. Through this exchange of MT capabilities,
a router is able to advertise the IS TLVs in LSPs with common MT set
over those adjacencies.
The case of adjacency contains multiple MTs on an interface, and if
there exists an overlapping IP address space among the topologies,
additional mechanisms MUST be used to resolve the topology identity
of the incoming IP packets on the interface. See further discussion
in Section 8.2.2 of this document.
Przygienda, et al. Standards Track PAGE 2
RFC 5120 M-ISIS February 2008
2.1. Forming Adjacencies on Point-to-Point Interfaces
Adjacencies on point-to-point interfaces are formed as usual with
IS-IS routers not implementing MT extensions. If a local router does
not participate in certain MTs, it will not advertise those MT IDs in
its IIHs and thus will not include that neighbor within its LSPs. On
the other hand, if an MT ID is not detected in the remote side's
IIHs, the local router MUST NOT include that neighbor within its
LSPs. The local router SHOULD NOT form an adjacency if they don't
have at least one common MT over the interface.
2.2. Forming Adjacencies on Broadcast Interfaces
On a LAN, all the routers on the LAN that implement the MT extension
MAY advertise their MT capability TLV in their IIHs. If there is at
least one adjacency on the LAN interface that belongs to this MT, the
MT capable router MUST include the corresponding MT IS Reachable TLV
in its LSP, otherwise it MAY include this MT IS Reachable TLV in its
LSP if the LAN interface participates in this MT set.
Two routers on a LAN SHALL always establish adjacency, regardless of
whether or not they have a common MT. This is to ensure all the
routers on the LAN can correctly elect the same DIS. The IS SHOULD
NOT include the MT IS TLV in its LSP if none of the adjacencies on
the LAN contain this MT.
The DIS, CSNP, and PSNP functions are not changed by MT extension.
3. Advertising MT Reachable Intermediate Systems in LSPs
A router MUST include within its LSPs in the Reachable Intermediate
Systems TLV-only adjacent nodes that are participating in the
corresponding topology and advertise such TLVs only if it
participates itself in the corresponding topology. The Standard
Reachable Intermediate Systems TLV is acting here as MT ID #0, the
equivalent of the newly introduced MT Reachable Intermediate Systems
TLV. A router MUST announce the MT IS TLV when there is at least one
adjacency on the interface that belongs to this MT, otherwise it MAY
announce the MT IS TLV of an adjacency for a given MT if this
interface participates in the LAN.
Since it is not possible to prevent a router that does not understand
MT extensions from being responsible for the generation of the
according pseudo-node, it is possible to neither introduce special
TLVs in the pseudo-node LSPs, nor run distinct DIS elections per MT.
Therefore, a generated pseudo-node LSP by DIS MUST contain
Przygienda, et al. Standards Track PAGE 3
RFC 5120 M-ISIS February 2008
in its IS Reachable TLV all nodes on the LAN as usual, regardless of
their MT capabilities. In other words, there is no change to the
pseudo-node LSP construction.
4. MTs and Overload, Partition, and Attached Bits
For each of the MTs, a router could become potentially partitioned,
overloaded, and attached independently. To prevent unnecessary
complexity, MT extensions do not support MT based partition repair.
The overload, partition, and attached bits in the LSP header only
reflect the status of the default topology.
Attached bit and overload bit are part of the MT TLV being
distributed within a node's LSP fragment zero. Since each adjacency
can belong to different MTs, it is possible that some MTs are L2
attached, and others are not on the same router. The overload bit in
the MT TLV can be used to signal the topology being overloaded. An
MT-based system is considered overloaded if the overload bit in the
MT is set.
Route leaking between the levels SHOULD only be performed within the
same MT.
5. Advertising MT Specific IP Prefixes
Each of the MTs commands its own address space so a new TLV is
necessary for prefixes stored in MTs other than MT ID #0. To make
the encoding less confusing when same prefixes are present in
multiple MTs and accelerate SPF per MT, rather than adding a sub-TLV
in Traffic Engineered (TE) extensions, a new TLV is introduced for
that purpose that closely follows TE encoding [RFC 3784].
6. MT SPF Computation
Each MT MUST run its own instance of the decision process. The
pseudo-node LSPs are used by all topologies during computation. Each
non-default topology MAY have its attached bit and overload bit set
in the MT TLV. A reverse-connectivity check within SPF MUST follow
the according MT to assure the bi-directional reachability within the
same MT.
The results of each computation SHOULD be stored in a separate
Routing Information Base (RIB), in normal cases, otherwise
overlapping addresses in different topologies could lead to
undesirable routing behavior, such as forwarding loops. The
forwarding logic and configuration need to ensure the same MT is
traversed from the source to the destination for packets. The
nexthops derived from the MT SPF MUST belong to the adjacencies
Przygienda, et al. Standards Track PAGE 4
RFC 5120 M-ISIS February 2008
conforming to the same MT for correct forwarding. It is recommended
for the administrators to ensure consistent configuration of all
routers in the domain to prevent undesirable forwarding behavior.
No attempt is made in this document to allow one topology to
calculate routes using the routing information from another topology
inside SPF. Even though it is possible to redistribute and leak
routes from another IS-IS topology or from external sources, the
exact mechanism is beyond the scope of this document.
7. Packet Encoding
Four new TLVs are added to support MT extensions. One of them is
common for the LSPs and IIHs. Encoding of Intermediate System TLV
and IPv4 Reachable Prefixes is tied to traffic engineering extensions
[RFC 3784] to simplify the implementation effort. The main reasons we
chose to use new TLVs instead of using sub-TLVs inside existing TLV
type-22 and type-135 are:
1. In many cases, multi-topologies are non-congruent, using the
sub-TLV approach will not save LSP space;
2. Many sub-TLVs are already being used in TLV type-22, and many
more are being proposed while there is a maximum limit on the
TLV size, from the existing TLVs;
3. If traffic engineering or some other applications are being
applied per topology level later, the new TLVs can
automatically inherit the same attributes already defined for
the "standard" topology without going through long standard
process to redefine them per topology.
7.1. Multi-Topology TLV
The TLV number of this TLV is 229. It contains one or more MTs; the
router is participating in the following structure:
x CODE - 229
x LENGTH - total length of the value field, it SHOULD be 2
times the number of MT components.
x VALUE - one or more 2-byte MT components, structured
as follows:
No. of Octets
+--------------------------------+
|O |A |R |R | MT ID | 2
+--------------------------------+
Przygienda, et al. Standards Track PAGE 5
RFC 5120 M-ISIS February 2008
Bit O represents the OVERLOAD bit for the MT (only valid in LSP
fragment zero for MTs other than ID #0, otherwise SHOULD be set to
0 on transmission and ignored on receipt).
Bit A represents the ATTACH bit for the MT (only valid in LSP
fragment zero for MTs other than ID #0, otherwise SHOULD be set to
0 on transmission and ignored on receipt).
Bits R are reserved, SHOULD be set to 0 on transmission and
ignored on receipt.
MT ID is a 12-bit field containing the ID of the topology being
announced.
This MT TLV can advertise up to 127 MTs. It is announced in IIHs and
LSP fragment 0, and can occur multiple times. The resulting MT set
SHOULD be the union of all the MT TLV occurrences in the packet. Any
other IS-IS PDU occurrence of this TLV MUST be ignored. Lack of MT
TLV in hellos and fragment zero LSPs MUST be interpreted as
participation of the advertising interface or router in MT ID #0
only. If a router advertises MT TLV, it has to advertise all the MTs
it participates in, specifically including topology ID #0 also.
7.2. MT Intermediate Systems TLV
The TLV number of this TLV is 222. It is aligned with extended IS
reachability TLV type 22 beside an additional two bytes in front at
the beginning of the TLV.
x CODE - 222
x LENGTH - total length of the value field
x VALUE - 2-byte MT membership plus the format of extended IS
reachability TLV, structured as follows:
No. of Octets
+--------------------------------+
|R |R |R |R | MT ID | 2
+--------------------------------+
| extended IS TLV format | 11 - 253
+--------------------------------+
. .
. .
+--------------------------------+
| extended IS TLV format | 11 - 253
+--------------------------------+
Bits R are reserved, SHOULD be set to 0 on transmission and
ignored on receipt.
Przygienda, et al. Standards Track PAGE 6
RFC 5120 M-ISIS February 2008
MT ID is a 12-bit field containing the non-zero MT ID of the
topology being announced. The TLV MUST be ignored if the ID is
zero. This is to ensure the consistent view of the standard
unicast topology.
After the 2-byte MT membership format, the MT IS content is in the
same format as extended IS TLV, type 22 [RFC 3784]. It can contain
up to 23 neighbors of the same MT if no sub-TLVs are used.
This TLV can occur multiple times.
7.3. Multi-Topology Reachable IPv4 Prefixes TLV
The TLV number of this TLV is 235. It is aligned with extended IP
reachability TLV type 135 beside an additional two bytes in front.
x CODE - 235
x LENGTH - total length of the value field
x VALUE - 2-byte MT membership plus the format of
extended IP reachability TLV, structured as follows:
No. of Octets
+--------------------------------+
|R |R |R |R | MT ID | 2
+--------------------------------+
| extended IP TLV format | 5 - 253
+--------------------------------+
. .
. .
+--------------------------------+
| extended IP TLV format | 5 - 253
+--------------------------------+
Bits R are reserved, SHOULD be set to 0 on transmission and
ignored on receipt.
MT ID is a 12-bit field containing the non-zero ID of the topology
being announced. The TLV MUST be ignored if the ID is zero. This
is to ensure the consistent view of the standard unicast topology.
After the 2-byte MT membership format, the MT IPv4 content is in
the same format as extended IP reachability TLV, type 135
[RFC 3784].
This TLV can occur multiple times.
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RFC 5120 M-ISIS February 2008
7.4. Multi-Topology Reachable IPv6 Prefixes TLV
The TLV number of this TLV is 237. It is aligned with IPv6
Reachability TLV type 236 beside an additional two bytes in front.
x CODE - 237
x LENGTH - total length of the value field
x VALUE - 2-byte MT membership plus the format of IPv6
Reachability TLV, structured as follows:
No. of Octets
+--------------------------------+
|R |R |R |R | MT ID | 2
+--------------------------------+
| IPv6 Reachability format | 6 - 253
+--------------------------------+
. .
+--------------------------------+
| IPv6 Reachability format | 6 - 253
+--------------------------------+
Bits R are reserved, SHOULD be set to 0 on transmission and
ignored on receipt.
MT ID is a 12-bit field containing the ID of the topology being
announced. The TLV MUST be ignored if the ID is zero.
After the 2-byte MT membership format, the MT IPv6 context is in
the same format as IPv6 Reachability TLV, type 236 [H01].
This TLV can occur multiple times.
7.5. Reserved MT ID Values
Certain MT topologies are assigned to serve predetermined purposes:
- MT ID #0: Equivalent to the "standard" topology.
- MT ID #1: Reserved for IPv4 in-band management
purposes.
- MT ID #2: Reserved for IPv6 routing topology.
- MT ID #3: Reserved for IPv4 multicast routing topology.
- MT ID #4: Reserved for IPv6 multicast routing topology.
- MT ID #5: Reserved for IPv6 in-band management
purposes.
- MT ID #6-#3995: Reserved for IETF consensus.
- MT ID #3996-#4095: Reserved for development, experimental and
proprietary features [RFC 3692].
Przygienda, et al. Standards Track PAGE 8
RFC 5120 M-ISIS February 2008
8. MT IP Forwarding Considerations
Using MT extension for IS-IS routing can result in multiple RIBs on
the system. In this section, we list some of the known
considerations for IP forwarding in various MT scenarios. Certain
deployment scenarios presented here imply different trade-offs in
terms of deployment difficulties and advantages obtained.
8.1. Each MT Belongs to a Distinct Address Family
In this case, each MT related route is installed into a separate RIB.
Multiple topologies can share the same IS-IS interface on detecting
the incoming packet address family. As an example, IPv4 and IPv6 can
share the same interface without any further considerations under MT
ISIS.
8.2. Some MTs Belong to the Same Address Family
8.2.1. Each Interface Belongs to One and Only One MT
In this case, MTs can be used to forward packets from the same
address family, even with overlapping addresses, since the MTs have
their dedicated interfaces, and those interfaces can be associated
with certain MT RIBs and FIBs.
8.2.2. Multiple MTs Share an Interface with Overlapping Addresses
Some additional mechanism is needed to select the correct RIBs for
the incoming IP packets to determine the correct RIB to make a
forwarding decision. For example, if the topologies are Quality of
Service (QoS) partitioned, then the Differentiated Services Code
Point (DSCP) bits in the IP packet header can be utilized to make the
decision. Some IP headers, or even packet data information, MAY be
checked to make the forwarding table selection, for example, the
source IP address in the header can be used to determine the desired
forwarding behavior.
This topic is not unique to IS-IS or even to Multi-topology, it is a
local policy and configuration decision to make sure the inbound
traffic uses the correct forwarding tables. For example, preferred
customer packets are sent through a Layer 2 Tunneling Protocol (L2TP)
towards the high-bandwidth upstream provider, and other packets are
sent through a different L2TP to a normal-bandwidth provider. Those
mechanisms are not part of the L2TP protocol specifications.
The generic approach of packet to multiple MT RIB mapping over the
same inbound interface is outside the scope of this document.
Przygienda, et al. Standards Track PAGE 9
RFC 5120 M-ISIS February 2008
8.2.3. Multiple MTs Share an Interface with Non-Overlapping Addresses
When there is no overlap in the address space among all the MTs,
strictly speaking, the destination address space classifies the
topology to which a packet belongs. It is possible to install routes
from different MTs into a shared RIB. As an example of such a
deployment, a special IS-IS topology can be set up for certain
External Border Gateway Protocol (eBGP) nexthop addresses.
8.3. Some MTs Are Not Used for Forwarding Purposes
MT in IS-IS MAY be used even if the resulting RIB is not used for
forwarding purposes. As an example, multicast Reverse Path
Forwarding (RPF) check can be performed on a different RIB than the
standard unicast RIB, albeit an entirely different RIB is used for
the multicast forwarding. However, an incoming packet MUST still be
clearly identified as belonging to a unique topology.
9. MT Network Management Considerations
When multiple IS-IS topologies exist within a domain, some of the
routers can be configured to participate in a subset of the MTs in
the network. This section discusses some of the options we have to
enable operations or the network management stations to access those
routers.
9.1. Create Dedicated Management Topology to Include All the Nodes
This approach is to set up a dedicated management topology or 'in-
band' management topology. This 'mgmt' topology will include all the
routers need to be managed. The computed routes in the topology will
be installed into the 'mgmt' RIB. In the condition that the 'mgmt'
topology uses a set of non-overlapping address space with the default
topology, those 'mgmt' routes can also be optionally installed into
the default RIB. The advantages of duplicate 'mgmt' routes in both
RIBs include: the network management utilities on the system does
not have to be modified to use a specific RIB other than the default
RIB; the 'mgmt' topology can share the same link with the default
topology if so designed.
Przygienda, et al. Standards Track PAGE 10
RFC 5120 M-ISIS February 2008
9.2. Extend the Default Topology to All the Nodes
Even in the case that default topology is not used on some of the
nodes in the IP forwarding, we MAY want to extend the default
topology to those nodes for the purpose of network management.
Operators SHOULD set high costs on the links that belong to the
extended portion of the default topology. This way, the IP data
traffic will not be forwarded through those nodes during network
topology changes.
10. Acknowledgments
The authors would like to thank Andrew Partan, Dino Farinacci, Derek
Yeung, Alex Zinin, Stefano Previdi, Heidi Ou, Steve Luong, Pekka
Savola, Mike Shand, Shankar Vemulapalli, and Les Ginsberg for the
discussion, their review, comments, and contributions to this
document.
11. Security Considerations
IS-IS security applies to the work presented. No specific security
issues with the proposed solutions are known. The authentication
procedure for IS-IS PDUs is the same regardless of MT information
inside the IS-IS PDUs.
Note that an authentication mechanism, such as the one defined in
[RFC 3567], SHOULD be applied if there is high risk resulting from
modification of multi-topology information.
As described in Section 8.2.2, multiple topologies share an interface
in the same address space, some mechanism beyond IS-IS needs to be
used to select the right forwarding table for an inbound packet. A
misconfiguration on the system or a packet with a spoofed source
address, for example, can lead to packet loss or unauthorized use of
premium network resource.
12. IANA Considerations
This document defines the following new IS-IS TLV types, which have
already been reflected in the IANA IS-IS TLV code-point registry:
Name Value
MT-ISN 222
M-Topologies 229
MT IP. Reach 235
MT IPv6 IP. Reach 237
Przygienda, et al. Standards Track PAGE 11
RFC 5120 M-ISIS February 2008
IANA has created a new registry, "IS-IS Multi-Topology Parameters",
with the assignments listed in Section 7.5 of this document and
registration policies [RFC 2434] for future assignments. The MT ID
values range 6-3995 are allocated through Expert Review; values in
the range of 3996-4095 are reserved for Private Use. In all cases,
assigned values are to be registered with IANA.
13. References
13.1. Normative References
[ISO10589] ISO. Intermediate System to Intermediate System Routing
Exchange Protocol for Use in Conjunction with the
Protocol for Providing the Connectionless-Mode Network
Service. ISO 10589, 1992.
[RFC 1195] Callon, R., "Use of OSI IS-IS for routing in TCP/IP and
dual environments", RFC 1195, December 1990.
[RFC 2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC 3692] Narten, T., "Assigning Experimental and Testing Numbers
Considered Useful", BCP 82, RFC 3692, January 2004.
[RFC 2434] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 2434,
October 1998.
13.2. Informative References
[RFC 3567] Li, T. and R. Atkinson, "Intermediate System to
Intermediate System (IS-IS) Cryptographic
Authentication", RFC 3567, July 2003.
[RFC 3784] Smit, H. and T. Li, "Intermediate System to Intermediate
System (IS-IS) Extensions for Traffic Engineering (TE)",
RFC 3784, June 2004.
[H01] C. Hopps, "Routing IPv6 with IS-IS", Work in Progress.
Przygienda, et al. Standards Track PAGE 12
RFC 5120 M-ISIS February 2008
Authors' Addresses
Tony Przygienda
Z2 Sagl
Via Rovello 32
CH-6942 Savosa
EMail: prz@net4u.ch
Naiming Shen
Cisco Systems
225 West Tasman Drive
San Jose, CA, 95134 USA
EMail: naiming@cisco.com
Nischal Sheth
Juniper Networks
1194 North Mathilda Avenue
Sunnyvale, CA 94089 USA
EMail: nsheth@juniper.net
Przygienda, et al. Standards Track PAGE 13
RFC 5120 M-ISIS February 2008
Full Copyright Statement
Copyright © The IETF Trust (2008).
This document is subject to the rights, licenses and restrictions
contained in BCP 78, and except as set forth therein, the authors
retain all their rights.
This document and the information contained herein are provided on an
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Przygienda, et al. Standards Track PAGE 14
M-ISIS: Multi Topology (MT) Routing in Intermediate System to Intermediate Systems (IS-ISs)
RFC TOTAL SIZE: 30318 bytes
PUBLICATION DATE: Friday, February 8th, 2008
LEGAL RIGHTS: The IETF Trust (see BCP 78)
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