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IETF RFC 7524
Last modified on Thursday, May 14th, 2015
 Permanent link to RFC 7524
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Internet Engineering Task Force (IETF) Y. Rekhter
Request for Comments: 7524 E. Rosen
Category: Standards Track Juniper Networks
ISSN: 2070-1721 R. Aggarwal
Arktan
T. Morin
I. Grosclaude
Orange
N. Leymann
Deutsche Telekom AG
S. Saad
AT&T
May 2015
Inter-Area Point-to-Multipoint (P2MP)
Segmented Label Switched Paths (LSPs)
 Abstract
This document describes procedures for building inter-area point-to-
multipoint (P2MP) segmented service label switched paths (LSPs) by
partitioning such LSPs into intra-area segments and using BGP as the
inter-area routing and Label Distribution Protocol (LDP). Within
each IGP area, the intra-area segments are either carried over intra-
area P2MP LSPs, using P2MP LSP hierarchy, or instantiated using
ingress replication. The intra-area P2MP LSPs may be signaled using
P2MP RSVP-TE or P2MP multipoint LDP (mLDP). If ingress replication
is used within an IGP area, then (multipoint-to-point) LDP LSPs or
(point-to-point) RSVP-TE LSPs may be used in the IGP area. The
applications/services that use such inter-area service LSPs may be
BGP Multicast VPN, Virtual Private LAN Service (VPLS) multicast, or
global table multicast over MPLS.
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 5741.
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 7524.
Rekhter, et al. Standards Track PAGE 1 
RFC 7524 Inter-Area P2MP Segmented LSPs May 2015
Copyright Notice
Copyright (c) 2015 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.
Rekhter, et al. Standards Track PAGE 2
RFC 7524 Inter-Area P2MP Segmented LSPs May 2015
Table of Contents
1. Introduction ....................................................5
2. Specification of Requirements ...................................5
3. General Assumptions and Terminology .............................6
4. Inter-Area P2MP Segmented Next-Hop Extended Community ...........7
5. Discovering P2MP FEC of Inter-Area P2MP Service LSP .............8
5.1. BGP MVPN ...................................................8
5.1.1. Routes Originated by PE or ASBR .....................9
5.1.2. Routes Re-advertised by PE or ASBR ..................9
5.1.3. Inter-Area Routes ...................................9
5.2. LDP VPLS with BGP Auto-discovery or BGP VPLS ..............10
5.2.1. Routes Originated by PE or ASBR ....................10
5.2.2. Routes Re-advertised by PE or ASBR .................11
5.2.3. Inter-Area Routes ..................................11
5.3. Global Table Multicast over MPLS ..........................12
6. Egress PE/ASBR Signaling Procedures ............................13
6.1. Determining the Upstream ABR/PE/ASBR (Upstream Node) ......13
6.1.1. Upstream Node for MVPN or VPLS .....................13
6.1.2. Upstream Node for Global Table Multicast ...........14
6.2. Originating a Leaf A-D Route ..............................15
6.2.1. Leaf A-D Route for MVPN and VPLS ...................15
6.2.2. Leaf A-D Route for Global Table Multicast ..........15
6.2.3. Constructing the Rest of the Leaf A-D Route ........17
6.3. PIM-SM in ASM Mode for Global Table Multicast .............18
6.3.1. Option 1 ...........................................18
6.3.1.1. Originating Source Active A-D Routes ......18
6.3.1.2. Receiving BGP Source Active A-D
Route by PE ...............................19
6.3.1.3. Handling (S,G,rpt) State ..................19
6.3.2. Option 2 ...........................................19
6.3.2.1. Originating Source Active A-D Routes ......19
6.3.2.2. Receiving BGP Source Active A-D Route .....20
6.3.2.3. Pruning Sources Off the Shared Tree .......20
6.3.2.4. More on Handling (S,G,rpt) State ..........21
7. Egress ABR Procedures ..........................................21
7.1. Handling Leaf A-D Route on Egress ABR .....................21
7.2. P2MP LSP as the Intra-Area LSP in the Egress Area .........23
7.2.1. Received Leaf A-D Route Is for MVPN or VPLS ........23
7.2.2. Received Leaf A-D Route Is for Global Table
Multicast ..........................................24
7.2.2.1. Global Table Multicast and S-PMSI
A-D Routes ................................24
7.2.2.2. Global Table Multicast and
Wildcard S-PMSI A-D Routes ................25
7.2.3. Global Table Multicast and the Expected
Upstream Node ......................................25
Rekhter, et al. Standards Track PAGE 3
RFC 7524 Inter-Area P2MP Segmented LSPs May 2015
7.2.4. P2MP LDP LSP as the Intra-Area P2MP LSP ............26
7.2.5. P2MP RSVP-TE LSP as the Intra-Area P2MP LSP ........26
7.3. Ingress Replication in the Egress Area ....................26
8. Ingress ABR Procedures .........................................27
8.1. P2MP LSP as the Intra-Area LSP in the Backbone Area .......27
8.2. Ingress Replication in the Backbone Area ..................27
9. Ingress PE/ASBR Procedures .....................................28
9.1. P2MP LSP as the Intra-Area LSP in the Ingress Area ........28
9.2. Ingress Replication in the Ingress Area ...................29
10. Common Tunnel Type in the Ingress and Egress Areas ............29
11. Placement of Ingress and Egress PEs ...........................30
12. MVPN with Virtual Hub-and-Spoke ...............................31
13. Data Plane ....................................................31
13.1. Data Plane Procedures on ABRs ............................31
13.2. Data Plane Procedures on Egress PEs ......................32
13.3. Data Plane Procedures on Ingress PEs .....................33
13.4. Data Plane Procedures on Transit Routers .................33
14. Support for Inter-Area Transport LSPs .........................33
14.1. "Transport Tunnel" Tunnel Type ...........................33
14.2. Discovering Leaves of the Inter-Area P2MP Service LSP ....34
14.3. Discovering P2MP FEC of P2MP Transport LSP ...............34
14.4. Egress PE Procedures for P2MP Transport LSP ..............35
14.5. ABRs and Ingress PE Procedures for P2MP Transport LSP ....35
14.6. Discussion ...............................................36
15. IANA Considerations ...........................................38
16. Security Considerations .......................................38
17. References ....................................................39
17.1. Normative References .....................................39
17.2. Informative References ...................................41
Acknowledgements ..................................................41
Authors' Addresses ................................................42
Rekhter, et al. Standards Track PAGE 4
RFC 7524 Inter-Area P2MP Segmented LSPs May 2015
1. Introduction
This document describes procedures for building inter-area point-to-
multipoint (P2MP) segmented service LSPs by partitioning such LSPs
into intra-area segments and using BGP as the inter-area routing and
label distribution protocol. Within each IGP area, the intra-area
segments are either carried over intra-area P2MP LSPs, potentially
using P2MP LSP hierarchy, or instantiated using ingress replication.
The intra-area P2MP LSPs may be signaled using P2MP RSVP-TE [RFC 4875]
or P2MP mLDP [RFC 6388]. If ingress replication is used in an IGP
area, then (multipoint-to-point) LDP LSPs [RFC 5036] or (point-to-
point) RSVP-TE LSPs [RFC 3209] may be used within the IGP area. The
applications/services that use such inter-area service LSPs may be
BGP Multicast VPN (BGP MVPN), VPLS multicast, or global table
multicast over MPLS.
The primary use case of such segmented P2MP service LSPs is when the
Provider Edge (PE) routers are in different areas but in the same
Autonomous System (AS) and thousands or more of PEs require P2MP
connectivity. For instance, this may be the case when MPLS is pushed
further to the metro edge and the metros are in different IGP areas.
This may also be the case when a service provider's network comprises
multiple IGP areas in a single AS, with a large number of PEs.
Seamless MPLS is the industry term to address this case
[SEAMLESS-MPLS]. Thus, one of the applicabilities of this document
is that it describes the multicast procedures for seamless MPLS.
It is to be noted that [RFC 6514] and [RFC 7117] already specify
procedures for building segmented inter-AS P2MP service LSPs. This
document complements those procedures, as it extends the segmented
P2MP LSP model such that it is applicable to inter-area P2MP service
LSPs as well. In fact, an inter-AS deployment could use inter-AS
segmented P2MP LSPs as specified in [RFC 6514] and [RFC 7117] where
each intra-AS segment is constructed using inter-area segmented P2MP
LSPs, as specified in this document.
2. Specification of Requirements
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].
Rekhter, et al. Standards Track PAGE 5
RFC 7524 Inter-Area P2MP Segmented LSPs May 2015
3. General Assumptions and Terminology
The reader is assumed to be familiar with MVPN procedures and
terminology [RFC 6513] [RFC 6514] and VPLS procedures and terminology
[RFC 7117].
This document allows Area Border Routers (ABRs), acting as Route
Reflectors, to follow the procedures specified in [SEAMLESS-MPLS]
when handling the BGP Next Hop of the routes to the PEs.
Specifically, when reflecting such routes from the non-backbone areas
into the backbone area, the ABRs MUST set the BGP Next Hop to their
own loopback addresses (next-hop-self), while when reflecting such
routes from the backbone area into the non-backbone areas, the ABRs
SHOULD NOT change the BGP Next Hop addresses (next-hop-unchanged).
While this document allows ABRs to follow the procedures specified in
[SEAMLESS-MPLS], procedures specified in this document are applicable
even when ABRs do not follow the procedures specified in
[SEAMLESS-MPLS].
This document specifies a particular way of supporting the global
table multicast service. Although the document refers to this
approach simply as "global table multicast", it does not mean to
imply that there are no other ways to support global table multicast.
An alternative way to support global table multicast is to use the
procedures for MVPN that are specified in [RFC 6514] and in this
document. That alternative is discussed in more detail in [GTM].
However, that alternative is not further considered in the current
document.
This document assumes that, in the context of global table multicast,
ABRs do not carry routes to the destinations external to their own
AS. Furthermore, in the context of global table multicast, this
document assumes that an Autonomous System Border Router (ASBR), when
re-advertising into Internal BGP (IBGP) routes received from an
external speaker (received via External BGP (EBGP)), may not change
the BGP Next Hop to self.
Within an AS, a P2MP service LSP is partitioned into three segments:
ingress area segment, backbone area segment, and egress area segment.
Within each area, a segment is carried over an intra-area P2MP LSP or
instantiated using ingress replication.
When intra-area P2MP LSPs are used to instantiate the intra-area
segments, there could be either 1:1 or n:1 mapping between intra-area
segments of the inter-area P2MP service LSP and a given intra-area
P2MP LSP. The latter is realized using P2MP LSP hierarchy with
Rekhter, et al. Standards Track PAGE 6
RFC 7524 Inter-Area P2MP Segmented LSPs May 2015
upstream-assigned labels [RFC 5331]. For simplicity of presentation,
we assume that P2MP LSP hierarchy is used even with 1:1 mapping; in
which case, an Implicit NULL is used as the upstream-assigned label.
When intra-area segments of the inter-area P2MP service LSP are
instantiated using ingress replication, multiple such segments may be
carried in the same P2P RSVP-TE or MP2P LDP LSP. This can be
achieved using downstream-assigned labels alone.
The ingress area segment of a P2MP service LSP is rooted at a PE (or
at an ASBR in the case where the P2MP service LSP spans multiple
ASes). The leaves of this segment are other PEs/ASBRs and ABRs in
the same area as the root PE.
The backbone area segment is rooted at either an ABR that is
connected to the ingress area (ingress ABR), an ASBR if the ASBR is
present in the backbone area, or a PE if the PE is present in the
backbone area. The backbone area segment has its leaf ABRs that are
connected to the egress area(s) or PEs in the backbone area, or ASBRs
in the backbone area.
The egress area segment is rooted at an ABR in the egress area
(egress ABR), and has its leaf PEs and ASBR in that egress area (the
latter covers the case where the P2MP service LSP spans multiple
ASes). For a given P2MP service LSP, note that there may be more
than one backbone segment, each rooted at its own ingress ABR, and
more than one egress area segment, each rooted at its own egress ABR.
This document uses the term "A-D routes" for "auto-discovery routes".
An implementation that supports this document MUST implement the
procedures described in the following sections to support inter-area
P2MP segmented service LSPs.
4. Inter-Area P2MP Segmented Next-Hop Extended Community
This document defines a new Transitive IPv4-Address-Specific Extended
Community Sub-Type: "Inter-Area P2MP Next-Hop". This document also
defines a new BGP Transitive IPv6-Address-Specific Extended Community
Sub-Type: "Inter-Area P2MP Next-Hop".
Rekhter, et al. Standards Track PAGE 7
RFC 7524 Inter-Area P2MP Segmented LSPs May 2015
A PE, an ABR, or an ASBR constructs the Inter-Area P2MP Segmented
Next-Hop Extended Community as follows:
- The Global Administrator field MUST be set to an IP address of the
PE, ABR, or ASBR that originates or advertises the route carrying
the P2MP Next-Hop Extended Community. For example this address
may be the loopback address or the PE, ABR, or ASBR that
advertises the route.
- The Local Administrator field MUST be set to 0.
If the Global Administrator field is an IPv4 address, the
IPv4-Address-Specific Extended Community is used; if the Global
Administrator field is an IPv6 address, the IPv6-Address-Specific
Extended Community is used.
The detailed usage of these Extended Communities is described in
the following sections.
5. Discovering P2MP FEC of Inter-Area P2MP Service LSP
Each inter-area P2MP service LSP has associated with it P2MP
Forwarding Equivalence Class (FEC). The egress PEs need to learn
this P2MP FEC in order to initiate the creation of the egress area
segment of the P2MP inter-area service LSP.
The P2MP FEC of the inter-area P2MP LSP is learned by the egress PEs
either by configuration or based on the application-specific
procedures (e.g., MVPN-specific procedures or VPLS-specific
procedures).
5.1. BGP MVPN
Egress PEs and/or ASBRs discover the P2MP FEC of the service LSPs
used by BGP MVPN using the Inclusive Provider Multicast Service
Interface (I-PMSI) or Selective PMSI (S-PMSI) A-D routes that are
originated by the ingress PEs or ASBRs following the procedures of
[RFC 6514], along with modifications as described in this document.
The Network Layer Reachability Information (NLRI) of such routes
encodes the P2MP FEC.
The procedures in this document require that at least one ABR in a
given IGP area act as a Route Reflector for MVPN A-D routes. Such a
Router Reflector is responsible for re-advertising MVPN A-D routes
across area boundaries. When re-advertising these routes across area
boundaries, this Route Reflector MUST follow the procedures in this
Rekhter, et al. Standards Track PAGE 8
RFC 7524 Inter-Area P2MP Segmented LSPs May 2015
document. Note that such a Route Reflector may also re-advertise
MVPN A-D routes within the same area; in which case, it follows the
plain BGP Route Reflector procedures [RFC 4456].
5.1.1. Routes Originated by PE or ASBR
The "Leaf Information Required" flag MUST be set in the PMSI Tunnel
attribute carried in the MVPN A-D routes, when originated by the
ingress PEs or ASBRs, except for the case where (a) as a matter of
policy (provisioned on the ingress PEs or ASBRs) there is no
aggregation of ingress area segments of the service LSPs and (b) mLDP
is used as the protocol to establish intra-area transport LSPs in the
ingress area. Before any Leaf A-D route is advertised by a PE or ABR
in the same area, as described in the following sections, an
I-PMSI/S-PMSI A-D route is advertised either with an explicit Tunnel
Type and Tunnel Identifier in the PMSI Tunnel attribute, if the
Tunnel Identifier has already been assigned, or with a special Tunnel
Type of "No tunnel information present" otherwise.
5.1.2. Routes Re-advertised by PE or ASBR
When the I-PMSI/S-PMSI A-D routes are re-advertised by an ingress
ABR, the "Leaf Information Required" flag MUST be set in the PMSI
Tunnel attribute present in the routes, except for the case where
(a) as a matter of policy (provisioned on the ingress ABR) there is
no aggregation of backbone area segments of the service LSPs and
(b) mLDP is used as the protocol to establish intra-area transport
LSPs in the backbone area. Likewise, when the I-PMSI/S-PMSI A-D
routes are re-advertised by an egress ABR, the "Leaf Information
Required" flag MUST be set in the PMSI Tunnel attribute present in
the routes, except for the case where (a) as a matter of policy
(provisioned on the egress ABR) there is no aggregation of egress
area segments of the service LSPs and (b) mLDP is used as the
protocol to establish intra-area transport LSPs in the egress area.
Note that the procedures in the above paragraph apply when intra-area
segments are realized by either intra-area P2MP LSPs or by ingress
replication.
5.1.3. Inter-Area Routes
When BGP MVPN I-PMSI or S-PMSI A-D routes are advertised or
propagated to signal inter-area P2MP service LSPs, to indicate that
these LSPs should be segmented using the procedures specified in this
document, these routes MUST carry the Inter-Area P2MP Segmented
Next-Hop Extended Community. This Extended Community MUST be
included in the I-PMSI/S-PMSI A-D route by the PE that originates
such a route, or an ASBR that re-advertises such a route into its own
Rekhter, et al. Standards Track PAGE 9
RFC 7524 Inter-Area P2MP Segmented LSPs May 2015
AS. The Global Administrator field in this Extended Community MUST
be set to the advertising PE or ASBR's IP address. This Extended
Community MUST also be included by ABRs as they re-advertise such
routes. An ABR MUST set the Global Administrator field of the Inter-
Area P2MP Segmented Next-Hop Extended Community to its own IP
address. Presence of this Extended Community in the I-PMSI/S-PMSI
A-D routes indicates to ABRs and PEs/ASBRs that they have to follow
the procedures in this document when these procedures differ from
those in [RFC 6514].
If an ASBR receives from an IBGP peer an I-PMSI or S-PMSI A-D route
that carries the Inter-Area P2MP Segmented Next-Hop Extended
Community, then before re-advertising this route to an EBGP peer, the
ASBR SHOULD remove this Extended Community from the route.
Suppose an ASBR receives an I-PMSI/S-PMSI A-D route from an EBGP
peer, and this route carries the Inter-Area P2MP Segmented Next-Hop
Extended Community. If the inter-area P2MP service LSP signaled by
this route should not be segmented, then before re-advertising this
route to its IBGP peers, the ASBR MUST remove this Extended Community
from the route.
To avoid requiring ABRs to participate in the propagation of
C-multicast routes, this document requires that ABRs MUST NOT modify
the BGP Next Hop when re-advertising Inter-AS I-PMSI A-D routes. For
consistency, this document requires that ABRs MUST NOT modify the BGP
Next Hop when re-advertising either Intra-AS or Inter-AS
I-PMSI/S-PMSI A-D routes. The egress PEs may advertise the
C-multicast routes to RRs that are different than the ABRs. However,
ABRs can still be configured to be the Route Reflectors for
C-multicast routes; in which case, they will participate in the
propagation of C-multicast routes.
5.2. LDP VPLS with BGP Auto-discovery or BGP VPLS
Egress PEs discover the P2MP FEC of the service LSPs used by VPLS,
using the VPLS A-D routes that are originated by the ingress PEs
[RFC 4761] [RFC 6074] or VPLS S-PMSI A-D routes that are originated by
the ingress PEs [RFC 7117]. The NLRI of such routes encodes the
P2MP FEC.
5.2.1. Routes Originated by PE or ASBR
The "Leaf Information Required" flag MUST be set in the PMSI Tunnel
attribute carried in the VPLS A-D routes or VPLS S-PMSI A-D routes,
when originated by the ingress PEs or ASBRs, except for the case
where (a) as a matter of policy (provisioned on the ingress PEs or
ASBRs) there is no aggregation of ingress area segments of the
Rekhter, et al. Standards Track PAGE 10
RFC 7524 Inter-Area P2MP Segmented LSPs May 2015
service LSPs and (b) mLDP is used as the protocol to establish intra-
area transport LSPs in the ingress area. Before any Leaf A-D route
is advertised by a PE or ABR in the same area, as described in the
following sections, a VPLS/S-PMSI A-D route is advertised either with
an explicit Tunnel Type and Tunnel Identifier in the PMSI Tunnel
attribute, if the Tunnel Identifier has already been assigned, or
with a special Tunnel Type of "No tunnel information present"
otherwise.
5.2.2. Routes Re-advertised by PE or ASBR
When the VPLS/S-PMSI A-D routes are re-advertised by an ingress ABR,
the "Leaf Information Required" flag MUST be set in the PMSI Tunnel
attribute present in the routes, except for the case where (a) as a
matter of policy (provisioned on the ingress ABR) there is no
aggregation of backbone area segments of the service LSPs and (b)
mLDP is used as the protocol to establish intra-area transport LSPs
in the backbone area. Likewise, when the VPLS/S-PMSI A-D routes are
re-advertised by an egress ABR, the "Leaf Information Required" flag
MUST be set in the PMSI Tunnel attribute present in the routes,
except for the case where (a) as a matter of policy (provisioned on
the egress ABR) there is no aggregation of egress area segments of
the service LSPs and (b) mLDP is used as the protocol to establish
intra-area transport LSPs in the egress area.
5.2.3. Inter-Area Routes
When VPLS A-D routes or S-PMSI A-D routes are advertised or
propagated to signal inter-area P2MP service LSPs, to indicate that
these LSPs should be segmented using the procedures specified in this
document, these routes MUST carry the Inter-Area P2MP Segmented
Next-Hop Extended Community. This Extended Community MUST be
included in the A-D route by the PE or ASBR that originates such a
route, and the Global Administrator field MUST be set to the
advertising PE or ASBR's IP address. This Extended Community MUST
also be included by ABRs as they re-advertise such routes. An ABR
MUST set the Global Administrator field of the Inter-Area P2MP
Segmented Next-Hop Extended Community to its own IP address.
Presence of this Extended Community in the I-PMSI/S-PMSI A-D routes
indicates to ABRs and PEs/ASBRs that they have to follow the
procedures in this document when these procedures differ from those
in [RFC 7117].
Note that the procedures in the above paragraph apply when intra-area
segments are realized by either intra-area P2MP LSPs or by ingress
replication.
Rekhter, et al. Standards Track PAGE 11
RFC 7524 Inter-Area P2MP Segmented LSPs May 2015
The procedures in this document require that at least one ABR in a
given area act as a Route Reflector for VPLS A-D routes. Such a
Router Reflector is responsible for re-advertising VPLS A-D routes
across areas boundaries. When re-advertising these routes across
areas boundaries, this Route Reflector MUST follow the procedures
in this document. Note that such a Route Reflector may also
re-advertise VPLS A-D routes within the same area; in which case,
it follows plain BGP Route Reflector procedures [RFC 4456].
When re-advertising VPLS A-D routes, a Route Reflector MUST NOT
modify the BGP Next Hop of these routes.
5.3. Global Table Multicast over MPLS
This section describes how the egress PEs discover the P2MP FEC when
the application is global table multicast over an MPLS-capable
infrastructure. In the rest of the document, we will refer to this
application as "global table multicast".
When Protocol Independent Multicast - Sparse Mode (PIM-SM) is used
for non-bidirectional ASM ("Any Source Multicast") group addresses,
this document refers to this as "PIM-SM in ASM mode".
In the case where global table multicast uses PIM-SM in ASM mode, the
following assumes that an inter-area P2MP service LSP could be used
to carry traffic either on a shared (*,G) or a source (S,G) tree.
An egress PE learns the (S/*,G) of a multicast stream as a result of
receiving IGMP or PIM messages on one of its IP multicast interfaces.
This (S/*,G) forms the P2MP FEC of the inter-area P2MP service LSP.
For each such P2MP FEC, there MAY exist a distinct inter-area P2MP
service LSP, or multiple such FECs MAY be carried over a single P2MP
service LSP using a wildcard (*,*) S-PMSI [RFC 6625].
Note that this document does not require the use of (*,G) inter-area
P2MP service LSPs when global table multicast uses PIM-SM in ASM
mode. In fact, PIM-SM in ASM mode may be supported entirely by using
only (S,G) inter-area P2MP service LSPs.
Rekhter, et al. Standards Track PAGE 12
RFC 7524 Inter-Area P2MP Segmented LSPs May 2015
6. Egress PE/ASBR Signaling Procedures
This section describes the egress PE/ASBR procedures for constructing
segmented inter-area P2MP LSPs. The procedures in this section apply
irrespective of whether the egress PE/ASBR is in a leaf IGP area, the
backbone area, or even in the same IGP area as the ingress PE/ASBR.
An egress PE/ASBR applies procedures specified in this section to
MVPN I-PMSI or S-PMSI A-D routes only if these routes carry the
Inter-Area P2MP Segmented Next-Hop Extended Community. An egress PE
applies procedures specified in this section to VPLS A-D routes or
VPLS S-PMSI A-D routes only if these routes carry the Inter-Area P2MP
Segmented Next-Hop Extended Community.
In order to support global table multicast, an egress PE MUST be
auto-configured to import routes that carry an AS-specific Route
Target Extended Community ([RFC 4360]) with the Global Administrator
field set to the AS of the PE and the Local Administrator field set
to 0.
Once an egress PE/ASBR discovers the P2MP FEC of an inter-area
segmented P2MP service LSP, it MUST propagate this P2MP FEC in BGP in
order to construct the segmented inter-area P2MP service LSP. This
propagation uses BGP Leaf A-D routes.
6.1. Determining the Upstream ABR/PE/ASBR (Upstream Node)
An egress PE/ASBR discovers the P2MP FEC of an inter-area P2MP
segmented service LSP as described in Section 5. Once the egress
PE/ASBR discovers this P2MP FEC, it MUST determine the upstream node
to reach such a FEC. If the egress PE/ASBR and the ingress PE/ASBR
are not in the same area, and the egress PE/ASBR is not in the
backbone IGP area, then this upstream node would be an egress ABR.
If the egress PE/ASBR is in the backbone area and the ingress PE/ASBR
is not in the backbone area, then this upstream node would be an
ingress ABR. If the egress PE/ASBR is in the same area as the
ingress PE/ASBR, then this upstream node would be the ingress
PE/ASBR.
6.1.1. Upstream Node for MVPN or VPLS
If the application is MVPN or VPLS, then the upstream node's IP
address is the IP address determined from the Global Administrator
field of the Inter-Area P2MP Segmented Next-Hop Extended Community.
As described in Section 5, this Extended Community MUST be carried in
the MVPN or VPLS A-D route from which the P2MP FEC of the inter-area
P2MP segmented service LSP is determined.
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RFC 7524 Inter-Area P2MP Segmented LSPs May 2015
6.1.2. Upstream Node for Global Table Multicast
If the application is global table multicast, then the unicast routes
to multicast sources/RPs SHOULD carry the "VRF Route Import" Extended
Community [RFC 6514] where the IP address in the Global Administrator
field is set to the IP address of the PE or ASBR advertising the
unicast route. The Local Administrator field of this Extended
Community MUST be set to 0 (note that this is in contrast to the case
of MVPN, where the Local Administrator field carries a non-zero value
that identifies a particular VRF on a PE that originates VPN-IP
routes). If it is not desirable to advertise the VRF Route Import
Extended Community in unicast routes, then unicast routes to
multicast sources/RPs MUST be advertised using the multicast
Subsequent Address Family Identifier (SAFI), i.e., SAFI 2, and such
routes MUST carry the VRF Route Import Extended Community.
Further, if the application is global table multicast, then the BGP
unicast routes that advertise the routes to the IP addresses of
PEs/ASBRs/ABRs SHOULD carry the Inter-Area P2MP Segmented Next-Hop
Extended Community. The IP address in the Global Administrator field
of this Extended Community MUST be set to the IP address of the PE,
ASBR, or ABR advertising the unicast route. The Local Administrator
field of this Extended Community MUST be set to 0. If it is not
desirable to advertise the Inter-Area P2MP Segmented Next-Hop
Extended Community in BGP unicast routes, then the BGP unicast routes
to the IP addresses of PEs/ASBRs/ABRs MUST be advertised using the
multicast SAFI, i.e., SAFI 2, and such routes MUST carry the Inter-
Area P2MP Segmented Next-Hop Extended Community. The procedures for
handling the BGP Next Hop attribute of SAFI 2 routes are the same as
those of handling regular unicast routes and MAY follow
[SEAMLESS-MPLS].
If the application is global table multicast, then in order to
determine the upstream node address, the egress PE first determines
the ingress PE. In order to determine the ingress PE, the egress PE
determines the best route to reach the S/RP. The ingress PE address
is the IP address determined from the Global Administrator field of
the VRF Route Import Extended Community that is carried in this
route. Then, the egress PE finds the best unicast route to reach the
ingress PE. The upstream node address is the IP address determined
from the Global Administrator field of the Inter-Area P2MP Segmented
Next-Hop Extended Community that is carried in this route.
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6.2. Originating a Leaf A-D Route
If the P2MP FEC was derived from an MVPN or VPLS A-D route, and if
the route carries a PMSI Tunnel attribute with the "Leaf Information
Required" flag set, then the egress PE MUST originate a Leaf A-D
route.
If the P2MP FEC was derived from a global table multicast (S/*,G),
and the upstream node's address is not the same as the egress PE,
then the egress PE MUST originate a Leaf A-D route.
6.2.1. Leaf A-D Route for MVPN and VPLS
If the P2MP FEC was derived from MVPN or VPLS A-D routes, then the
Route Key field of the Leaf A-D route contains the NLRI of the A-D
route from which the P2MP FEC was derived. This follows procedures
for constructing Leaf A-D routes described in [RFC 6514] [RFC 7117].
6.2.2. Leaf A-D Route for Global Table Multicast
If the application is global table multicast, then the MCAST-VPN NLRI
of the Leaf A-D route is constructed as follows.
The Route Key field of the MCAST-VPN NLRI has the following format:
+-----------------------------------+
| RD (8 octets) |
+-----------------------------------+
| Multicast Source Length (1 octet) |
+-----------------------------------+
| Multicast Source (Variable) |
+-----------------------------------+
| Multicast Group Length (1 octet) |
+-----------------------------------+
| Multicast Group (Variable) |
+-----------------------------------+
| Ingress PE's IP Address |
+-----------------------------------+
RD is set to 0 for (S,G) state and all ones for (*,G) state,
Multicast Source is set to S for (S,G) state or RP for (*,G) state,
Multicast Group is set to G, and Multicast Source Length and
Multicast Group Length are set to either 4 or 16 (depending on
whether S/RP and G are IPv4 or IPv6 addresses).
The Ingress PE's IP address is determined as described in
Section 6.1.
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RFC 7524 Inter-Area P2MP Segmented LSPs May 2015
The Originating Router's IP Address field of the MCAST-VPN NLRI is
set to the address of the local PE (the PE that originates the
route).
Thus, the entire MCAST-VPN NLRI of the route has the following
format:
+-----------------------------------+
| Route Type = 4 (1 octet) |
+-----------------------------------+
| Length (1 octet) |
+-----------------------------------+
| RD (8 octets) |
+-----------------------------------+
| Multicast Source Length (1 octet) |
+-----------------------------------+
| Multicast Source (Variable) |
+-----------------------------------+
| Multicast Group Length (1 octet) |
+-----------------------------------+
| Multicast Group (Variable) |
+-----------------------------------+
| Ingress PE's IP Address |
+-----------------------------------+
| Originating Router's IP Address |
+-----------------------------------+
Note that the encoding of the MCAST-VPN NLRI for the Leaf A-D routes
used for global table multicast is different from the encoding used
by the Leaf A-D routes originated in response to S-PMSI or I-PMSI A-D
routes. A router that receives a Leaf A-D route can distinguish
between these two cases by examining the third octet of the MCAST-VPN
NLRI of the route. If the value of this octet is 0x01, 0x02, or
0x03, then this Leaf A-D route was originated in response to an
S-PMSI or I-PMSI A-D route. If the value of this octet is either
0x00 or 0xff, and octets 3 through 10 contain either all 0x00 or all
0xff, then this is a Leaf A-D route used for global table multicast.
When the PE deletes (S,G)/(*,G) state that was created as a result of
receiving PIM or IGMP messages on one of its IP multicast interfaces,
if the PE previously originated a Leaf A-D route for that state, the
PE SHOULD withdraw that route.
An AS with an IPv4 network may provide global table multicast service
for customers that use IPv6, and an AS with an IPv6 network may
provide global table multicast service for customers that use IPv4.
Therefore, the address family of the Ingress PE's IP Address field
and the Originating Router's IP Address field in the Leaf A-D routes
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RFC 7524 Inter-Area P2MP Segmented LSPs May 2015
used for global table multicast MUST NOT be inferred from the AFI
field of the associated MP_REACH_NLRI/MP_UNREACH_NLRI attribute of
these routes. The address family is determined from the length of
the address (a length of 4 octets for IPv4 addresses or a length of
16 octets for IPv6 addresses).
For example, if an AS with an IPv4 network is providing IPv6
multicast service to a customer, the Ingress PE's IP Address and
Originating Router's IP Address in the Leaf A-D routes used for IPv6
global table multicast will be a 4-octet IPv4 address, even though
the AFI of those routes will have the value 2.
Note that the Ingress PE's IP Address and the Originating Router's IP
Address must be either both IPv4 or both IPv6 addresses; thus, they
must be of the same length. Since the two variable-length fields
(Multicast Source and Multicast Group) in the Leaf A-D routes used
for global table multicast have their own Length field, from these
two Length fields, and the Length field of the MCAST-VPN NLRI, one
can compute the length of the Ingress PE's IP Address field and the
Originating Router's IP Address field. If the computed length of
these fields is neither 4 nor 16, the MP_REACH_NLRI attribute MUST be
considered to be "incorrect", and MUST be handled as specified in
Section 7 of [RFC 4760].
6.2.3. Constructing the Rest of the Leaf A-D Route
The Next Hop field of the MP_REACH_NLRI attribute of the route SHOULD
be set to the same IP address as the one carried in the Originating
Router's IP Address field of the route.
When ingress replication is used to instantiate the egress area
segment, the Leaf A-D route MUST carry a downstream-assigned label in
the PMSI Tunnel attribute where the PMSI Tunnel Type is set to
ingress replication. A PE MUST assign a distinct MPLS label for each
Leaf A-D route originated by the PE.
To constrain distribution of this route, the originating PE
constructs an IP-based Route Target Extended Community by placing the
IP address of the upstream node in the Global Administrator field of
the Extended Community, with the Local Administrator field of this
community set to 0. The originating PE then adds this Route Target
Extended Community to this Leaf A-D route. The upstream node's
address is determined as specified in Section 6.1.
The PE then advertises this route to the upstream node.
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6.3. PIM-SM in ASM Mode for Global Table Multicast
This specification allows two options for supporting global table
multicast that are initiated using PIM-SM in ASM mode. The first
option does not carry IP multicast shared trees over the MPLS
network. The second option does carry shared trees over the MPLS
network and provides support for switching from shared trees to
source trees.
6.3.1. Option 1
This option does not carry IP multicast shared trees over the MPLS
network. Therefore, when an (egress) PE creates (*,G) state (as a
result of receiving PIM or IGMP messages on one of its IP multicast
interfaces), the PE does not propagate this state using Leaf A-D
routes.
6.3.1.1. Originating Source Active A-D Routes
Whenever an RP that is co-located with a PE discovers a new multicast
source (as a result of receiving PIM Register or MSDP messages), the
RP/PE SHOULD originate a BGP Source Active A-D route. Similarly,
whenever, as a result of receiving MSDP messages, a PE that is not
configured as an RP discovers a new multicast source, the PE SHOULD
originate a BGP Source Active A-D route. The BGP Source Active A-D
route carries a single MCAST-VPN NLRI constructed as follows:
+ The RD in this NLRI is set to 0.
+ The Multicast Source field MUST be set to S. The Multicast Source
Length field is set appropriately to reflect this.
+ The Multicast Group field MUST be set to G. The Multicast Group
Length field is set appropriately to reflect this.
The Route Target of this Source Active A-D route is an AS-specific
Route Target Extended Community with the Global Administrator field
set to the AS of the advertising RP/PE and the Local Administrator
field set to 0.
To constrain distribution of the Source Active A-D route to the AS of
the advertising RP, this route SHOULD carry the NO_EXPORT Community
([RFC 1997]).
Using the normal BGP procedures, the Source Active A-D route is
propagated to all other PEs within the AS.
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Whenever the RP/PE discovers that the source is no longer active, the
RP MUST withdraw the Source Active A-D route, if such a route was
previously advertised by the RP.
6.3.1.2. Receiving BGP Source Active A-D Route by PE
As a result of receiving PIM or IGMP messages on one of its IP
multicast interfaces, when an egress PE creates in its Tree
Information Base (TIB) a new (*,G) entry with a non-empty outgoing
interface list that contains one or more IP multicast interfaces, the
PE MUST check if it has any Source Active A-D routes for that G. If
there is such a route, S of that route is reachable via an MPLS
interface, and the PE does not have (S,G) state in its TIB for (S,G)
carried in the route, then the PE originates a Leaf A-D route
carrying that (S,G), as specified in Section 6.2.2.
When an egress PE receives a new Source Active A-D route, the PE MUST
check if its TIB contains an (*,G) entry with the same G as carried
in the Source Active A-D route. If such an entry is found, S is
reachable via an MPLS interface, and the PE does not have (S,G) state
in its TIB for (S,G) carried in the route, then the PE originates a
Leaf A-D route carrying that (S,G), as specified in Section 6.2.2.
6.3.1.3. Handling (S,G,rpt) State
Creation and deletion of (S,G,rpt) state on a PE that resulted from
receiving PIM messages on one of its IP multicast interfaces do not
result in any BGP actions by the PE.
6.3.2. Option 2
This option does carry IP multicast shared trees over the MPLS
network. Therefore, when an egress PE creates (*,G) state (as a
result of receiving PIM or IGMP messages on one of its IP multicast
interfaces), the PE does propagate this state using Leaf A-D routes.
6.3.2.1. Originating Source Active A-D Routes
Whenever a PE creates an (S,G) state as a result of receiving Leaf
A-D routes associated with the global table multicast service, if S
is reachable via one of the IP multicast-capable interfaces, and the
PE determines that G is in the PIM-SM in ASM mode range, the PE MUST
originate a BGP Source Active A-D route. The route carries a single
MCAST-VPN NLRI constructed as follows:
+ The RD in this NLRI is set to 0.
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RFC 7524 Inter-Area P2MP Segmented LSPs May 2015
+ The Multicast Source field MUST be set to S. The Multicast Source
Length field is set appropriately to reflect this.
+ The Multicast Group field MUST be set to G. The Multicast Group
Length field is set appropriately to reflect this.
The Route Target of this Source Active A-D route is an AS-specific
Route Target Extended Community with the Global Administrator field
set to the AS of the advertising PE and the Local Administrator field
set to 0.
To constrain distribution of the Source Active A-D route to the AS of
the advertising PE, this route SHOULD carry the NO_EXPORT Community
[RFC 1997].
Using the normal BGP procedures, the Source Active A-D route is
propagated to all other PEs within the AS.
Whenever the PE deletes the (S,G) state that was previously created
as a result of receiving a Leaf A-D route for (S,G), the PE that
deletes the state MUST also withdraw the Source Active A-D route, if
such a route was advertised when the state was created.
6.3.2.2. Receiving BGP Source Active A-D Route
Procedures for receiving BGP Source Active A-D routes are the same as
with Option 1.
6.3.2.3. Pruning Sources Off the Shared Tree
After receiving a new Source Active A-D route for (S,G), if a PE
determines that (a) it has the (*,G) entry in its TIB, (b) the
incoming interface (iif) list for that entry contains one of the IP
interfaces, (c) an MPLS LSP is in the outgoing interface (oif) list
for that entry, and (d) the PE does not originate a Leaf A-D route
for (S,G), then the PE MUST transition the (S,G,rpt) downstream state
to the Prune state. [Conceptually the PIM state machine on the PE
will act "as if" it had received Prune(S,G,Rpt) from some other PE,
without actually having received one.] Depending on the (S,G,rpt)
state on the iifs, this may result in the PE using PIM procedures to
prune S off the Shared (*,G) tree.
Transitioning the state machine to the Prune state SHOULD be done
after a delay that is controlled by a timer. The value of the timer
MUST be configurable. The purpose of this timer is to ensure that S
is not pruned off the shared tree until all PEs have had time to
receive the Source Active A-D route for (S,G).
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RFC 7524 Inter-Area P2MP Segmented LSPs May 2015
The PE MUST keep the (S,G,rpt) downstream state machine in the Prune
state for as long as (a) the outgoing interface list (oif) for (*,G)
contains an MPLS LSP, (b) the PE has at least one Source Active A-D
route for (S,G), and (c) the PE does not originate the Leaf A-D route
for (S,G). Once any of these conditions become no longer valid, the
PE MUST transition the (S,G,rpt) downstream state machine to the
NoInfo state.
Note that, except for the scenario described in the first paragraph
of this section, in all scenarios relying solely on PIM procedures on
the PE is sufficient to ensure the correct behavior when pruning
sources off the shared tree.
6.3.2.4. More on Handling (S,G,rpt) State
Creation and deletion of (S,G,rpt) state on a PE that resulted from
receiving PIM messages on one of its IP multicast interfaces do not
result in any BGP actions by the PE.
7. Egress ABR Procedures
This section describes the egress ABR procedures for constructing
segmented inter-area P2MP LSPs.
7.1. Handling Leaf A-D Route on Egress ABR
When an egress ABR receives a Leaf A-D route and the Route Target
Extended Community carried by the route contains the IP address of
this ABR, the following procedures will be executed.
If the value of the third octet of the MCAST-VPN NLRI of the received
Leaf A-D route is either 0x01, 0x02, or 0x03, this indicates that the
Leaf A-D route was originated in response to an S-PMSI or I-PMSI A-D
route (see Section 6.2.2). In this case, the egress ABR MUST find an
S-PMSI or I-PMSI route whose NLRI has the same value as the Route Key
field of the received Leaf A-D route. If such a matching route is
found, then the Leaf A-D route MUST be accepted. If the Leaf A-D
route is accepted and if it is the first Leaf A-D route update for
the Route Key field in the route, or the withdrawal of the last Leaf
A-D route for the Route Key field, then the following procedures will
be executed.
If the RD of the received Leaf A-D route is set to all zeros or all
ones, then the received Leaf A-D route is for the global table
multicast service.
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If the received Leaf A-D route is the first Leaf A-D route update for
the Route Key field carried in the route, then the egress ABR
originates a Leaf A-D route, whose MCAST-VPN NLRI is constructed as
follows.
The Route Key field of the MCAST-VPN NLRI is the same as the Route
Key field of the MCAST-VPN NLRI of the received Leaf A-D route. The
Originating Router's IP Address field of the MCAST-VPN NLRI is set to
the address of the local ABR (the ABR that originates the route).
The Next Hop field of the MP_REACH_NLRI attribute of the route SHOULD
be set to the same IP address as the one carried in the Originating
Router's IP Address field of the route.
To constrain distribution of this route, the originating egress ABR
constructs an IP-based Route Target Extended Community by placing the
IP address of the upstream node in the Global Administrator field of
the Extended Community, with the Local Administrator field of this
Extended Community set to 0, and sets the Extended Communities
attribute of this Leaf A-D route to that Extended Community.
The upstream node's IP address is the IP address determined from the
Global Administrator field of the Inter-Area P2MP Segmented Next-Hop
Extended Community, where this Extended Community is obtained as
follows. When the Leaf A-D route is for MVPN or VPLS, this Extended
Community is the one carried in the I-PMSI/S-PMSI A-D route that
matches the Leaf A-D route. When the Leaf A-D route is for global
table multicast, this Extended Community is the one carried in the
best unicast route to the Ingress PE. The Ingress PE address is
determined from the received Leaf A-D route. The best unicast route
MUST first be determined from multicast SAFI, i.e., SAFI 2 routes, if
present.
The ABR then advertises this Leaf A-D route to the upstream node in
the backbone area.
Mechanisms specified in [RFC 4684] for constrained BGP route
distribution can be used along with this specification to ensure that
only the needed PE/ABR will have to process a said Leaf A-D route.
When ingress replication is used to instantiate the backbone area
segment, the Leaf A-D route originated by the egress ABR MUST carry a
downstream-assigned label in the PMSI Tunnel attribute where the
Tunnel Type is set to ingress replication. The ABR MUST assign a
distinct MPLS label for each Leaf A-D route that it originates.
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In order to support global table multicast, an egress ABR MUST auto-
configure an import AS-based Route Target Extended Community with the
Global Administrator field set to the AS of the ABR and the Local
Administrator field set to 0.
If the received Leaf A-D route is the withdrawal of the last Leaf A-D
route for the Route Key carried in the route, then the egress ABR
must withdraw the Leaf A-D route associated with that Route Key that
has been previously advertised by the egress ABR in the backbone
area.
7.2. P2MP LSP as the Intra-Area LSP in the Egress Area
This section describes procedures for using intra-area P2MP LSPs in
the egress area. The procedures that are common to both P2MP RSVP-TE
and P2MP LDP are described first, followed by procedures that are
specific to the signaling protocol.
When P2MP LSPs are used as the intra-area LSPs, note that an existing
intra-area P2MP LSP may be used solely for a particular inter-area
P2MP service LSP or for other inter-area P2MP service LSPs as well.
The choice between the two options is purely local to the egress ABR.
The first option provides one-to-one mapping between inter-area P2MP
service LSPs and intra-area P2MP LSPs; the second option provides
many-to-one mapping, thus allowing the aggregation of forwarding
state.
7.2.1. Received Leaf A-D Route Is for MVPN or VPLS
If the value of the third octet of the MCAST-VPN NLRI of the received
Leaf A-D route is either 0x01, 0x02, or 0x03, this indicates that the
Leaf A-D route was originated in response to an MVPN or VPLS S-PMSI
or I-PMSI A-D route (see Section 6.2.2). In this case, the ABR MUST
re-advertise in the egress area the MVPN/VPLS A-D route that matches
the Leaf A-D route to signal the binding of the intra-area P2MP LSP
to the inter-area P2MP service LSP. This must be done if and only if
(a) such a binding hasn't already been advertised or (b) the binding
has changed. The re-advertised route MUST carry the Inter-area P2MP
Segmented Next-Hop Extended Community.
The PMSI Tunnel attribute of the re-advertised route specifies either
an intra-area P2MP RSVP-TE LSP or an intra-area P2MP LDP LSP rooted
at the ABR and MUST also carry an upstream-assigned MPLS label. The
upstream-assigned MPLS label MUST be set to Implicit NULL if the
mapping between the inter-area P2MP service LSP and the intra-area
P2MP LSP is one-to-one. If the mapping is many-to-one, the intra-
area segment of the inter-area P2MP service LSP (referred to as the
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RFC 7524 Inter-Area P2MP Segmented LSPs May 2015
"inner" P2MP LSP) is constructed by nesting the inter-area P2MP
service LSP in an intra-area P2MP LSP (referred to as the "outer"
intra-area P2MP LSP), by using P2MP LSP hierarchy based on upstream-
assigned MPLS labels [RFC 5332].
If segments of multiple MVPN or VPLS S-PMSI service LSPs are carried
over a given intra-area P2MP LSP, each of these segments MUST carry a
distinct upstream-assigned label, even if all these service LSPs are
for (C-S/*,C-G/*)s from the same MVPN/VPLS. Therefore, an ABR
maintains a Label Forwarding Information Base (LFIB) state for each
such S-PMSI traversing the ABR (that applies to both the ingress and
the egress ABRs).
7.2.2. Received Leaf A-D Route Is for Global Table Multicast
When the RD of the received Leaf A-D route is set to all zeros or all
ones, this is the case of inter-area P2MP service LSP being
associated with the global table multicast service. The procedures
for this are described below.
7.2.2.1. Global Table Multicast and S-PMSI A-D Routes
This section applies only if it is desired to send a particular (S,G)
or (*,G) global table multicast flow to only those egress PEs that
have receivers for that multicast flow.
If the egress ABR has not previously received (and re-advertised) an
S-PMSI A-D route for (S,G) or (*,G) that has been originated by an
ingress PE/ASBR (see Section 9.1), then the egress ABR MUST originate
an S-PMSI A-D route. The PMSI Tunnel attribute of the route MUST
contain the identity of the intra-area P2MP LSP and an upstream-
assigned MPLS label (although this label may be an Implicit NULL --
see Section 3). The RD, Multicast Source Length, Multicast Source,
Multicast Group Length (1 octet), and Multicast Group fields of the
NLRI of this route are the same as those of the received Leaf A-D
route. The Originating Router's IP Address field in the S-PMSI A-D
route is the same as the Ingress PE's IP Address field in the
received Leaf A-D route. The Route Target of this route is an AS-
specific Route Target Extended Community with the Global
Administrator field set to the AS of the advertising ABR and the
Local Administrator field set to 0. The route MUST carry the Inter-
Area P2MP Segmented Next-Hop Extended Community. This Extended
Community is constructed following the procedures in Section 4.
The egress ABR MUST advertise this route into the egress area. PEs
in the egress area that participate in the global table multicast
will import this route based on the Route Target carried by the
route.
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RFC 7524 Inter-Area P2MP Segmented LSPs May 2015
A PE in the egress area that originated the Leaf A-D route SHOULD
join the P2MP LSP advertised in the PMSI Tunnel attribute of the
S-PMSI A-D route.
7.2.2.2. Global Table Multicast and Wildcard S-PMSI A-D Routes
It may be desirable for an ingress PE to carry multiple multicast
flows associated with the global table multicast over the same inter-
area P2MP service LSP. This can be achieved using wildcard, i.e.,
(*,*) S-PMSI A-D routes [RFC 6625]. An ingress PE MAY advertise a
wildcard S-PMSI A-D route as described in Section 9.
If the ingress PE originates a wildcard S-PMSI A-D route, and the
egress ABR receives this route from the ingress ABR, then the egress
ABR either (a) MUST re-advertise this route into the egress area with
the PMSI Tunnel attribute containing the identifier of the intra-area
P2MP LSP in the egress area and an upstream-assigned label (note that
this label may be an Implicit NULL -- see Section 3) assigned to the
inter-area wildcard S-PMSI or (b) MUST be able to disaggregate
traffic carried over the wildcard S-PMSI onto the egress area (S,G)
or (*,G) S-PMSIs. The procedures for such disaggregation require IP
processing on the egress ABRs.
If the egress ABR advertises a wildcard S-PMSI A-D route into the
egress area, this route MUST carry an AS-specific Route Target
Extended Community with the Global Administrator field set to the AS
of the advertising ABR and the Local Administrator field set to 0.
PEs in the egress area that participate in the global table multicast
will import this route.
A PE in the egress area SHOULD join the P2MP LSP advertised in the
PMSI Tunnel attribute of the wildcard S-PMSI A-D route if (a) the
Originating Router's IP Address field in the S-PMSI A-D route has the
same value as the Ingress PE's IP Address in at least one of the Leaf
A-D routes for global table multicast originated by the PE and (b)
the upstream ABR for the Ingress PE's IP address in that Leaf A-D
route is the egress ABR that advertises the wildcard S-PMSI A-D
route.
7.2.3. Global Table Multicast and the Expected Upstream Node
If the mapping between the inter-area P2MP service LSP for global
table multicast service and the intra-area P2MP LSP is many-to-one,
then an egress PE must be able to determine whether a given multicast
packet for a particular (S,G) is received from the "expected"
upstream node. The expected node is the node towards which the Leaf
A-D route is sent by the egress PE. Packets received from another
upstream node for that (S,G) MUST be dropped. To allow the egress PE
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to determine the sender upstream node, the intra-area P2MP LSP MUST
be signaled with no Penultimate Hop Popping (PHP), when the mapping
between the inter-area P2MP service LSP for global table multicast
service and the intra-area P2MP LSP is many-to-one.
Further, the egress ABR MUST first push onto the label stack the
upstream-assigned label advertised in the S-PMSI A-D route, if the
label is not the Implicit NULL.
7.2.4. P2MP LDP LSP as the Intra-Area P2MP LSP
The above procedures are sufficient if P2MP LDP LSPs are used as the
intra-area P2MP LSP in the egress area.
7.2.5. P2MP RSVP-TE LSP as the Intra-Area P2MP LSP
If P2MP RSVP-TE LSP is used as the intra-area LSP in the egress area,
then the egress ABR can either (a) graft the leaf (whose IP address
is specified in the received Leaf A-D route) into an existing P2MP
LSP rooted at the egress ABR, and use that LSP for carrying traffic
for the inter-area segmented P2MP service LSP or (b) originate a new
P2MP LSP to be used for carrying (S,G).
When the RD of the received Leaf A-D route is all zeros or all ones,
the procedures are as described in Section 7.2.2.
Note also that the SESSION object that the egress ABR would use for
the intra-area P2MP LSP need not encode the P2MP FEC from the
received Leaf A-D route.
7.3. Ingress Replication in the Egress Area
When ingress replication is used to instantiate the egress area
segment, the Leaf A-D route advertised by the egress PE MUST carry a
downstream-assigned label in the PMSI Tunnel attribute where the
Tunnel Type is set to ingress replication. We will call this label
the egress PE downstream-assigned label.
The egress ABR MUST forward packets received from the backbone area
intra-area segment, for a particular inter-area P2MP LSP, to all the
egress PEs from which the egress ABR has imported a Leaf A-D route
for the inter-area P2MP LSP. A packet to a particular egress PE is
encapsulated, by the egress ABR, using an MPLS label stack the bottom
label of which is the egress PE downstream-assigned label. The top
label is the P2P RSVP-TE or the MP2P LDP label to reach the
egress PE.
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Note that these procedures ensure that an egress PE always receives
packets only from the upstream node expected by the egress PE.
8. Ingress ABR Procedures
When an ingress ABR receives a Leaf A-D route and the Route Target
Extended Community carried by the route contains the IP address of
this ABR, the ingress ABR follows the same procedures as in Section
7, with egress ABR replaced by ingress ABR, backbone area replaced by
ingress area, and backbone area segment replaced by ingress area
segment.
In order to support global table multicast, the ingress ABR MUST be
auto-configured with an import AS-based Route Target Extended
Community whose Global Administrator field is set to the AS of the
ABR and whose Local Administrator field is set to 0.
8.1. P2MP LSP as the Intra-Area LSP in the Backbone Area
The procedures for binding the backbone area segment of an inter-area
P2MP LSP to the intra-area P2MP LSP in the backbone area are the same
as in Sections 7 and 7.2, with egress PE being replaced by egress
ABR, egress ABR being replaced by ingress ABR, and egress area being
replaced by backbone area. This applies to the inter-area P2MP LSPs
associated with either MVPN, VPLS, or global table multicast.
It is to be noted that, in the case of global table multicast, if the
backbone area uses wildcard S-PMSI, then the egress area also SHOULD
use wildcard S-PMSI for global table multicast, or the egress ABRs
MUST be able to disaggregate traffic carried over the wildcard S-PMSI
onto the egress area (S,G) or (*,G) S-PMSIs. The procedures for such
disaggregation require IP processing on the egress ABRs.
8.2. Ingress Replication in the Backbone Area
When ingress replication is used to instantiate the backbone area
segment, the Leaf A-D route advertised by the egress ABR MUST carry a
downstream-assigned label in the PMSI Tunnel attribute where the
Tunnel Type is set to ingress replication. We will call this the
egress ABR downstream-assigned label. The egress ABR MUST assign a
distinct MPLS label for each Leaf A-D route originated by the ABR.
The ingress ABR MUST forward packets received from the ingress area
intra-area segment, for a particular inter-area P2MP LSP, to all the
egress ABRs from which the ingress ABR has imported a Leaf A-D route
for the inter-area P2MP LSP. A packet to a particular egress ABR is
encapsulated, by the ingress ABR, using an MPLS label stack the
bottom label of which is the egress ABR downstream-assigned label.
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The top label is the P2P RSVP-TE or the MP2P LDP label to reach the
egress ABR.
9. Ingress PE/ASBR Procedures
This section describes the ingress PE/ASBR procedures for
constructing segmented inter-area P2MP LSPs.
When an ingress PE/ASBR receives a Leaf A-D route and the Route
Target Extended Community carried by the route contains the IP
address of this PE/ASBR, the following procedures will be executed.
If the value of the third octet of the MCAST-VPN NLRI of the received
Leaf A-D route is either 0x01, 0x02, or 0x03, this indicates that the
Leaf A-D route was originated in response to an S-PMSI or I-PMSI A-D
route (see Section 6.2.2). In this case, the ingress PE/ASBR MUST
find an S-PMSI or I-PMSI route whose NLRI has the same value as the
Route Key field of the received Leaf A-D route. If such a matching
route is found, then the Leaf A-D route MUST be accepted or else it
MUST be discarded. If the Leaf A-D route is accepted, then it MUST
be processed as per MVPN or VPLS procedures.
If the RD of the received A-D route is set to all zeros or all ones,
then the received Leaf A-D route is for the global table multicast
service. If this is the first Leaf A-D route for the Route Key
carried in the route, the PIM implementation MUST set its upstream
(S/RP,G) state machine to Joined state for the (S/RP,G) received via
a Leaf A-D route update. Likewise, if this is the withdrawal of the
last Leaf A-D route whose Route Key matches a particular (S/RP,G)
state, the PIM implementation MUST set its upstream (S/RP,G) state
machine to Prune state for the (S/RP,G).
9.1. P2MP LSP as the Intra-Area LSP in the Ingress Area
If the value of the third octet of the MCAST-VPN NLRI of the received
Leaf A-D route is either 0x01, 0x02, or 0x03 (which indicates that
the Leaf A-D route was originated in response to an S-PMSI or I-PMSI
A-D route), and P2MP LSP is used as the intra-area LSP in the ingress
area, then the procedures for binding the ingress area segment of the
inter-area P2MP LSP to the intra-area P2MP LSP in the ingress area
are the same as in Sections 7 and 7.2.
When the RD of the received Leaf A-D route is all zeros or all ones,
as is the case where the inter-area service P2MP LSP is associated
with the global table multicast service, the ingress PE MAY originate
an S-PMSI A-D route with the RD, multicast source, and multicast
group fields being the same as those in the received Leaf A-D route.
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Further, in the case of global table multicast, an ingress PE MAY
originate a wildcard S-PMSI A-D route as per the procedures in
[RFC 6625] with the RD set to 0. This route may be originated by the
ingress PE based on configuration or based on the import of a Leaf
A-D route with the RD set to 0. If an ingress PE originates such a
route, then the ingress PE MAY decide not to originate (S,G) or (*,G)
S-PMSI A-D routes.
The wildcard S-PMSI A-D route MUST carry the Inter-Area P2MP
Segmented Next-Hop Extended Community. This Extended Community is
constructed following the procedures in Section 4.
It is to be noted that, in the case of global table multicast, if the
ingress area uses wildcard S-PMSI, then the backbone area also SHOULD
use wildcard S-PMSI for global table multicast, or the ingress ABRs
MUST be able to disaggregate traffic carried over the wildcard S-PMSI
onto the backbone area (S,G) or (*,G) S-PMSIs. The procedures for
such disaggregation require IP processing on the ingress ABRs.
9.2. Ingress Replication in the Ingress Area
When ingress replication is used to instantiate the ingress area
segment, the Leaf A-D route advertised by the ingress ABR MUST carry
a downstream-assigned label in the PMSI Tunnel attribute where the
Tunnel Type is set to ingress replication. We will call this the
ingress ABR downstream-assigned label. The ingress ABR MUST assign a
distinct MPLS label for each Leaf A-D route originated by the ABR.
The ingress PE/ASBR MUST forward packets received from the CE, for a
particular inter-area P2MP LSP, to all the ingress ABRs from which
the ingress PE/ASBR has imported a Leaf A-D route for the inter-area
P2MP LSP. A packet to a particular ingress ABR is encapsulated, by
the ingress PE/ASBR, using an MPLS label stack the bottom label of
which is the ingress ABR downstream-assigned label. The top label is
the P2P RSVP-TE or the MP2P LDP label to reach the ingress ABR.
10. Common Tunnel Type in the Ingress and Egress Areas
For a given inter-area service P2MP LSP, the PE/ASBR that is the root
of that LSP controls the type of the intra-area P-tunnel that carries
the ingress area segment of that LSP. However, the type of the
intra-area P-tunnel that carries the backbone area segment of that
LSP may be different from the type of the intra-area P-tunnels that
carry the ingress area segment and the egress area segment of that
LSP. In that situation, if, for a given inter-area P2MP LSP, it is
desirable/necessary to use the same type of tunnel for the intra-area
P-tunnels that carry the ingress area segment and for the intra-area
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P-tunnels that carry the egress area segment of that LSP, then the
following procedures on the ingress ABR and egress ABR provide this
functionality.
When an ingress ABR re-advertises into the backbone area a BGP MVPN
I-PMSI, S-PMSI A-D route, or VPLS A-D route, the ingress ABR places
the PMSI Tunnel attribute of this route into the ATTR_SET BGP
attribute [RFC 6368], adds this attribute to the re-advertised route,
and then replaces the original PMSI Tunnel attribute with a new one
(note that the Tunnel Type of the new attribute may be different from
the Tunnel Type of the original attribute).
When an egress ABR re-advertises into the egress area a BGP MVPN
I-PMSI or S-PMSI A-D route, or VPLS A-D route, if the route carries
the ATTR_SET BGP attribute [RFC 6368], the ABR sets the Tunnel Type of
the PMSI Tunnel attribute in the re-advertised route to the Tunnel
Type of the PMSI Tunnel attribute carried in the ATTR_SET BGP
attribute, and removes the ATTR_SET from the route.
11. Placement of Ingress and Egress PEs
As described in the earlier sections, procedures in this document
allow the placement of ingress and egress PEs in the backbone area.
They also allow the placement of egress PEs in the ingress area or
the placement of ingress PEs in the egress area.
For instance, suppose that in the ingress and egress areas, a global
table multicast service is being provided, and the data is being sent
over PIM-based IP/GRE P-tunnels. Suppose also that it is desired to
carry that data over the backbone area through MPLS P-tunnels. This
can be done if the ABR connecting the ingress area to the backbone
follows the procedures that this document specifies for ingress PEs
providing the global table multicast service, and if the ABR
connecting the egress area to the backbone follows the procedures
that this document specifies for egress PEs providing the global
table multicast service.
If MVPN service is being provided in the ingress and egress areas,
with the MVPN data carried in PIM-based IP/GRE P-tunnels, this same
technique enables the MVPN data to be carried over the backbone in
MPLS P-tunnels. The PIM-based IP/GRE P-tunnels in the ingress and
egress areas are treated as global table multicasts, and the ABRs
provide the ingress and egress PE functionality.
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12. MVPN with Virtual Hub-and-Spoke
Procedures described in this document could be used in conjunction
with the Virtual Hub-and-Spoke procedures [RFC 7024].
This document does not place any restrictions on the placement of
Virtual Hubs and Virtual Spokes.
In addition to I-PMSI/S-PMSI A-D routes, the procedures described in
this document are applicable to Associated-V-spoke-only I-PMSI A-D
routes.
In the scenario where a V-hub, as a result of importing an S-PMSI A-D
route in its VRF, originates an S-PMSI A-D route targeted to its
V-spokes (as specified in Section 7.8.2 of [RFC 7024]), the V-hub
SHOULD be able to control via configuration whether the Inter-Area
P2MP Segmented Next-Hop Extended Community, if present in the
received S-PMSI A-D route, should also be carried in the originated
S-PMSI A-D route. By default, if the received S-PMSI A-D route
carries the Inter-Area P2MP Segmented Next-Hop Extended Community,
then the originated S-PMSI A-D route SHOULD also carry this Extended
Community.
13. Data Plane
This section describes the data plane procedures on the ABRs, ingress
PEs, egress PEs, and transit routers.
13.1. Data Plane Procedures on ABRs
When procedures in this document are followed to signal inter-area
P2MP segmented LSPs, ABRs are required to perform only MPLS
switching. When an ABR receives an MPLS packet from an "incoming"
intra-area segment of the inter-area P2MP segmented LSP, it forwards
the packet, based on MPLS switching, on to another "outgoing" intra-
area segment of the inter-area P2MP segmented LSP.
If the outgoing intra-area segment is instantiated using a P2MP LSP,
and if there is a one-to-one mapping between the outgoing intra-area
segment and the P2MP LSP, then the ABR MUST pop the incoming
segment's label stack and push the label stack of the outgoing P2MP
LSP. If there is a many-to-one mapping between outgoing intra-area
segments and the P2MP LSP, then the ABR MUST pop the incoming
segment's label stack and first push the upstream-assigned label
corresponding to the outgoing intra-area segment, if such a label has
been assigned, and then push the label stack of the outgoing P2MP
LSP.
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If the outgoing intra-area segment is instantiated using ingress
replication, then the ABR must pop the incoming segment's label stack
and replicate the packet once to each leaf ABR or PE of the outgoing
intra-area segment. The label stack of the packet sent to each such
leaf MUST first include a downstream-assigned label assigned by the
leaf to the segment, followed by the label stack of the P2P or MP2P
LSP to the leaf.
13.2. Data Plane Procedures on Egress PEs
An egress PE must first identify the inter-area P2MP segmented LSP
based on the incoming label stack. After this identification, the
egress PE must forward the packet using the application that is bound
to the inter-area P2MP segmented LSP.
Note that the application-specific forwarding for MVPN service may
require the egress PE to determine whether the packets were received
from the expected sender PE. When the application is MVPN, the FEC
of an inter-area P2MP segmented LSP is at the granularity of the
sender PE. Note that MVPN intra-AS I-PMSI A-D routes and S-PMSI A-D
routes both carry the Originating Router's IP Address. Thus, an
egress PE could associate the data arriving on P-tunnels advertised
by these routes with the Originating Router's IP Address carried by
these routes, which is the same as the ingress PE. Since a unique
label stack is associated with each such FEC, the egress PE can
determine the sender PE from the label stack.
Likewise for VPLS service, for the purposes of Media Access Control
(MAC) learning the egress, the PE must be able to determine the
"VE-ID" (VPLS Edge Device Identifier) from which the packets have
been received. The FEC of the VPLS A-D routes carries the VE-ID.
Thus, an egress PE could associate the data arriving on P-tunnels
advertised by these routes with the VE-ID carried by these routes.
Since a unique label stack is associated with each such FEC, the
egress PE can perform MAC learning for packets received from a given
VE-ID.
When the application is global table multicast, it is sufficient for
the label stack to include identification of the sender upstream
node. When P2MP LSPs are used, this requires that PHP MUST be turned
off. When ingress replication is used, the egress PE knows the
incoming downstream-assigned label to which it has bound a particular
(S/*,G) and must accept packets with only that label for that
(S/*,G).
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13.3. Data Plane Procedures on Ingress PEs
An Ingress PE must perform application-specific forwarding procedures
to identify the outgoing intra-area segment of an incoming packet.
If the outgoing intra-area segment is instantiated using a P2MP LSP,
and if there is a one-to-one mapping between the outgoing intra-area
segment and the P2MP LSP, then the ingress PE MUST encapsulate the
packet in the label stack of the outgoing P2MP LSP. If there is a
many-to-one mapping between outgoing intra-area segments and the P2MP
LSP, then the PE MUST first push the upstream-assigned label
corresponding to the outgoing intra-area segment, if such a label
has been assigned, and then push the label stack of the outgoing
P2MP LSP.
If the outgoing intra-area segment is instantiated using ingress
replication, then the PE must replicate the packet once to each leaf
ABR or PE of the outgoing intra-area segment. The label stack of the
packet sent to each such leaf MUST first include a downstream-
assigned label assigned by the leaf to the segment, followed by the
label stack of the P2P or MP2P LSP to the leaf.
13.4. Data Plane Procedures on Transit Routers
When procedures in this document are followed to signal inter-area
P2MP segmented LSPs, transit routers in each area perform only MPLS
switching.
14. Support for Inter-Area Transport LSPs
This section describes OPTIONAL procedures that allow multiple
(inter-area) P2MP LSPs to be aggregated into a single inter-area P2MP
"transport LSP". The segmentation procedures, as specified in this
document, are then applied to these inter-area P2MP transport LSPs,
rather than being applied directly to the individual LSPs that are
aggregated into the transport. In the following, the individual LSPs
that are aggregated into a single transport LSP will be known as
"service LSPs".
14.1. "Transport Tunnel" Tunnel Type
For the PMSI Tunnel attribute, we define a new Tunnel Type, called
"Transport Tunnel", whose Tunnel Identifier is a tuple <Source PE
Address, Local Number>. This Tunnel Type is assigned a value of 8.
The Source PE Address is the address of the PE that originates the
(service) A-D route that carries this attribute, and the Local Number
is a number that is unique to the Source PE. The length of the Local
Number part is the same as the length of the Source PE Address.
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Thus, if the Source PE Address is an IPv4 address, then the Local
Number part is 4 octets; if the Source PE Address is an IPv6 address,
then the Local Number part is 16 octets.
14.2. Discovering Leaves of the Inter-Area P2MP Service LSP
When aggregating multiple P2MP LSPs using P2MP LSP hierarchy,
determining the leaf nodes of the LSPs being aggregated is essential
for being able to trade-off the overhead due to the P2MP LSPs versus
suboptimal use of bandwidth due to the partial congruency of the LSPs
being aggregated.
Therefore, if a PE that is a root of a given service P2MP LSP wants
to aggregate this LSP with other (service) P2MP LSPs rooted at the
same PE into an inter-area P2MP transport LSP, the PE should first
determine all the leaf nodes of that service LSP, as well as those of
the other service LSPs being aggregated.
To accomplish this, the PE sets the PMSI Tunnel attribute of the
service A-D route (an I-PMSI or S-PMSI A-D route for MVPN or VPLS
service) associated with that LSP as follows. The Tunnel Type is set
to "No tunnel information present", and the "Leaf Information
Required" flag is set to 1. The route MUST NOT carry the Inter-Area
P2MP Segmented Next-Hop Extended Community. In contrast to the
procedures for the MVPN and VPLS A-D routes described so far, the
Route Reflectors that participate in the distribution of this route
need not be ABRs.
14.3. Discovering P2MP FEC of P2MP Transport LSP
Once the ingress PE determines all the leaves of a given P2MP service
LSP, the PE (using some local criteria) selects a particular inter-
area transport P2MP LSP to be used for carrying the (inter-area)
service P2MP LSP.
Once the PE selects the transport P2MP LSP, the PE (re-)originates
the service A-D route. The PMSI Tunnel attribute of this route now
carries the Tunnel Identifier of the selected transport LSP, with the
Tunnel Type set to "Transport Tunnel". If the transport LSP carries
multiple P2MP service LSPs, then the MPLS Label field in the
attribute carries an upstream-assigned label assigned by the PE that
is bound to the P2MP FEC of the inter-area P2MP service LSP.
Otherwise, this field is set to Implicit NULL.
As described earlier, this service A-D route MUST NOT carry the
Inter-Area P2MP Segmented Next-Hop Extended Community, and the Route
Reflectors that participate in the distribution of this route need
not be ABRs.
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14.4. Egress PE Procedures for P2MP Transport LSP
When an egress PE receives and accepts an MVPN or VPLS service A-D
route, if the "Leaf Information Required" flag in the PMSI Tunnel
attribute of the received A-D route is set to 1, and the route does
not carry the Inter-Area P2MP Segmented Next-Hop Extended Community,
then the egress PE, following the "regular" MVPN or VPLS procedures
associated with the received A-D route (as specified in [RFC 6514] and
[RFC 7117]), originates a Leaf A-D route.
In addition, if the Tunnel Type in the PMSI Tunnel attribute of the
received service A-D route is set to "Transport Tunnel", the egress
PE originates yet another Leaf A-D route.
The format of the Route Key field of the MCAST-VPN NLRI of this
additional Leaf A-D route is the same as defined in Section 6.2.2.
The Route Key field of the MCAST-VPN NLRI of this route is
constructed as follows:
RD (8 octets) - set to 0
Multicast Source Length, Multicast Source - constructed from the
Source PE Address part of the Tunnel Identifier carried in the
PMSI Tunnel attribute of the received service S-PMSI A-D
route.
Multicast Group Length, Multicast Group - constructed from the
Local Number part of the Tunnel Identifier carried in the PMSI
Tunnel attribute of the received service S-PMSI A-D route.
Ingress PE IP Address - set to the Source PE Address part of the
Tunnel Identifier carried in the PMSI Tunnel attribute of the
received service S-PMSI A-D route.
The egress PE, when determining the upstream ABR, follows the
procedures specified in Section 6.1 for global table multicast.
The egress PE constructs the rest of the Leaf A-D route following the
procedures specified in Section 6.2.3.
From that point on we follow the procedures used for the Leaf A-D
routes for global table multicast, as outlined below.
14.5. ABRs and Ingress PE Procedures for P2MP Transport LSP
In this section, we specify ingress and egress ABRs, as well as
ingress PE procedures for P2MP transport LSPs.
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When an egress ABR receives the Leaf A-D route, and P2MP LSP is used
to instantiate the egress area segment of the inter-area transport
LSP, the egress ABR will advertise into the egress area an S-PMSI A-D
route. This route is constructed following the procedures in Section
7.2.2.1. The egress PE(s) will import this route.
The egress ABR will also propagate, with appropriate modifications,
the received Leaf A-D route into the backbone area. This is
irrespective of whether the egress area segment is instantiated using
P2MP LSP or ingress replication.
If P2MP LSP is used to instantiate the backbone area segment of the
inter-area transport LSP, then an ingress ABR will advertise into the
backbone area an S-PMSI A-D route. This route is constructed
following the procedures in Section 7.1.2.1. The egress ABR(s) will
import this route.
The ingress ABR will also propagate, with appropriate modifications,
the received Leaf A-D route into the ingress area towards the
ingress/root PE. This is irrespective of whether the backbone area
segment is instantiated using P2MP LSP or ingress replication.
Finally, if P2MP LSP is used to instantiate the ingress area segment,
the ingress PE will advertise into the ingress area an S-PMSI A-D
route with the RD, multicast source, and multicast group fields being
the same as those in the received Leaf A-D route. The PMSI Tunnel
attribute of this route contains the identity of the intra-area P2MP
LSP used to instantiate the ingress area segment, and an upstream-
assigned MPLS label. The ingress ABR(s) and other PE(s) in the
ingress area, if any, will import this route. The ingress PE will
use the (intra-area) P2MP LSP advertised in this route for carrying
traffic associated with the original service A-D route advertised by
the PE.
14.6. Discussion
Use of inter-area transport P2MP LSPs, as described in this section,
creates a level of indirection between (inter-area) P2MP service
LSPs, and intra-area transport LSPs that carry the service LSPs.
Rather than segmenting (inter-area) service P2MP LSPs, and then
aggregating (intra-area) segments of these service LSPs into intra-
area transport LSPs, this approach first aggregates multiple (inter-
area) service P2MP LSPs into a single inter-area transport P2MP LSP,
then segments such inter-area transport P2MP LSPs, and then
aggregates (intra-area) segments of these inter-area transport LSPs
into intra-area transport LSPs.
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RFC 7524 Inter-Area P2MP Segmented LSPs May 2015
On one hand, this approach could result in reducing the state (and
the overhead associated with maintaining the state) on ABRs. This is
because instead of requiring ABRs to maintain state for individual
P2MP service LSPs, ABRs would need to maintain state only for the
inter-area P2MP transport LSPs. Note, however, that this reduction
is possible only if a single inter-area P2MP transport LSP aggregates
more than one (inter-area) service LSP. In the absence of such
aggregation, use of inter-area transport LSPs provides no benefits,
yet results in extra overhead. And while such aggregation does allow
reduced state on ABRs, it comes at a price, as described below.
As we mentioned before, aggregating multiple P2MP service LSPs into a
single inter-area P2MP transport LSP requires the PE rooted at these
LSPs to determine all the leaf nodes of the service LSPs to be
aggregated. This means that the root PE has to track all the leaf
PEs of these LSPs. In contrast, when one applies segmentation
procedures directly to the P2MP service LSPs, the root PE has to
track only the leaf PEs in its own IGP area, plus the ingress ABR(s).
Likewise, an ingress ABR has to track only the egress ABRs. Finally,
an egress ABR has to track only the leaf PEs in its own area.
Therefore, while the total overhead of leaf tracking due to the P2MP
service LSPs is about the same in both approaches, the distribution
of this overhead is different. Specifically, when one uses inter-
area P2MP transport LSPs, this overhead is concentrated on the
ingress PE. When one applies segmentation procedures directly to the
P2MP service LSPs, this overhead is distributed among the ingress PE
and ABRs.
Moreover, when one uses inter-area P2MP transport LSPs, ABRs have to
bear the overhead of leaf tracking for inter-area P2MP transport
LSPs. In contrast, when one applies segmentation procedures directly
to the P2MP service LSPs, there is no such overhead (as there are no
inter-area P2MP transport LSPs).
Use of inter-area P2MP transport LSPs may also result in more
bandwidth inefficiency, as compared to applying segmentation
procedures directly to the P2MP service LSPs. This is because with
inter-area P2MP transport LSPs the ABRs aggregate segments of inter-
area P2MP transport LSPs, rather than segments of (inter-area) P2MP
service LSPs. To illustrate this, consider the following example.
Assume PE1 in Area 1 is an ingress PE, with two multicast streams,
(C-S1, C-G1) and (C-S2, C-G2), originated by an MVPN site connected
to PE1. Assume that PE2 in Area 2 has an MVPN site with receivers
for (C-S1, C-G1), PE3 and PE4 in Area 3 have an MVPN site with
receivers for both (C-S1, C-G1) and (C-S2, C-G2). Finally, assume
that PE5 in Area 4 has an MVPN site with receivers for (C-S2, C-G2).
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RFC 7524 Inter-Area P2MP Segmented LSPs May 2015
When segmentation applies directly to the P2MP service LSPs, Area 2
would have just one intra-area transport LSP that would carry the
egress area segment of the P2MP service LSP for (C-S1, C-G1); Area 3
would have just one intra-area transport LSP that would carry the
egress area segments of both the P2MP service LSP for (C-S1, C-G1)
and the P2MP service LSP for (C-S2, C-G2); Area 4 would have just one
intra-area transport LSP that would carry the egress area segment of
the P2MP service LSP for (C-S2, C-G2). Note that there is no
bandwidth inefficiency in this case at all.
When using inter-area P2MP transport LSPs, to achieve the same state
overhead on the routers within each of the egress areas (except for
egress ABRs), PE1 would need to aggregate the P2MP service LSP for
(C-S1, C-G1) and the P2MP service LSP for (C-S2, C-G2) into the same
inter-area P2MP transport LSP. While such aggregation would reduce
state on ABRs, it would also result in bandwidth inefficiency, as
(C-S1, C-G1) will be delivered not just to PE2, PE3, and PE4, but
also to PE5, which has no receivers for this stream. Likewise,
(C-S2, C-G2) will be delivered not just to PE3, PE4, and PE5, but
also to PE2, which has no receivers for this stream.
15. IANA Considerations
This document defines a new BGP Extended Community called "Inter-Area
P2MP Segmented Next-Hop" (see Section 4). This may be either a
Transitive IPv4-Address-Specific Extended Community or a Transitive
IPv6-Address-Specific Extended Community. IANA has assigned the
value 0x12 in the "Transitive IPv4-Address-Specific Extended
Community Sub-Types" registry, and IANA has assigned the value 0x0012
in the "Transitive IPv6-Address-Specific Extended Community Types"
registry. This document is the reference for both code points.
IANA has assigned the value 0x08 in the "P-Multicast Service
Interface Tunnel (PMSI Tunnel) Tunnel Types" registry [RFC 7385] as
"Transport Tunnel" (see Section 14).
This document makes use of a Route Distinguisher whose value is all
ones. The two-octet type field of this Route Distinguisher thus has
the value 65535. IANA has assigned this value in the "Route
Distinguisher Type Field" registry as "For Use Only in Certain Leaf
A-D Routes", with this document as the reference.
16. Security Considerations
Procedures described in this document are subject to security threats
similar to those experienced by any MPLS deployment. It is
recommended that baseline security measures are considered as
described in "Security Framework for MPLS and GMPLS Networks"
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RFC 7524 Inter-Area P2MP Segmented LSPs May 2015
[RFC 5920], in the mLDP specification [RFC 6388], and in the P2MP
RSVP-TE specification [RFC 3209]. The security considerations of
[RFC 6513] are also applicable.
17. References
17.1. Normative References
[RFC 1997] Chandra, R., Traina, P., and T. Li, "BGP Communities
Attribute", RFC 1997, DOI 10.17487/RFC 1997, August 1996,
<http://www.rfc-editor.org/info/RFC 1997>.
[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 3209] Awduche, D., Berger, L., Gan, D., Li, T., Srinivasan, V.,
and G. Swallow, "RSVP-TE: Extensions to RSVP for LSP
Tunnels", RFC 3209, DOI 10.17487/RFC 3209, December 2001,
<http://www.rfc-editor.org/info/RFC 3209>.
[RFC 4360] Sangli, S., Tappan, D., and Y. Rekhter, "BGP Extended
Communities Attribute", RFC 4360, DOI 10.17487/RFC 4360,
February 2006, <http://www.rfc-editor.org/info/RFC 4360>.
[RFC 4456] Bates, T., Chen, E., and R. Chandra, "BGP Route
Reflection: An Alternative to Full Mesh Internal BGP
(IBGP)", RFC 4456, DOI 10.17487/RFC 4456, April 2006,
<http://www.rfc-editor.org/info/RFC 4456>.
[RFC 4684] Marques, P., Bonica, R., Fang, L., Martini, L., Raszuk,
R., Patel, K., and J. Guichard, "Constrained Route
Distribution for Border Gateway Protocol/MultiProtocol
Label Switching (BGP/MPLS) Internet Protocol (IP) Virtual
Private Networks (VPNs)", RFC 4684, DOI 10.17487/RFC 4684,
November 2006, <http://www.rfc-editor.org/info/RFC 4684>.
[RFC 4760] Bates, T., Chandra, R., Katz, D., and Y. Rekhter,
"Multiprotocol Extensions for BGP-4", RFC 4760,
DOI 10.17487/RFC 4760, January 2007,
<http://www.rfc-editor.org/info/RFC 4760>.
[RFC 4761] Kompella, K., Ed., and Y. Rekhter, Ed., "Virtual Private
LAN Service (VPLS) Using BGP for Auto-Discovery and
Signaling", RFC 4761, DOI 10.17487/RFC 4761, January 2007,
<http://www.rfc-editor.org/info/RFC 4761>.
Rekhter, et al. Standards Track PAGE 39
RFC 7524 Inter-Area P2MP Segmented LSPs May 2015
[RFC 4875] Aggarwal, R., Ed., Papadimitriou, D., Ed., and S.
Yasukawa, Ed., "Extensions to Resource Reservation
Protocol - Traffic Engineering (RSVP-TE) for Point-to-
Multipoint TE Label Switched Paths (LSPs)", RFC 4875,
DOI 10.17487/RFC 4875, May 2007,
<http://www.rfc-editor.org/info/RFC 4875>.
[RFC 5036] Andersson, L., Ed., Minei, I., Ed., and B. Thomas, Ed.,
"LDP Specification", RFC 5036, DOI 10.17487/RFC 5036,
October 2007, <http://www.rfc-editor.org/info/RFC 5036>.
[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>.
[RFC 5332] Eckert, T., Rosen, E., Ed., Aggarwal, R., and Y. Rekhter,
"MPLS Multicast Encapsulations", RFC 5332,
DOI 10.17487/RFC 5332, August 2008,
<http://www.rfc-editor.org/info/RFC 5332>.
[RFC 6074] Rosen, E., Davie, B., Radoaca, V., and W. Luo,
"Provisioning, Auto-Discovery, and Signaling in Layer 2
Virtual Private Networks (L2VPNs)", RFC 6074,
DOI 10.17487/RFC 6074, January 2011,
<http://www.rfc-editor.org/info/RFC 6074>.
[RFC 6368] Marques, P., Raszuk, R., Patel, K., Kumaki, K., and T.
Yamagata, "Internal BGP as the Provider/Customer Edge
Protocol for BGP/MPLS IP Virtual Private Networks
(VPNs)", RFC 6368, DOI 10.17487/RFC 6368, September 2011,
<http://www.rfc-editor.org/info/RFC 6368>.
[RFC 6388] Wijnands, IJ., Ed., Minei, I., Ed., Kompella, K., and B.
Thomas, "Label Distribution Protocol Extensions for
Point-to-Multipoint and Multipoint-to-Multipoint Label
Switched Paths", RFC 6388, DOI 10.17487/RFC 6388, November
2011, <http://www.rfc-editor.org/info/RFC 6388>.
[RFC 6513] Rosen, E., Ed., and R. Aggarwal, Ed., "Multicast in
MPLS/BGP IP VPNs", RFC 6513, DOI 10.17487/RFC 6513,
February 2012, <http://www.rfc-editor.org/info/RFC 6513>.
[RFC 6514] Aggarwal, R., Rosen, E., Morin, T., and Y. Rekhter, "BGP
Encodings and Procedures for Multicast in MPLS/BGP IP
VPNs", RFC 6514, DOI 10.17487/RFC 6514, February 2012,
<http://www.rfc-editor.org/info/RFC 6514>.
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RFC 7524 Inter-Area P2MP Segmented LSPs May 2015
[RFC 6625] Rosen, E., Ed., Rekhter, Y., Ed., Hendrickx, W., and R.
Qiu, "Wildcards in Multicast VPN Auto-Discovery Routes",
RFC 6625, DOI 10.17487/RFC 6625, May 2012,
<http://www.rfc-editor.org/info/RFC 6625>.
[RFC 7117] Aggarwal, R., Ed., Kamite, Y., Fang, L., Rekhter, Y., and
C. Kodeboniya, "Multicast in Virtual Private LAN Service
(VPLS)", RFC 7117, DOI 10.17487/RFC 7117, February 2014,
<http://www.rfc-editor.org/info/RFC 7117>.
[RFC 7385] Andersson, L. and G. Swallow, "IANA Registry for
P-Multicast Service Interface (PMSI) Tunnel Type Code
Points", RFC 7385, DOI 10.17487/RFC 7385, October 2014,
<http://www.rfc-editor.org/info/RFC 7385>.
17.2. Informative References
[GTM] Zhang, J, Giuliano, L, Rosen, E., Ed., Subramanian, K.,
Pacella, D., and J. Schiller, "Global Table Multicast
with BGP-MVPN Procedures", Work in Progress, draft-ietf-
bess-mvpn-global-table-mcast-00, November 2014.
[RFC 5920] Fang, L., Ed., "Security Framework for MPLS and GMPLS
Networks", RFC 5920, DOI 10.17487/RFC 5920, July 2010,
<http://www.rfc-editor.org/info/RFC 5920>.
[RFC 7024] Jeng, H., Uttaro, J., Jalil, L., Decraene, B., Rekhter,
Y., and R. Aggarwal, "Virtual Hub-and-Spoke in BGP/MPLS
VPNs", RFC 7024, DOI 10.17487/RFC 7024, October 2013,
<http://www.rfc-editor.org/info/RFC 7024>.
[SEAMLESS-MPLS]
Leymann, N., Ed., Decraene, B., Filsfils, C.,
Konstantynowicz, M., Ed., and D. Steinberg, "Seamless
MPLS Architecture", Work in Progress,
draft-ietf-mpls-seamless-mpls-07, June 2014.
Acknowledgements
We would like to thank Loa Andersson and Qin Wu for their review and
comments.
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RFC 7524 Inter-Area P2MP Segmented LSPs May 2015
Authors' Addresses
Yakov Rekhter
Juniper Networks
1194 North Mathilda Ave.
Sunnyvale, CA 94089
United States
Eric C Rosen
Juniper Networks
10 Technology Park Drive
Westford, MA 01886
United States
EMail: erosen@juniper.net
Rahul Aggarwal
EMail: raggarwa_1@yahoo.com
Thomas Morin
Orange
2, avenue Pierre-Marzin
22307 Lannion Cedex
France
EMail: thomas.morin@orange.com
Irene Grosclaude
Orange
2, avenue Pierre-Marzin
22307 Lannion Cedex
France
EMail: irene.grosclaude@orange.com
Nicolai Leymann
Deutsche Telekom AG
Winterfeldtstrasse 21
Berlin 10781
Germany
EMail: n.leymann@telekom.de
Samir Saad
AT&T
EMail: samirsaad1@outlook.com
Rekhter, et al. Standards Track PAGE 42
RFC TOTAL SIZE: 105804 bytes
PUBLICATION DATE: Thursday, May 14th, 2015
LEGAL RIGHTS: The IETF Trust (see BCP 78)
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