Inter-Area Point-to-Multipoint (P2MP) Segmented Label Switched Paths (LSPs)
updated by
- rfc-8534 — Explicit Tracking with Wildcard Routes in Multicast VPN
Extracted elements (27)
BGP is used as the inter-area routing and label distribution protocol rather than extending intra-area P2MP signaling (RSVP-TE or mLDP) across area boundaries. This enables scalable deployment across thousands of PEs in multiple IGP areas within a single AS (Seamless MPLS), without requiring area-spanning P2MP state.
P2MP LSP hierarchy with upstream-assigned labels is used even for 1:1 mapping between intra-area segments and P2MP LSPs by employing Implicit NULL as the upstream-assigned label. This simplifies the model by providing a uniform mechanism for both 1:1 and n:1 mappings.
A router receiving a Leaf A-D route distinguishes between global table multicast routes and those originated in response to S-PMSI or I-PMSI A-D routes by examining the third octet of the MCAST-VPN NLRI: values 0x01, 0x02, or 0x03 indicate response to S-PMSI/I-PMSI A-D routes; values 0x00 or 0xff with octets 3-10 all zeros or all ones indicate global table multicast.
This document complements RFC 6514 and RFC 7117 for inter-AS segmented P2MP service LSPs. An inter-AS deployment can nest the two: inter-AS segmented P2MP LSPs (per RFC6514/RFC7117) where each intra-AS segment is itself constructed using inter-area segmented P2MP LSPs as specified in this document.
ABRs MUST NOT modify the BGP Next Hop when re-advertising either Intra-AS or Inter-AS I-PMSI/S-PMSI A-D routes, so that ABRs are not required to participate in the propagation of C-multicast routes.
An implementation that supports this document MUST implement the procedures described in Sections 4 through 13 to support inter-area P2MP segmented service LSPs.
For global table multicast, an egress PE MUST be auto-configured to import routes carrying an AS-specific Route Target Extended Community with the Global Administrator field set to the AS of the PE and Local Administrator set to 0. Similarly, an egress ABR MUST auto-configure an import AS-based Route Target Extended Community for the same purpose.
For global table multicast, unicast routes to multicast sources/RPs SHOULD carry the VRF Route Import Extended Community [RFC6514] with Local Administrator set to 0. If not, these routes MUST be advertised using multicast SAFI (SAFI 2) with this community. BGP unicast routes to PE/ASBR/ABR addresses SHOULD carry the Inter-Area P2MP Segmented Next-Hop Extended Community, or else MUST be advertised using SAFI 2.
If segments of multiple MVPN or VPLS S-PMSI service LSPs are carried over a given intra-area P2MP LSP, each segment MUST carry a distinct upstream-assigned label even if all service LSPs belong to the same MVPN/VPLS. The ABR maintains an LFIB state for each such S-PMSI traversing it.
The address family of the Ingress PE's IP Address and Originating Router's IP Address in Leaf A-D routes for global table multicast MUST NOT be inferred from the AFI field of the MP_REACH_NLRI/MP_UNREACH_NLRI attribute; instead, address family is determined by field length (4 octets = IPv4, 16 octets = IPv6). If the computed length is neither 4 nor 16, the MP_REACH_NLRI MUST be treated as incorrect per Section 7 of RFC 4760.
The 'Leaf Information Required' flag MUST be set in VPLS A-D routes and VPLS S-PMSI A-D routes when re-advertised by ingress or egress ABRs, with the same exception as for MVPN (no aggregation policy and mLDP used for intra-area transport). ABRs re-advertising VPLS A-D routes MUST NOT modify the BGP Next Hop.
To constrain distribution of a Leaf A-D route, the originating PE constructs an IP-based Route Target Extended Community with the upstream node's IP address in the Global Administrator field and Local Administrator set to 0, and adds this to the Leaf A-D route before advertising it to the upstream node.
To support a common tunnel type in ingress and egress areas, when re-advertising MVPN I-PMSI/S-PMSI or VPLS A-D routes into the backbone, an ingress ABR places the original PMSI Tunnel attribute into the ATTR_SET BGP attribute [RFC6368], adds it to the re-advertised route, and replaces the PMSI Tunnel attribute with a new one potentially of a different Tunnel Type. The egress ABR reverses this on re-advertisement into the egress area.
When ABRs reflect BGP routes from non-backbone areas into the backbone, they MUST set the BGP Next Hop to their own loopback addresses (next-hop-self). When reflecting from the backbone area into non-backbone areas, ABRs SHOULD NOT change the BGP Next Hop (next-hop-unchanged).
When an ASBR receives from an IBGP peer an I-PMSI or S-PMSI A-D route carrying the Inter-Area P2MP Segmented Next-Hop Extended Community, before re-advertising to an EBGP peer, the ASBR SHOULD remove this Extended Community. If such a route is received from an EBGP peer and the LSP should not be segmented, the ASBR MUST remove this Extended Community before re-advertising to IBGP peers.
When ingress replication is used for the egress area segment, the Leaf A-D route MUST carry a downstream-assigned label in the PMSI Tunnel attribute with Tunnel Type set to ingress replication. A PE MUST assign a distinct MPLS label for each Leaf A-D route it originates.
When MVPN I-PMSI/S-PMSI A-D routes are advertised to signal inter-area P2MP service LSPs, they MUST carry the Inter-Area P2MP Segmented Next-Hop Extended Community. The originating PE or ASBR MUST include this community and set its Global Administrator field to their own IP address; ABRs re-advertising such routes MUST also include and update this community to their own IP address.
When the egress ABR re-advertises a wildcard (*,*) S-PMSI A-D route into the egress area, the route MUST carry an AS-specific Route Target Extended Community with the Global Administrator field set to the AS of the advertising ABR and Local Administrator set to 0, so egress area PEs participating in global table multicast can import it.
When the mapping between an inter-area P2MP service LSP for global table multicast and the intra-area P2MP LSP is many-to-one, the intra-area P2MP LSP MUST be signaled with no Penultimate Hop Popping (PHP), enabling egress PEs to determine the sender upstream node from the preserved label stack.
A new PMSI Tunnel Type 'Transport Tunnel' (value 8) is defined, with Tunnel Identifier encoded as a tuple <Source PE Address, Local Number>. The Source PE Address is the address of the PE originating the service A-D route; the Local Number is unique to that PE. Both fields have equal length: 4 octets for IPv4 or 16 octets for IPv6.
An inter-area P2MP service LSP is partitioned into three intra-area segments: an ingress area segment rooted at the ingress PE/ASBR, a backbone area segment rooted at the ingress ABR, and one or more egress area segments each rooted at an egress ABR. Each segment is carried over an intra-area P2MP LSP or instantiated using ingress replication.
Two new BGP Extended Community Sub-Types are defined: 'Inter-Area P2MP Next-Hop' as both a Transitive IPv4-Address-Specific and a Transitive IPv6-Address-Specific Extended Community. The Global Administrator field MUST be set to the IP address of the PE, ABR, or ASBR originating or advertising the route; the Local Administrator field MUST be set to 0.
IANA Considerations (Section 15) registers two new BGP Extended Community Sub-Types for 'Inter-Area P2MP Next-Hop' (one each for Transitive IPv4-Address-Specific and Transitive IPv6-Address-Specific Extended Communities), and a new PMSI Tunnel Type value of 8 for 'Transport Tunnel'.
Section 16 covers security considerations for inter-area P2MP segmented LSPs. The use of BGP as the inter-area signaling mechanism means that the security of the overall system depends on the security of the BGP sessions used to distribute Leaf A-D routes and I-PMSI/S-PMSI A-D routes. Standard BGP security mechanisms and the security properties defined in RFC 6514 and RFC 7117 apply.
For global table multicast, when the first Leaf A-D route for a Route Key is received, the PIM implementation MUST set the upstream (S/RP,G) state machine to Joined state. When the last Leaf A-D route for a Route Key is withdrawn, the PIM implementation MUST set the upstream (S/RP,G) state machine to Prune state.
In PIM-SM ASM Option 2, when a PE has a (*,G) TIB entry with an MPLS LSP in the outgoing interface list, receives a Source Active A-D route for (S,G), and does not originate a Leaf A-D route for (S,G), the PE MUST transition the (S,G,rpt) downstream state machine to Prune state after a configurable timer. The Prune state MUST be maintained until one of the three triggering conditions becomes invalid.
The MCAST-VPN NLRI Route Key field for Leaf A-D routes used for global table multicast encodes: RD (8 octets; 0 for S,G or all-ones for *,G), Multicast Source Length (1 octet), Multicast Source (variable), Multicast Group Length (1 octet), Multicast Group (variable), and Ingress PE's IP Address (4 or 16 octets). The full NLRI additionally prepends Route Type=4 (1 octet) and Length (1 octet) and appends Originating Router's IP Address.