Encoding Multipoint LDP (mLDP) Forwarding Equivalence Classes (FECs) in the NLRI of BGP MCAST-VPN Routes
updates
- rfc-6514 — BGP Encodings and Procedures for Multicast in MPLS/BGP IP VPNs
Extracted elements (17)
RFC 6514's encoding of mLDP FECs in MCAST-VPN NLRI required three restrictive conditions: (1) the customer must not use both PIM/IGMP and mLDP simultaneously, (2) only P2MP FEC Elements (not MP2MP) are used, and (3) the Opaque Value must be exactly 32 or 128 bits. This document removes all three restrictions.
The existing RFC 6514 encoding placed the mLDP Root Node in the Multicast Source field and the Opaque Value in the Multicast Group field, but provided no way to distinguish whether the Multicast Group field contained an IP address or an mLDP Opaque Value, breaking mixed PIM/mLDP deployments.
The new mLDP route type codepoints are assigned so that the high-order two bits are 0x01 and the low-order six bits match the corresponding Generic/PIM route type codepoints from RFC 6514. This pairing scheme allows implementations to correlate mLDP and PIM variants of the same route type.
The registry divides the space into a Generic/PIM range (0x01-0x3F), an mLDP range (0x43-0x7F), and a reserved range (0x80-0xFF). Corresponding PIM and mLDP route types share the same low-order 6 bits. When requesting new codepoints, both a Generic/PIM and an mLDP codepoint should be requested simultaneously, even if one will only be marked Reserved.
Per BGP-ERR Section 5.4 (Typed NLRI), if the Route Type field of a received MCAST-VPN NLRI contains an unrecognized value, the route is discarded. This applies to the new mLDP route types when received by implementations that do not yet support them.
The C-multicast Shared Tree Join route and Source Active A-D route are not used for mLDP signaling because mLDP does not have unidirectional shared trees. The Intra-AS and Inter-AS I-PMSI A-D routes do not carry protocol-specific multicast state and are unaffected by this document.
The use of wildcards in S-PMSI A-D route NLRIs (as specified in RFC 6625) is defined only for PIM-based customer multicast. Wildcard support when the customer uses mLDP is explicitly out of scope for this document.
The AFI and mLDP FEC address families are considered to correspond if they are identical, or if BGP AFI is IPv4 and FEC identifies 'Multi-Topology IPv4', or if BGP AFI is IPv6 and FEC identifies 'Multi-Topology IPv6'.
When an mLDP FEC Element is encoded in the NLRI of a BGP Update with SAFI MCAST-VPN, the address family of the Root Node Address (as indicated in the FEC Element) MUST correspond to the address family in the AFI field of that BGP Update.
An mLDP FEC Element (as defined in RFC 6388) contains a FEC Type, a Root Node Address, an Opaque Value, and an address family field identifying the address family of the Root Node Address. The FEC Type distinguishes P2MP LSPs from MP2MP LSPs.
This document defines three new MCAST-VPN route types: C-Multicast Source Tree Join route for C-multicast mLDP (0x47), S-PMSI A-D route for C-multicast mLDP (0x43), and Leaf A-D route for C-multicast mLDP (0x44). These carry mLDP FEC Elements directly in their NLRI instead of Multicast Source/Group fields.
IANA created the 'BGP MCAST-VPN Route Types' registry under the 'Border Gateway Protocol (BGP) Parameters' top-level registry. Allocation policy is Standards Action. Initial assignments include values 0x01-0x07 for Generic/PIM types (RFC 6514), 0x43, 0x44, 0x47 for mLDP types, and 0x80-0xFF reserved.
This document introduces no new security considerations. The security considerations of RFC 6388 (mLDP) and RFC 6514 (MVPN-BGP) are applicable, as this document only specifies a method for encoding mLDP FEC Elements in BGP Update NLRI fields already covered by those specifications.
For a Leaf A-D route for C-multicast mLDP not derived from an S-PMSI A-D, the Route Key field consists of: Route Distinguisher, an mLDP FEC Element, and the IP address of the Ingress PE router (per SEAMLESS-MCAST conventions, with Multicast Source/Group fields replaced).
In the three new C-multicast mLDP route types, the Multicast Source Length, Multicast Source, Multicast Group Length, and Multicast Group fields from RFC 6514 are omitted entirely and replaced by a single mLDP FEC Element as defined in RFC 6388 Section 2.2.
The NLRI of a C-multicast Source Tree Join route for C-multicast mLDP consists of: Route Distinguisher, followed by the Source AS, followed by the mLDP FEC Element.
The NLRI of an S-PMSI A-D route for C-multicast mLDP consists of: Route Distinguisher, followed by the mLDP FEC Element, followed by the Originating Router's IP Address field.