PIM Group-to-Rendezvous-Point Mapping
updates
- rfc-4601 — Protocol Independent Multicast - Sparse Mode (PIM-SM): Protocol Specification (Revised)
Extracted elements (21)
Embedded-RP mappings (RFC 3956) always have the highest priority and cannot be overridden by static configuration or dynamic mechanisms, because RFC 3956 specifies they MUST be treated as the longest possible match and highest priority for ff70::/12.
The BSR hash function is ignored for BIDIR-PIM RP mappings because BIDIR-PIM is designed to avoid data-driven events; RP mapping is determined solely based on group mask from dynamic protocols or static configuration.
The existing RFC 4601 Section 4.7.1 algorithm does not consider the origin of a Group-to-RP mapping or the PIM mode, treating all mappings equally and providing no flexibility to prefer BIDIR-PIM over PIM-SM entries.
The new algorithm supersedes RFC 4601 Section 4.7.1 and the PIM-STD-MIB algorithm to enable management stations to deterministically predict RP selection, fixing vendor inconsistencies in how Group-to-RP mappings are resolved to create multicast forwarding state.
Implementations continue to be manageable via PIM-STD-MIB (RFC 5060); SSM and Dense Mode group ranges are represented in pimGroupMappingTable with pimGroupMappingRPAddressType set to unknown(0) and pimGroupMappingPimMode set to ssm(2) or dm(5).
The pimGroupMappingPrecedence and pimStaticRPPrecedence objects from PIM-STD-MIB (RFC 5060) are not used by the new algorithm; their values are retained for legacy implementations but MUST be ignored by implementations supporting this algorithm.
This document updates RFC 4601 Section 4.7.1; the new algorithm is backward compatible and produces the same result only when Group-to-RP mappings are learned from a single source. Full benefits require widespread deployment.
A Group-to-RP mapping entry with PIM Mode BIDIR is preferred over an entry with PIM Mode PIM-SM, as stipulated in RFC 5059 Section 3.3.
An implementation of PIM SHOULD support configuration to filter specific dynamic mechanisms for a valid group prefix range, such as permitting Auto-RP but filtering BSR for the same administratively scoped range (e.g., 239/8).
An implementation of PIM that supports only one mechanism for learning Group-to-RP mappings MUST also use the algorithm defined in this document.
For addresses in the range ff70::/12, the Embedded-RP mapping MUST be considered the longest possible match and higher priority than any other mechanism, as specified in RFC 3956 Section 7.1.
If the multicast group address is in the SSM range or is configured for Dense Mode, no Group-to-RP mapping is selected and the algorithm terminates; the pimGroupMappingRPAddressType is set to unknown(0) in the MIB.
Interoperation of BSR and Auto-RP within a single administrative domain is OPTIONAL; it is not usually necessary to run both dynamic Group-to-RP mapping mechanisms simultaneously.
It is RECOMMENDED that network operators configure only one PIM-BIDIR RP for each RP Priority, because the BSR hash is ignored for BIDIR-PIM mappings.
When a router receives overlapping Group-to-RP mappings from multiple sources (e.g., Auto-RP and BSR), the algorithm defined in this document MUST be used on all routers in the domain.
When all other tiebreakers are exhausted, the RP with the numerically highest IP address is selected as the final tiebreaker.
When remaining mappings are from BSR, the RP with the lowest Candidate-RP-Advertisement priority value (indicating highest priority per RFC 5059) is selected; if tied, the BSR hash function from RFC 4601 Section 4.7.2 is applied for PIM-SM groups.
The pimGroupMappingTable (RFC 5060) stores all Group-to-RP mapping entries including origin type, PIM mode, group prefix, RP address, and precedence. The new algorithm operates on this table, using origin and mode fields rather than the precedence field.
Different routers may select different Group-to-RP mappings for the same group if their learned mappings are inconsistent (e.g., BSR not enabled on one router), breaking multicast connectivity. Consistent, secure mapping distribution is required.
Secure transport of Group-to-RP mapping protocols can be accomplished by using authentication with IPsec, as described in RFC 4601 Section 6.3, to prevent inconsistent or malicious mappings from being injected.
The RP selection algorithm proceeds through ten ordered steps: (1) Embedded-RP extraction, (2) SSM/Dense Mode termination with no mapping, (3) subset selection by matching group prefix, (4) termination if no entries, (5) longest prefix match, (6) BIDIR preferred over PIM-SM, (7) dynamic preferred over static with BSR preferred over Auto-RP, (8) lowest BSR Candidate-RP-Advertisement priority value wins, (9) BSR hash function for PIM-SM tiebreak, (10) highest RP IP address as final tiebreaker.