PIM Multi-Topology ID (MT-ID) Join Attribute
Extracted elements (20)
PIM MT-ID extends PIM signaling to identify the topology used for RPF when constructing multicast distribution trees, enabling disjoint multicast paths for resilience. For example, two source trees (S1,G1) and (S1,G2) can be built using separate network segments so a single failure does not interrupt the stream.
The MT-ID naming spaces for PIM and IGPs are intentionally independent: a PIM RPF topology's MT-ID need not match the IGP topology ID that contributes routes to it. This allows flexible aggregation (e.g., a PIM topology may draw from MBGP plus one IGP topology), though alignment is recommended when only one IGP is in use to simplify operations.
A router that supports RFC 5384 but not this document will skip the PIM MT-ID Join Attribute and proceed to the next attribute without altering its RPF decision. A router that supports neither RFC 5384 will ignore the entire Join/Assert message. A router supporting both but administratively suppressing the options SHOULD ignore Join/Assert messages containing the PIM MT-ID attribute.
A downstream router seeing conflicting PIM MT-ID attributes in Join packets MUST follow RFC 4601 as if the attribute did not exist (e.g., suppress its own Join) and MUST NOT use RFC 5384 rules to select among downstream MT-IDs. If the Assert winner uses a different MT-ID, the downstream router SHOULD choose the winner as RPF neighbor but MUST NOT encode PIM MT-ID in Joins sent to it.
A router SHOULD NOT include the PIM MT-ID attribute in Join/Assert packets if PIM MT-ID is 0, if the upstream router does not advertise the PIM Join Attribute or PIM MT-ID Hello option, or if the source is pruned (the attribute MUST be ignored for a pruned source).
A router that supports this document MUST include the PIM MT-ID Hello Option in its PIM Hello packets and MUST include both the Join Attribute Option (RFC 5384) and the PIM MT-ID Option.
An upstream router receiving conflicting PIM MT-ID attributes from Join packets MUST follow RFC 5384 to select one; it MAY ignore incoming Join packets altogether to minimize transient forwarding loops. For Assert packets, the RFC 4601 tiebreaker rules apply and PIM MT-ID is not considered in selecting the Assert winner.
The MT-ID value 0 is reserved as the default and MUST NOT be included in the Join Attribute encoding.
The PIM MT-ID value MUST be unique and consistent within the network for the same topology; for example, PIM 500 must refer to the same topology on all routers that use it.
Upon receiving a PIM MT-ID Join Attribute, a router MUST validate the encoding, apply conflict resolution rules to determine which MT-ID to use, and then use the identified topology for RPF unless overridden by local configuration. Local configuration always takes precedence.
Validity checks on a received PIM MT-ID: only the last of multiple MT-ID attributes is accepted; if the Length field is not 2 the rest of the Join message including the current (S,G)/(*,G) entry MUST be ignored; if the Value is 0 the attribute is ignored but processing continues.
When assigning a PIM MT-ID, if only one unicast routing protocol contributes routes to a topology, the PIM MT-ID is RECOMMENDED to be assigned the same value as the IGP topology identifier to reduce management overhead.
When comparing two PIM MT-IDs during conflict resolution, only the 12-bit Value field is compared; the four reserved bits and other TLV fields MUST NOT be used in the comparison.
A PIM MT-ID is a numerical identifier (1–4095) assigned per PIM domain to uniquely identify a PIM RPF topology. It is used only in the PIM control plane for RPF lookup selection and does not function as a forwarding-plane differentiator; forwarding differentiation is achieved via group and/or source addresses.
The PIM MT-ID attribute applies only to PIM Join/Assert packets used by PIM SM, SSM, and Bidir. It is not used in any other PIM packets and can only build shared or source trees for those modes.
IANA assigned PIM Hello Option type value 30 for the PIM MT-ID Hello Option in the 'PIM-Hello Options' sub-registry under 'Protocol Independent Multicast (PIM) Parameters', with OptionLength=0, allocated via First Come First Served policy.
IANA assigned value 2 for the PIM MT-ID Join Attribute in the 'PIM Join Attribute Types' sub-registry under 'Protocol Independent Multicast (PIM) Parameters'.
The security of the PIM MT-ID Join Attribute is only as strong as the security of the PIM packet carrying it; no independent authentication is provided. Malicious alteration of the PIM MT-ID Hello Option or the MT-ID value in a packet could cause PIM resiliency goals to be violated, so the security considerations of RFC 4601 and RFC 5384 apply.
The PIM MT-ID Hello Option is a 4-byte TLV: OptionType=30 (16 bits), OptionLength=0 (16 bits), with no value payload.
The PIM MT-ID Join Attribute TLV is 4 bytes: F-bit=0 (non-transitive), E-bit per RFC 5384, Attr Type=2, Length=2, 4 reserved bits set to zero, and a 12-bit Value field carrying the MT-ID (range 1–4095).