3. Steering Traffic into an SR P2MP Policy

The Replication-SID at the Root node is the Tree-SID of a PTI. It is RECOMMENDED that the Tree-SID also be used as the Replication-SID at intermediate Replication nodes and Leaf nodes, because that simplifies operation and troubleshooting. Other Replication-SIDs MAY be used. For SRv6, the Replication-SID is the FUNCT portion of the SRv6 SID; the LOC portion differs per node.

An SR P2MP Policy has a Binding SID (BSID). A BSID steers traffic toward the Root node when the Root is not the ingress node of the SR domain. The BSID is the last segment in the segment list used to reach the Root. It is RECOMMENDED that the BSID remain constant for the policy lifetime. The BSID MAY be the Tree-SID of the active PTI of the active CP, but in that case it changes when the active CP or PTI changes.

The Root node can steer incoming packets into a policy by:

• Local-policy-based forwarding: map the packet to the active PTI of the active CP using local forwarding policy. Implementations should provide a way to inspect which traffic maps to an SR P2MP Policy and which active CP/PTI is selected.
• Tree-SID-based forwarding: use the incoming BSID, possibly the Tree-SID, to map the packet to the active PTI. The BSID is replaced by the Tree-SID of the active PTI, and the packet is replicated using downstream Replication-SIDs.

For local-policy-based SR-MPLS forwarding, the Root node SHOULD set the encapsulating MPLS header TTL so the replicated packet can reach the furthest Leaf node. For SRv6, RFC 9524 gives guidance for the IPv6 Hop Limit.