Prefix SID resolution for segment routing mapping server

The following processing rules apply for prefix SID resolution.

• For a specific prefix, IGP selects the SID value among multiple advertised values in the following order:

  1. local intra-area SID owned by this router

  2. prefix SID sub-TLV advertised within an IP Reachability TLV

     If multiple SIDs exist, select the SID corresponding to the destination router or the ABR with the lowest system ID that is reachable using the first next hop of the prefix.

  3. IS-IS SID and label binding TLV from the mapping server

     If multiple SIDs exist, select the following, using the preference rules in draft-ietf-spring-conflict-resolution-05 [sid-conflict-resolution] when applied to the SRMS entries of the conflicting SIDs. The order of these rules is as follows:

     1. smallest range
     2. smallest starting address
     3. smallest algorithm
     4. smallest starting SID
  Note: If an L1L2 router acts as a mapping server and also re-advertises the mapping server prefix SID from other mapping servers, the redistributed mapping server prefix SID is preferred by other routers resolving the prefix, which may result in not selecting the mapping server respecting these rules.

• The selected SID is used with all ECMP next-hops from step 1 toward all destination nodes or ABR nodes which advertised the prefix.

• If duplicate prefix SIDs exist for different prefixes after the above steps, the first SID that is processed is programmed for its corresponding prefix. Subsequent SIDs cause a duplicate SID trap message and are not programmed. The corresponding prefixes are still resolved and programmed normally using IP next-next-hops.