When the Point-of-Local Repair (PLR) node activates the bypass LSP by sending a PATH message to refresh the path state of protected LSP at the Merge-Point (MP) node, it must use an IPv4 tunnel sender address in the sender template object that is different than the one used by the ingress LER in the PATH message. These are the procedures specified in RFC 4090 that are followed in the SR OS implementation.
The router uses the address of the outgoing interface of the bypass LSP as the IPv4 tunnel sender address in the sender template object. This address is different from the system interface address used in the sender template of the protected LSP by the ingress LER and so, there are no conflicts when the ingress LER acts as a PLR.
When the PLR is the ingress LER node and the outgoing interface of the bypass LSP is unnumbered, it is required that the user assigns to the interface a borrowed IP address that is different from the system interface. If not, the bypass LSP does not come up.
In addition, the PLR node includes the IPv4 RSVP_HOP object (C-Type=1) or the IF_ID RSVP_HOP object (C-Type=3) in the PATH message if the outgoing interface of the bypass LSP is numbered or unnumbered respectively.
When the MP node receives the PATH message over the bypass LSP, it creates the merge-point context for the protected LSP and associate it with the existing state if any of the following is satisfied:
Change in C-Type of the RSVP_HOP object
C-Type is IF_ID RSVP_HOP and did not change but IF_ID TLV is different
Change in IPv4 Next/Previous Hop Address in RSVP_HOP object regardless of the C-Type value.
These procedures at the PLR and MP nodes are followed in both link-protect and node-protect FRR. If the MP node is running a pre-Release 11.0 implementation, it rejects the new IF_ID C-Type and drops the PATH over bypass. This results in the protected LSP state expiring at the MP node, which tears down the path. This is the case in general when node-protect FRR is enabled and the MP node does not support unnumbered RSVP interface.