Question

A service provider is deploying SR-MPLS across its IS-IS backbone. All routers are Cisco IOS XR except one legacy node, R5, which was manually configured with an SRGB of 20000-28000, while every other router uses the default SRGB of 16000-23999. R5 advertises Prefix-SID index 100 for its loopback. A neighboring router, R4 (SRGB 16000-23999), needs to forward traffic toward R5's loopback. Which outgoing label will R4 impose, and why?

Accepted Answer

R4 imposes label 20100, because the outgoing label toward a next hop is derived from that next hop's SRGB base plus the advertised Prefix-SID index.

Answer

R4 imposes label 16100, because each router computes the outgoing label using the remote node's advertised index added to the remote node's SRGB base.

Answer

R4 imposes label 16100, because the outgoing label is always calculated from the local SRGB base plus the advertised index regardless of the neighbor's block.

Answer

R4 drops the traffic, because a mismatched SRGB between adjacent routers prevents any Prefix-SID label programming from occurring.