Question

A service provider is deploying SR-MPLS across an IS-IS backbone. An engineer configures a Prefix-SID on the loopback of each PE router to identify it uniquely in the domain. Traffic destined to PE5's Node-SID enters at PE1, which has two equal-cost IGP paths toward PE5. How will transit routers forward a packet carrying only PE5's Node-SID as the top label?

Accepted Answer

Each transit router forwards the packet along the IGP shortest path to PE5, load-balancing across all equal-cost paths, swapping the label to the next-hop's outgoing label for that Node-SID.

Answer

The packet is dropped at the first transit router because a Node-SID requires an accompanying Adjacency-SID to specify the outgoing interface.

Answer

The ingress PE must impose a full label stack listing every hop's Node-SID, since transit routers cannot resolve a single Node-SID to a path.

Answer

Transit routers flood the packet out all interfaces because Node-SIDs are treated as broadcast identifiers within the SR domain.