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Interview Questions — Phase 3.5 · Segment Routing (SR-MPLS & SRv6)

These self-test questions target Google Network Infrastructure Engineer (NIE), FAANG WAN Systems Engineer, and Segment Routing Specialist technical interview scenarios.

SR-MPLS & SID Mechanics

What is the primary architectural advantage of Segment Routing over RSVP-TE and LDP?

Segment Routing shifts state from the network core (P routers) to the packet header at the headend ingress router (PE). Core routers carry zero soft state (no RSVP Refresh messages or signaling timers), eliminating signaling overhead and allowing networks to scale infinitely.

What is the difference between a Prefix SID, a Node SID, and an Adjacency SID?
  • Prefix SID: Global segment representing an IP prefix.
  • Node SID: Special Prefix SID allocated out of the SRGB (16000–23999) representing a specific router's loopback IP. Globally unique domain-wide.
  • Adjacency SID: Local segment representing a specific physical link. Allocated dynamically (24000+) and only locally significant to the originating router.

Ti-LFA & Sub-50ms Fast Reroute

Why does Ti-LFA guarantee 100% protection against link/node failure while classic LFA fails on ring topologies?

Classic LFA (RFC 5286) only evaluates basic neighbor inequalities. On ring topologies, neighbor routers would re-route traffic back across the failed link, creating micro-loops. Ti-LFA (Topology-Independent LFA) calculates exact P-Space and Q-Space node intersections and pushes explicit segment label stacks to force traffic along the pre-computed post-convergence path.

Explain P-Space, Q-Space, and PQ Node in Ti-LFA calculations.
  • P-Space: Set of routers reachable from the source router without traversing the failed link.
  • Q-Space: Set of routers from which the target destination can be reached without traversing the failed link.
  • PQ Node: The intersection node (\text{P} \cap \text{Q}) where traffic can be safely delivered without looping.

SR-PCE & BGP Color Steering

How does BGP Color Extended Community steering work in SR-TE?

The headend router evaluates inbound BGP routes carrying a Color Extended Community (e.g. color:100). If a matching Segment Routing TE policy exists for Color 100 + Endpoint IP, the router automatically installs the BGP route into the SR-TE tunnel forwarding entry, steering traffic into the explicit low-latency or high-bandwidth label stack.