IP routing
IP routing
This domain tests how Nokia SR OS routers build reachability, choose best paths, and carry MPLS transport across IS-IS/OSPF and BGP. Expect questions on route preference, BGP best-path, route policy, segment routing, LDP/RSVP-TE, and IPv6.
Core rules
- SR OS route selection: longest prefix match first; then lowest route preference (direct 0, static 5, OSPF 10, IS-IS 15, BGP 170); then lowest metric; equal results install ECMP.
- BGP compares paths in order: highest local-preference, shortest AS-path, lowest origin, lowest MED (same neighbor AS), eBGP over iBGP, lowest IGP cost to next hop, lowest router ID.
- iBGP does not re-advertise routes learned from iBGP; use route reflectors or full mesh. Next hop is unchanged across iBGP unless next-hop-self is configured.
- Segment routing (SR-IS-IS/SR-OSPF) uses prefix-SIDs; LDP and SR can coexist but require a deliberate transport design.
Worked example
A router has two BGP paths to 10.10.10.0/24. Path A has local-pref 200 and AS-path 65001 65002 65003. Path B has local-pref 100 and AS-path 65001. B is shorter, but local-preference is evaluated first, so A wins. If both had local-pref 100, B would win on AS-path length.
Traps
- Confusing route preference with BGP local-preference; they act at different stages.
- Assuming MED is compared across all paths; by default it is compared only among paths from the same neighboring AS.
- Forgetting that a route reflector adds CLUSTER_LIST and ORIGINATOR_ID, which affect loop detection.