IP routing
This domain covers how SR OS builds and selects routes: IGPs (OSPF, IS-IS), BGP, route preference, ECMP and route policy. The exam expects you to know why one route beats another, how areas/levels constrain topology, and how policy changes what is imported or exported.
Core rules
- Nokia route preference (lower wins): direct 0, static 5, OSPF internal 10, IS-IS L1 15, IS-IS L2 18, RIP 100, OSPF external 150, BGP 170.
- OSPF: areas must touch area 0; inter-area routes are type 3; external routes type 5 (or type 7 in NSSA). Metric is cost; default reference bandwidth needs tuning for high-speed links.
- IS-IS: Level 1 intra-area, Level 2 backbone; L1/L2 routers leak or attach. Wide metrics support large costs.
- BGP best-path order: highest local preference, shortest AS path, lowest origin, lowest MED, eBGP over iBGP, lowest IGP cost to next hop, lowest router ID.
- Route policies use prefix lists and policy statements with explicit default actions; an unhandled route falls to the default action.
Worked example
A prefix is learned from an OSPF external LSA and from iBGP. OSPF external preference is 150, BGP is 170, so the OSPF route is installed - even though the BGP path may be "better" by AS-path length. If the design requires BGP, either change preference or filter the OSPF external.
Traps
- Comparing BGP attributes before checking route preference; preference is evaluated first.
- Forgetting that iBGP requires full mesh or route reflectors, and that next-hop-self is needed on some sessions.
- Assuming OSPF cost automatically reflects link speed; it does not unless the reference bandwidth is changed.
- Route policy without a default action silently accepting routes you meant to drop.