IP Routing Protocols (OSPF, OSPFv3, BGP, IS-IS, RIP)
Expect questions that test protocol mechanics and configuration intent rather than rote commands. For OSPF, know area types (stub, totally stubby, NSSA), LSA types, DR/BDR election on broadcast networks, and how virtual links repair a discontiguous backbone. OSPFv3 runs per-link and uses link-local addresses. BGP questions focus on path selection order (weight, local preference, origin, AS-path length, MED, eBGP over iBGP), route reflectors vs. confederations, and the difference between network and import-route. IS-IS uses levels (L1/L2) and NET addresses; know DIS election and default route injection. RIP is simple but traps on split horizon, poison reverse, and metric infinity.
Common traps: assuming OSPF adjacency forms across mismatched hello/dead timers or MTU; forgetting that an NSSA blocks type-5 LSAs but allows type-7; and mis-ordering BGP attributes when weight and local preference conflict. Another trap is believing network in BGP advertises a connected route - it only matches an existing route in the table.
Worked rule of thumb for BGP selection: compare weight first (local to router), then local preference (AS-wide), then prefer locally originated, then shortest AS-path, then lowest origin, then lowest MED, then eBGP over iBGP, then lowest router ID. Memorize this ladder and apply it step by step to any “which path wins” question.
Verification
Use display ospf peer, display bgp routing-table, and display isis peer to confirm adjacencies before chasing route selection.