IP Connectivity
The IP Connectivity domain is the largest at 25% and tests the routing table, static routes, single-area OSPFv2, and first-hop redundancy. You must explain why a specific route is chosen, not just configure one.
Routing table and forwarding
show ip route lists connected (C), local (L), static (S), and dynamically learned routes. A router forwards using longest prefix match first; administrative distance breaks ties between sources for the same prefix; metric breaks ties within one protocol. Default AD values: connected 0, static 1, eBGP 20, EIGRP 90, OSPF 110, RIP 120. The default route 0.0.0.0/0 matches when no more specific route exists.
Static routing
ip route <network> <mask> {next-hop | exit-interface} adds a static route. A floating static uses a higher AD, such as 200, as a backup. Summary routes reduce table size but must not create black holes. Recursive lookup resolves the next hop through another route; an exit interface alone can cause proxy ARP issues on multi-access links.
OSPFv2
OSPF is link-state and uses cost = reference bandwidth / interface bandwidth (default reference 100 Mbps). Neighbors must match area, hello/dead timers, MTU, authentication, and stub flags. Broadcast networks elect a DR and BDR; set router-id explicitly. Passive interfaces stop hellos without blocking the subnet advertisement. default-information originate injects a default route.
FHRP
HSRPv2, VRRP, and GLBP provide a virtual gateway. HSRP uses priority, where higher wins, and preemption is off by default; the virtual MAC is 0000.0c9f.fxxx. VRRP is standards-based with a virtual MAC of 0000.5e00.01xx.
Worked example
Routes to 10.1.2.3 are 10.0.0.0/8 via A, 10.1.0.0/16 via B, 10.1.2.0/24 via C, and 0.0.0.0/0 via D. The /24 route via C wins on longest prefix match. AD is irrelevant because the prefixes differ. Only if two sources offered 10.1.2.0/24 would lower AD decide.
Traps
Choosing the lowest AD before longest match; forgetting OSPF MTU or timer mismatches block adjacency; using the wrong cost reference; and assuming HSRP preempts by default.