IP Connectivity
Reading the routing table
Each entry has a prefix, next hop or exit interface, administrative distance (AD), and metric. The router forwards on longest prefix match first; AD and metric only break ties between identical prefixes. Trap: a /24 learned through OSPF beats a /16 static route for any destination inside the /24 - longest match outranks trustworthiness.
Administrative distance
Connected 0, static 1, eBGP 20, EIGRP 90, OSPF 110, RIP 120, iBGP 200. A floating static route sets a higher AD than the dynamic protocol so it installs only when the primary disappears.
OSPFv2 single area
Neighbors must match area ID, subnet/mask, timers, and authentication; an MTU mismatch can stall database exchange at ExStart. On broadcast networks the DR/BDR uses highest priority (default 1), then highest router ID (highest loopback, else highest active interface IP). Election is not preemptive - a new router with better priority waits for an OSPF restart. Cost = reference bandwidth (100 Mbps by default) ÷ interface bandwidth. Passive interfaces suppress hellos without stopping advertisement; default-information originate injects a default route.
First-hop redundancy
HSRP (Cisco, active/standby, virtual MAC 0000.0c07.acXX, preemption off by default), VRRP (standard, master/backup), GLBP (load sharing).
Rule of thumb
Asked why a route is not installed, compare AD first, metric second. Asked which path a packet takes, run longest prefix match before anything else.