IP Connectivity
IP Connectivity
IP Connectivity covers how a router chooses a path: routing table contents, static and default routes, administrative distance, single-area OSPFv2, and first-hop redundancy protocols.
Core rules
- Longest prefix match wins before any other comparison. If two sources offer the same prefix, lowest administrative distance wins: connected 0, static 1, eBGP 20, EIGRP 90, OSPF 110, RIP 120, iBGP 200.
- Static syntax: ip route network mask {next-hop | exit-interface} [distance]. A floating static uses a distance above the dynamic protocol so it installs only when the primary fails.
- OSPF cost = reference bandwidth (default 100 Mbps) ÷ interface bandwidth, minimum 1. Gigabit and faster all cost 1 unless auto-cost reference-bandwidth is raised consistently on every router.
- OSPF neighbors must match area, hello/dead timers, subnet, MTU, and authentication, and must be reachable in the same broadcast domain or by unicast.
- FHRP: HSRP and VRRP elect the highest priority (default 100), then highest IP. Preempt must be enabled to reclaim the active role after recovery.
Worked example
Routes: 10.1.0.0/16 via 192.0.2.1; 10.1.5.0/24 via 192.0.2.2; 0.0.0.0/0 via 192.0.2.3. A packet to 10.1.5.9 matches all three, but /24 is longest, so it goes to 192.0.2.2. A packet to 10.1.9.9 matches /16 and the default, so /16 wins.
Traps
Comparing metrics across protocols when AD decides; forgetting a connected route beats any static; typing a subnet mask where OSPF expects a wildcard; and setting passive-interface on a link that must form an adjacency.