IP Routing and OSPF
Routing and OSPF fundamentals
Layer 3 forwarding uses connected routes, static routes, and dynamic protocols. An SVI or routed port is required for inter-VLAN routing; the default gateway must point to the correct next hop. Administrative distance ranks sources (connected, static, OSPF), while metric selects the best path within a protocol. OSPF builds adjacencies through Hellos, elects a DR/BDR on broadcast networks, and uses cost = reference bandwidth / interface bandwidth. Areas reduce LSDB size; area 0 is the backbone; ABRs connect areas; ASBRs redistribute. Passive interfaces advertise the subnet without forming adjacencies. Authentication and MTU must match.
Traps: placing an interface in the wrong area; mismatched hello/dead timers or MTU; making an uplink passive and breaking adjacency; forgetting default-information originate; and assuming OSPF cost uses actual link speed without checking reference bandwidth. Also watch DR election priority and neighbor state stuck in EXSTART/EXCHANGE.
Worked example/rule of thumb: With reference bandwidth 100 Mbps, a 10 Mbps interface has cost 10; a 100 Mbps interface has cost 1; a 1 Gbps interface also has cost 1 unless reference bandwidth is raised to 1000 or 10000. Rule: verify neighbors reach FULL, then check area, network type, authentication, and MTU.