Link-state IGP, metric = cost, Dijkstra (SPF), protocol 89, AD 110, multicast 224.0.0.5 / 224.0.0.6 (DR/BDR). Area 0 is the mandatory backbone.

Router ID & DR election

Router-ID: manual → highest loopback → highest active interface IP. DR election: highest priority (tie: highest RID). Hello/Dead must match between neighbors or no adjacency: 10/40 s on broadcast, 30/120 on NBMA.

LSAs you must know

Type	Name	Originator	Scope
1	Router	every router	area
2	Network	DR	area
3	Summary (inter-area prefix)	ABR	between areas
4	ASBR Summary	ABR	between areas
5	External	ASBR	whole AS
7	NSSA external	NSSA ASBR	NSSA (→5 at ABR)
8/9	Link / Intra-area prefix	OSPFv3 only	IPv6 moves addresses out of types 1/2

Packet types

Hello → DBD → LSR → LSU → LSAck. That’s the adjacency build in order.

Cost & the reference-bandwidth trap

Default reference 100 Mbps → every link ≥100 Mbps costs 1. Fix with auto-cost reference-bandwidth 10000 (on every router) — and per-interface ip ospf cost beats auto-cost.

Config essentials

router ospf 1
 router-id 2.2.2.2
 passive-interface default
 no passive-interface Gi1/0/1
 network 10.10.10.0 0.0.0.3 area 0
 default-information originate