OSPF Fundamentals and Protocol Operation
Domain 1 - OSPF Fundamentals and Protocol Operation
This domain tests whether you understand OSPF as a link-state protocol: how routers discover each other, exchange topology data, and build a common view of the network. Expect questions on packet types, neighbor states, area structure, and the roles of ABR and ASBR.
Core rules
- OSPF runs directly over IP with protocol number 89; hello packets go to 224.0.0.5 (AllSPFRouters), and DR/BDR listen on 224.0.0.6.
- Five packet types: Hello, Database Description (DBD), Link State Request (LSR), Link State Update (LSU), and Link State Acknowledgment.
- Routers in the same area must agree on hello/dead intervals, area ID, authentication, and stub flags before they become neighbors; only after LSDB exchange do they reach Full.
- The backbone is area 0. Every other area must connect to it, directly or via a virtual link. An ABR has interfaces in two or more areas; an ASBR injects external routes.
- Router ID is chosen explicitly or as the highest loopback/system address; it identifies the router in LSAs, not the adjacency.
Worked example
R1 and R2 sit in area 0 with a 1 Gbps link; R2 connects to R3 over 10 Gbps. SR OS uses a reference bandwidth of 100 Gbps, so cost = reference/bandwidth, minimum 1: R1–R2 = 100, R2–R3 = 10. A prefix behind R3 has intra-area cost 110 from R1. If a second path existed with lower total cost, SPF would install it; equal cost would give ECMP.
Traps
Confusing "neighbor" (Hello seen) with "adjacent" (Full). Assuming every neighbor becomes DR. Forgetting that the router ID is not the interface address used for forwarding. Treating area 0 as optional.