OSPF Fundamentals
OSPF Fundamentals
This domain tests the base mechanics: how SR OS routers form adjacencies, how areas scope flooding, how the router identity is chosen and how a DR/BDR is elected. Questions here are short but usually turn on one exact prerequisite.
Core rules
- OSPF is link-state: routers flood LSAs, each area keeps an identical LSDB, and Dijkstra runs per area.
- Area 0 is the backbone. Every non-backbone area must connect to it directly or through a virtual link. An ABR has at least one interface in the backbone and one in another area.
- Adjacency prerequisites: same area ID, matching hello/dead intervals, matching authentication, matching stub/NSSA flags, compatible MTU for database exchange, and unique router IDs.
- SR OS selects the router ID in this order: explicit
router-idunder the OSPF instance, then the system interface address, then the highest loopback, then the highest interface address. - On broadcast and NBMA segments the DR is the highest interface priority, then the highest router ID; the BDR is chosen the same way among the rest. Election is non-preemptive.
Worked example
R1 (priority 100) and R2 (priority 1) share a broadcast segment and both reach Full. Later R3 is added with priority 200. R3 does not become DR: OSPF does not preempt, so R3 stays DROther until the existing DR or BDR fails. The trap is assuming the highest priority always wins immediately.
Traps that cost marks
- An area-ID mismatch produces no adjacency at all, not a partial one, because hellos are dropped before the state machine starts.
- Changing the router ID does not take effect in a running instance until OSPF is restarted.
- On a broadcast segment, 2-Way between two DROthers is normal; it is not a fault.
- A stub flag mismatch keeps the neighbor in Init or drops it silently.