OSPF Neighbor Adjacencies and Network Types
Domain 2 - OSPF Neighbor Adjacencies and Network Types
This domain tests the state machine and the interface settings that decide whether two routers can synchronize databases.
States and timers
Down → Init → 2-Way → ExStart → Exchange → Loading → Full. On broadcast networks, routers that are not DR/BDR stop at 2-Way with each other, which is normal. Hello 10 s / dead 40 s on broadcast and point-to-point; 30/120 on NBMA. Any mismatch in hello, dead, area, authentication, MTU, or stub flag prevents Full.
DR and BDR
On broadcast and NBMA segments, a DR and BDR are elected by interface priority (default 1), then highest router ID. Priority 0 means never DR. The election is not preemptive: a new higher-priority router does not displace the DR until the DR fails.
Network types
- Broadcast: Ethernet with DR/BDR; multicast hellos.
- Point-to-point: no DR/BDR; used on PPP, HDLC, or Ethernet configured as point-to-point.
- Passive: advertises the interface prefix but sends no hellos; used for loops and access links. On SR OS, set the type under config>router>ospf>area>interface with interface-type point-to-point or broadcast, and add passive where required.
Worked example
R1 (priority 1, RID 10.0.0.1), R2 (priority 1, RID 10.0.0.2), R3 (priority 0). R2 becomes DR because its RID is highest among non-zero priorities; R1 becomes BDR; R3 never participates. If R4 with priority 200 joins later, it stays DROther until the DR fails.
Traps
Blaming area mismatch when the real problem is MTU (stuck in ExStart/Exchange). Expecting immediate preemption. Forgetting that point-to-point links skip DR election. Leaving an access interface active instead of passive.