Cisco CCNA (200-301)IP ConnectivityMedium
On a multi-access OSPF segment, R3 shows a neighbor state of '2WAY/DROTHER' for R4. Both routers are non-DR/BDR neighbors on the same broadcast network. What does this indicate about the network?
- AThis is the expected steady-state behavior; DROTHER routers do not form full adjacencies with each other
- BThe OSPF process has failed to elect a DR and BDR on this segment
- CThere is an MTU mismatch preventing full adjacency between R3 and R4
- DR3 and R4 have an area mismatch and will never form an adjacency
Show answer & explanationAnswer & explanation
Correct answer: A. This is the expected steady-state behavior; DROTHER routers do not form full adjacencies with each other
On broadcast multi-access networks, OSPF routers that are neither DR nor BDR (DROTHER) only reach the 2-WAY state with each other to reduce LSA flooding overhead; they form FULL adjacencies only with the DR and BDR. This is normal, expected behavior, not an error.
Why the other options are wrong
- B. A DR/BDR must already exist for DROTHER states to appear in the output.
- C. MTU mismatches typically cause routers to stall in EXSTART/EXCHANGE, not 2WAY.
- D. Area mismatches prevent even 2-WAY state from forming; here they've already reached 2-WAY.
OSPF DROTHER 2-WAY Behavior
On broadcast/NBMA multi-access segments, non-DR/BDR routers (DROTHERs) remain in 2-WAY state with each other while forming FULL adjacencies only with the DR and BDR.
- Reduces flooding overhead on multi-access networks
- FULL state only required with DR/BDR, not all neighbors
- Point-to-point links always go to FULL state, no DR/BDR needed
Memory trick: DROTHERs wave hello but only hug the DR