R1 and R2 are connected via a serial interface configured as OSPF network type 'point-to-point' on R1's side, but R2's corresponding interface remains at the default OSPF network type for that media. The link never forms an OSPF adjacency beyond the EXSTART state. What is the most likely explanation?
- AR1's interface is passive while R2's is active
- BThe routers have different OSPF process IDs configured
- CThe serial encapsulation types do not match between R1 and R2
- DR1 and R2 have mismatched OSPF network types, causing a DR/BDR election conflict on one side but not the other
Show answer & explanationAnswer & explanation
Correct answer: D. R1 and R2 have mismatched OSPF network types, causing a DR/BDR election conflict on one side but not the other
On point-to-point network type, no DR/BDR election occurs, but on broadcast/NBMA defaults for the same media a DR/BDR election is expected; a mismatch in the configured OSPF network type between the two ends leads to inconsistent handling of DR/BDR requirements, which commonly manifests as adjacency issues that stall in EXSTART or never progress past 2-WAY, since the two sides disagree on whether a DR is needed. This inconsistency is a documented cause of OSPF adjacency failures on serial links.
Why the other options are wrong
- A. OSPF passive-interface would prevent hello packets entirely, resulting in no neighbor relationship at all, not a stall at EXSTART.
- B. Process IDs are locally significant and don't need to match between neighbors.
- C. Encapsulation mismatches typically prevent Layer 2 connectivity entirely, not just OSPF-specific adjacency stalling at EXSTART.
OSPF Network Type Mismatch
OSPF network type (e.g., point-to-point vs broadcast) must match on both ends of a link; mismatches affect whether DR/BDR election occurs and can prevent full adjacency.
- Point-to-point requires no DR/BDR election
- Broadcast/NBMA networks require DR/BDR election
- Mismatched types often cause stuck EXSTART/2-WAY states
Memory trick: Two roads must agree: both point-to-point or both broadcast