What has to match for two routers to form an OSPF adjacency?
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In the hello: area ID, hello and dead intervals, subnet and mask on broadcast links, authentication type and key, and the stub area flags. Beyond the hello: unique router IDs, matching IP MTU for the DBD exchange, and compatible network types. And the interface must not be passive.
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Checked in the hello (no neighbor at all if wrong)
- Area ID. Both interfaces must be in the same area.
- Hello and dead intervals.
- Subnet and mask. Checked on broadcast and NBMA. Skipped on point-to-point links and virtual links.
- Authentication. Same type (null, plain text, MD5 or SHA) and same key and key ID.
- Stub area flags. Both sides agree on stub or NSSA. A mismatch logs
"mismatched Stub/Transit area option bit" in
debug ip ospf hello.
Checked later
- Router IDs must differ. A duplicate gives
%OSPF-4-DUP_RTRID_NBR. - IP MTU is compared in DBD packets. A mismatch leaves the neighbor stuck in ExStart or Exchange.
- Network type is not in the hello, but broadcast against non-broadcast fails on timers, and broadcast against point-to-point can reach Full while routes stay missing, because the two LSAs describe the link differently and the SPF two-way check fails.
Things that do not have to match
Process ID, interface cost, and router priority.
They will ask next
Does the OSPF process ID have to match?
No. It is locally significant. R1 can run
router ospf 1and R2router ospf 100and they peer fine.Which check is skipped on point-to-point links?
The network mask check. Two routers on a point-to-point link can even use unnumbered interfaces.
Traps
- Saying the process ID must match. It does not.
- Forgetting MTU, because it is not in the hello and the symptom appears later, in ExStart.
How did you do?Book 3.4 Routing