Enabling OSPF Neighbor Discovery on LANs
OSPF sends multicast OSPF messages, attempting to discover neighbor only when two requirements are met:
- network command or ip ospf area interface
- the interface has not been made passive
When both requirements are met, OSPF sends Hellos to the 224.0.0.5 multicast address. The helllo itself contains several parameters that must be checked, including OSPF RID and OSPF area with bunch of other stuff like hello and dead intervals, subnet, autehntication.
OPSF Timers
OSPF uses two timers to monitor the reachability of neighbors. Hello interval defines how often the router sends a Hello message in the interface.
We can change the interface hello interval
int f0/0
ip ospf hello-interval 9
sh ip ospf interface f0/0
In the end, all the neighbor relationships failed because those routers now have mismatched hello and dead interval. However, check this.
Neighbor Down: Dead timer expired
*Apr 28 00:06:58.495: %OSPF-5-ADJCHG: Process 4, Nbr 2.2.2.2 on FastEthernet0/0 from
FULL to DOWN,
It only failed only after the Dead timers expired.
Designated Routers and Backup Designated Routers
The database identifies routers based on their RID. By design, all OSPF RIDs in a domain should be unique. It is clear that routers with duplicate ID would not form a neighbor relationship. We would choose the DR with the highest router id and then the highest ip address of loobback