OSPF is a major topic on your CCNA exam, as well it should be. OSPF is
a widely-used WAN protocol, and you need to learn the fundamentals
before moving on to more complicated configurations. One such detail is
the OSPF Router ID, or RID.
The RID is the dotted decimal value by which other OSPF routers will
identify a given OSPF router. There are some interesting defaults for
this value, and a command you should know to hardcode the RID. You had
also better know what has to happen for this command to take effect, so
let's take a more detailed look at the OSPF RID.
In this example, R1 has an adjacency with R2 and R3 over the
172.12.123.0/24 frame network. R1 is the hub, with R2 and R3 as the
spokes. No other interfaces are OSPF-enabled on any of the routers.
Running show ip ospf neighbor on R1, we see some unusual values under
"Neighbor ID", which is another name for the OSPF RID.
R1#show ip ospf neighbor
Neighbor ID Pri State Dead Time Address Interface
3.3.3.3 0 FULL/DROTHER 00:01:57 172.12.123.3 Serial0
2.2.2.2 0 FULL/DROTHER 00:01:57 172.12.123.2 Serial0
Notice the Neighbor ID of each remote address is the loopback address. How can that be if they’re not OSPF-enabled?
When determining the Router ID (RID) of an OSPF-enabled router, OSPF
will always use the numerically highest IP address on the router’s
loopback interfaces, regardless of whether that loopback is
OSPF-enabled.
What if there is no loopback? OSPF will then use the numerically
highest IP address of the physical interfaces, regardless of whether
that interface is OSPF-enabled.
BOTTOM LINE: An interface does not have to be running OSPF to have its IP address used as the OSPF RID.
The OSPF RID can be changed, but it requires a restart or to
reinitialize the OSPF routing process. Use the router-id command to
change the default RID of each router as shown, and clear the OSPF
process to do so.
R1#conf t
Enter configuration commands, one per line. End with CNTL/Z.
R1(config)#router ospf 1
R1(config-router)#router-id 11.11.11.11
Reload or use "clear ip ospf process" command, for this to take effect
R1#clear ip ospf process
Reset ALL OSPF processes? [no]: yes
1d05h: %OSPF-5-ADJCHG: Process 1, Nbr 3.3.3.3 on Serial0 from 2WAY to
DOWN, Neighbor Down: Interface down or detached
1d05h: %OSPF-5-ADJCHG: Process 1, Nbr 2.2.2.2 on Serial0 from 2WAY to
DOWN, Neighbor Down: Interface down or detached
After entering the router-id command, the router console informed you
that you have to reload the router or reset the OSPF processes for this
to take effect. You enter the clear ip ospf process command to do this.
Notice that when you’re asked if you really want to do this, the prompt
is “no”? That’s because all the OSPF adjacencies on this router will be
lost and will have to begin the process again. That’s OK on a practice
rack, not good in a production network. Don’t use that one at work.
The OSPF RID is not a complicated concept, but the fact that an
interface doesn't have to be OSPF-enabled in order to have its IP
address act as the RID takes some getting used to. And remember - when
the router or switch asks you a question and the prompted answer is
"no", take one step back and make sure you really want to do what
you're about to do!