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OSPF is a key subject on your CCNA exam, as properly it must be. OSPF is a widely-utilised WAN protocol, and you require to discover the fundamentals just before moving on to far more difficult configurations. One such detail is the OSPF Router ID, or RID.

The RID is the dotted decimal value by which other OSPF routers will determine a provided OSPF router. There are some exciting defaults for this value, and a command you ought to know to hardcode the RID. You had also greater know what has to come about for this command to take effect, so let's take a more detailed look at the OSPF RID.

In this instance, R1 has an adjacency with R2 and R3 more than the 172.12.123./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. Operating show ip ospf neighbor on R1, we see some unusual values beneath "Neighbor ID", which is one more name for the OSPF RID.

R1#show ip ospf neighbor

Neighbor ID Pri State Dead Time Address Interface

3.3.3.three Full/DROTHER 00:01:57 172.12.123.3 Serial0

2.two.2.two Full/DROTHER 00:01:57 172.12.123.two Serial0

Notice the Neighbor ID of every single remote address is the loopback address. How can that be if theyre 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 routers 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 or not that interface is OSPF-enabled.

BOTTOM LINE: An interface does not have to be operating OSPF to have its IP address utilised as the OSPF RID.

The OSPF RID can be changed, but it demands a restart or to reinitialize the OSPF routing procedure. Use the router-id command to change the default RID of every router as shown, and clear the OSPF method to do so.

R1#conf t

Enter configuration commands, a single per line. Finish with CNTL/Z.

R1(config)#router ospf 1

R1(config-router)#router-id 11.11.11.11

Reload or use "clear ip ospf method" command, for this to take impact

R1#clear ip ospf approach

Reset ALL OSPF processes? [no]: yes

1d05h: %OSPF-five-ADJCHG: Method 1, Nbr three.three.three.3 on Serial0 from 2WAY to

DOWN, Neighbor Down: Interface down or detached

1d05h: %OSPF-five-ADJCHG: Approach 1, Nbr 2.two.2.two 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 impact. You enter the clear ip ospf method command to do this. Notice that when youre asked if you really want to do this, the prompt is no? Thats since all the OSPF adjacencies on this router will be lost and will have to commence the approach once more. Thats OK on a practice rack, not great in a production network. Dont use that one at function.

The OSPF RID is not a complex idea, but the fact that an interface does not have to be OSPF-enabled in order to have its IP address act as the RID takes some obtaining utilized to. And keep in mind - when the router or switch asks you a query and the prompted answer is "no", take one particular step back and make positive you really want to do what you are about to do! cnc waterjet