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OSPF is a major topic on your CCNA exam, as nicely it need to be. OSPF is a broadly-utilised WAN protocol, and you need to have to discover the fundamentals before moving on to far more complicated configurations. One particular such detail is the OSPF Router ID, or RID.
The RID is the dotted decimal worth by which other OSPF routers will determine a offered OSPF router. There are some fascinating defaults for this worth, and a command you must know to hardcode the RID. You had also better know what has to happen for this command to take impact, so let's take a much 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
three.three.3.3 Complete/DROTHER 00:01:57 172.12.123.three Serial0
two.2.2.2 Full/DROTHER 00:01:57 172.12.123.2 Serial0
Notice the Neighbor ID of every single remote address is the loopback address. How can that be if theyre not OSPF-enabled?
When figuring out the Router ID (RID) of an OSPF-enabled router, OSPF will constantly use the numerically highest IP address on the routers loopback interfaces, regardless of no matter 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 regardless of whether that interface is OSPF-enabled.
BOTTOM LINE: An interface does not have to be running OSPF to have its IP address utilized as the OSPF RID.
The OSPF RID can be changed, but it calls for a restart or to reinitialize the OSPF routing process. Use the router-id command to change the default RID of every 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 approach" command, for this to take impact
R1#clear ip ospf method
Reset ALL OSPF processes? [no]: yes
1d05h: %OSPF-5-ADJCHG: Process 1, Nbr three.3.3.three on Serial0 from 2WAY to
DOWN, Neighbor Down: Interface down or detached
1d05h: %OSPF-5-ADJCHG: Approach 1, Nbr two.two.two.two on Serial0 from 2WAY to
DOWN, Neighbor Down: Interface down or detached
After getting into 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 youre asked if you genuinely want to do this, the prompt is no? Thats because all the OSPF adjacencies on this router will be lost and will have to start the method once again. Thats OK on a practice rack, not good in a production network. Dont use that one at operate.
The OSPF RID is not a difficult notion, but the reality that an interface doesn't have to be OSPF-enabled in order to have its IP address act as the RID takes some getting utilised to. And bear in mind - when the router or switch asks you a query and the prompted answer is "no", take 1 step back and make sure you really want to do what you are about to do! cnc waterjet
