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Cisco CCNA Certification Exam Tutorial: The OSPF RID

OSPF is a major subject on your CCNA examination, as well it ought to be. OSPF is a extensively-used WAN protocol, and you could learn the fundamentals earlier than transferring on to more sophisticated configurations. One such element is the OSPF Router ID, or RID.

The RID is the dotted decimal worth by which different OSPF routers will establish a given OSPF router. There are some interesting defaults for this value, and a command it is best to know to hardcode the RID. You had also higher know what has to occur for this command to take impact, so let’s take a more detailed look at the OSPF RID.

On this example, R1 has an adjacency with R2 and R3 over the 172.12.123.0/24 body network. R1 is the hub, with R2 and R3 as the spokes. No different interfaces are OSPF-enabled on any of the routers. Running show ip ospf neighbor on R1, we see some uncommon values under “Neighbor ID”, which is another name for the OSPF RID.

R1show ip ospf neighbor

Neighbor ID     Pri   State                 Useless Time   Handle         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

Discover the Neighbor ID of every distant handle 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 at all times use the numerically highest IP address on the router’s loopback interfaces, regardless of whether that loopback is OSPF-enabled.

What if there isn’t a loopback?  OSPF will then use the numerically highest IP address of the physical interfaces, regardless of whether or not that interface is OSPF-enabled.
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BOTTOM LINE:  An interface does not must be working OSPF to have its IP handle used as the OSPF RID.

The OSPF RID will be changed, but it surely requires a restart or to reinitialize the OSPF routing process.  Use the router-id command to change the default RID of each router as proven, and clear the OSPF process to do so.

R1conf t

Enter configuration instructions, one per line.  End with CNTL/Z.

R1(config)router ospf 1

R1(config-router)router-id 11.11.11.eleven

Reload or use “clear ip ospf course of” command, for this to take impact

R1clear ip ospf process

Reset ALL OSPF processes? [no]: yes

1d05h: %OSPF-5-ADJCHG: Process 1, Nbr 3.3.3.three on Serial0 from 2WAY to
DOWN, Neighbor Down: Interface down or indifferent

1d05h: %OSPF-5-ADJCHG: Process 1, Nbr 2.2.2.2 on Serial0 from 2WAY to
DOWN, Neighbor Down: Interface down or detached

After coming into the router-id command, the router console knowledgeable 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 while you’re requested should you really need to do that, the prompt is “no”?  That’s because all the OSPF adjacencies on this router might be lost and must begin the method again.  That’s OK on a follow rack, not good in a production network.  Don’t use that one at work.

The OSPF RID is just not an advanced idea, however the fact that an interface doesn’t must be OSPF-enabled in an effort to have its IP handle act because the RID takes some getting used to. And keep in mind – when the router or switch asks you a question and the prompted answer is “no”, take one step again and ensure you actually need to do what you’re about to do!

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