This is just copy and paste from some website:

Description: Router ID is the tie-breaker for OSPF path selection. The path selection process uses a variety of metrics to select a route. If all other metrics (accessibility, administrative weight, local preference, etc.) are equal, OSPF determines the router ID using the following priority:

Use the address configured by the ospf router-id command

Use the address of the loopback 0 interface

Use the highest IP address of any interface

If no interface exists, set the router-ID to 0.0.0.0

If no OSPF router ID is explicitly configured, OSPF computes the router-ID based on the items 2, 3, and 4 and restarts OSPF (if the process is enabled and router-ID has changed).

Example: In the following example, the ospf router-id ip-address command configures the router ID as 5.1.1.49, and the show ip ospf command displays the setting:

router(config-router)#ospf router-id 5.1.1.49

router(config-router)#end

Thanks,

Mohamad

Hi Zeeshan,

I understand that your question is what comes after the router-id command?

It should be an IP Address, which you want to designate as the router ID while configuring OSPF.

In such scenarios try to look for Command Reference Guide, where all the detals about the usage of a command is given.

Have a look at this for details on router-id -

http://www.cisco.com/en/US/docs/ios/12_2/iproute/command/reference/1rfospf.html#wp1049279

-> Sushil

I agree with Mohammad. I always put the loopback IP Address (/32).

Hello Sarah,

I guess you are also interested on why we should configure an OSPF router-id:

the main reason is to assure consistency over time.

Without an explicit configuration the OSPF Router-ID can change:

example :

after some time a new loopback address with an higher ip address is added.

no changes is observed until the OSPF process is restarted or the whole router is reloaded.

After that, OSPF at startup picks the highest loop ip address as router id.

This is not desirable: we want to trace a node with a specific ip address.

To be noted that the router-id argument is an ip address but doesn't need to be an ip address associated to a real interface on the router.

Also the router-id could be an ip address that is not advertised in OSPF or other manner (doesn't need to be routed and reachable)

However, one of the best practices is to use a /24 space taken from address plan and to allocate a /32 ip address to each router and to advertise them in OSPF.

Other note: if using BGP and OSPF to have a successful redistribution of OSPF into BGP the two router-id must be the same.

Hope to help

Giuseppe