IP for router ID

Dear ,

What is the practical way to define the right IP for Router ID?
Suppose one of my router eth with ip address, can i use this ip as router id ?  and what about loopback ip ?

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Basically, if one does not take special measures a router can be accessed by telnetting to any configured IP address at any interface.
A cisco will chose the highest IP of its active interfaces on boot up as router ID, unless a specific IP is specified. Router ID can serve as a tiebreaker for OSPF designated router elections on a network segment, but it's most practical purpose is to identify a router for management purposes. Larger networks tend to assign a loopback address from a pool of networks used only for loopbacks and designate these as the router ID.  Router ID is usually specific to a particular routing protocol process.

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ikhmerAuthor Commented:
Well, I want to enable ospf protocol in my existing network.
the point is should i assign ip of another subnet just router id or if router-id can be manually assign to be the same ip address of any interface's ip?

e.g: my Router A, ethernet0/1:
and i manually assign as its router id ?

or should i set ( ip of other subnet ) for Router A ID ? in this case i will wast one ip address.
so, what is the most practical way?

Best regards,
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If you create a loopback interface, OSPF will automatically use it for its router ID. This is useful because if you add/change your interface addresses, the loopback won't change and so the router ID won't change. And, it's always up which is handy if you have more than one interface in use- you can always access the router via the loopback address regardless of whether a particular interface is working (of course, ONE of them must be up!).

You should include the loopback address in the routing process because functions like ASBR depend on the reachablility of the router ID to work. Many people take an unused subnet and break it up for loopback addresses only. You can address it with a 32-bit mask (

ikhmerAuthor Commented:
thank you! so, i will need to have another subnet for router id.
Well, that's the easiest way because it doesn't interfere with your current addressing plan.
I manage my loopbacks which are also the router IDs by making them /32s and adding those host routes to the networks OSPF is routing to make them reachable throughout the network.
Yep, that's what I was saying. I've done that everywhere I've ever worked.
ikhmerAuthor Commented:
well, i decide to assign router id manually and did not use loopback . the router id is the ip address of one eth interface of each router. so i don't need to have a separate subnet for just router id.

if you think it is not the appropriate way please give me your comments !
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