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Loopback addresses

I have a question regarding loopback addresses.

We currently have a network of over 150 routers and are looking at putting loopback addresses on each of them in order to help manage our network more efficiently. I know how to configure a loopback address but what will the overall effect be on our network. Will it increase our routing table size (we are running OSPF)? Will it cause any problems? I have seen implementations where users use an IP address that is completely outside of their normal operating network address scheme (for example, they use a 10.0.1.x address for the ethernet interface of a router but use 172.25.1.x for the loopback address). Is there any benefit to doing this?

Any help or advice on this loopback configuration would be helpful.

Cisco Employee

Re: Loopback addresses

As far as the impact on your network, this shouldn't be harmful since most service provider do it on flat networks (one area or level, no summarization). Using a separate range of addresses is beneficial only if you want to summarize on all addresses but the loopback addresses. Most SP don't use summarization but if they do they usually leak the loopback addresses anyway since these are use as the BGP next-hop address and leaking them is useful in selecting the best BGP path based on the IGP metric.

Hope this help

Harold Ritter
Sr. Technical Leader
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Re: Loopback addresses

Definitely the routing table entries are going to increase with the addition of loopback interface (once the loopback network is included in OSPF). Now it would be a nice idea to keep a separate network space for loopback networks, for ease of administration. (easy to remember).

You can infact assign, all routers in one single area with loopback addresses from a single subnet (with /32 masks), say, etc.

The ABR for each area, will have all the loopback addresses, for that area, in its routing table. You can configure summarization of these networks, to next higher supernet, to reduce the amount of routing table entries, on routers in other areas.

Configuration of loopbacks can also be helpful in identifying each router using its router-id. (router-id is usually highest ip add or loopback interface ip address). Though newer IOS versions have a "router-id" command to forcefully specify a router-id for the router.

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