Loopback

Unanswered Question
Jul 18th, 2007

Can someone please explain to me the purpose of using a loopback ip when configuring a router?

Thanks in advance,

SK

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Jon Marshall Wed, 07/18/2007 - 09:42

Hi

Loopbacks can be used for a number of things such as management of the router itself.

The thing about a loopback interface is that unlike a physical interface it cannot go down unless of course the router itself goes down. So you often use loopbacks as the source address of the router in routing protocols, tacacs queries etc. because it is a stable interface.

And you might need to readdress some fo your physcial interfaces from time to time. If you use the loopback for managing the router and this resolves in your DNS you wouldn't not need to update your DNS.

HTH

Jon

Richard Burts Wed, 07/18/2007 - 09:45

SK

There are a number of advantages possible with the use of a loopback interface. The loopback interface is a virtual interface which can be configured with an IP address. One of the advantages is that the loopback interface is not tied to or dependent on a physical interface. So the loopback interface will stay up no matter what happens to physical interfaces (unless someone enters a shutdown command on the loopback interface or the router itself goes down).

One of the practical aspects to consider is that if we have a router and we have 2 data paths through the network to get to it. If we attempt to access the router using one of the interface addresses, and that interface happens to be down, then we can not access the router. But if the router has a loopback interface and if the loopback is advertised over both physical interfaces then we can access by either interface that happens to be up.

Another aspect may be in the configuration of the router to use aaa to authenticate to an ACS server. The ACS server can configure only a single address to identify the router. If the router has 2 interfaces that could be used to get to the ACS server then it becomes ambiguous which address will be the source address of the packet to ACS. This creates a problem because if ACS receives a packet and the source address of the packet is not one of the addresses configured on the server then the server will reject the packet. So configuring a loopback will allow us to use the loopback as the source address of the ACS packet. So no matter which interface is used to send the packet it will always have the expected source address.

Loopback addresses are frequently used as router ID for routing protocols. This is based on the better stability of the loopback interface as compared with physical interfaces.

There are some other things we could talk about but this should be enough to get us started.

HTH

Rick

skhirbash Wed, 07/18/2007 - 10:43

Can i use the same loopback ip address, such as 1.1.1.1 255.255.255.0, on every router that I have, currently 20, or should i use an ip from each differnet subnet ip scope, such as 172.16.50.0 255.255.0.0?

Thanks,

sK

Richard Burts Wed, 07/18/2007 - 10:59

SK

You would not want to use the same IP address on multiple routers because this would create ambiguity about which router was which. One interesting aspect of loopback interfaces is that frequently they are configured with /32 masks (255.255.255.255) so you could take one of your subnets and use it to create addresses for loopback interfaces.

HTH

Rick

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