Why Loopback an Interface?

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What is the benefit of a loopback interface on a router?



Re: Why Loopback an Interface?
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The main advantage is that it never goes down. Therefore it's very useful
for management, router id's or such things like tunnel anchors.

BL
--
Making a cup of coffee is like making love to a beautiful woman. It's got to
be hot. You've got to take your time. You've got to stir... gently and
firmly. You've got to grind your beans until they squeak. And then you put
in the milk.
- Swiss Tony



Re: Why Loopback an Interface?

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Is it so that a router has an IP address which will not change
irrespective of which subnets its various interfaces are placed on from
time to time?

I think there's more reasons too, but I'd have to check some notes for
them.

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Andrew W. Young                                  news03@andrew-young.com
                                             http://www.andrew-young.com /

Re: Why Loopback an Interface?
As others have mentioned, it's useful in cases where you can't have an
interface go down.  OSPF, for example, behaves very badly when an
interface fails and it has to choose a new router ID.  The BGP process
won't even start if you don't have a loopback interface configured
that it can use as its router ID.

It also can allow you to avoid having multiple adjacencies, because a
router can be reached by a number of paths.  Without a loopback, you
would have to form an adjacency between every single interface on
every single router.  With a loopback, you just make one adjacency,
and it doesn't matter how traffic gets there, as long as it can.

Finally, it can allow you to do load-balancing on a protocol that
doesn't support it, or without the additional configuration that
usually goes along with it.  For example, you have two redundant links
between routers, and you're running BGP.  BGP will not load-balance
unless you create two adjacencies and use the maximum-paths option.
Instead, you can create a single adjacency to the loopback, set two
static routes to that loopback, one out each interface, and voila you
have BGP load-balancing, with one less memory-hogging neighbor
relationship to boot!

There's a bunch of other reasons, but suffice to say, loopback
interfaces==more gooder.


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