loopback question

[rhltechie]

Hi All,

One of the things that puzzles me most and always has is the use of the loopback address on routers. can someone tell me why this interface is needed?

also about the addressing of it, i noticed my loopback here is set to 172.16.255.255 255.255.255.255. How should I address the remote routers in my WAN? should they follow this same trend? I will be using BGP between these remotes and the mpls cloud if that matters.

TIA,

R

[c.lemaire]

Hello,

A loopback interface is always up. So if you have several path to a router and a routing protocol is announcing the loopback, you can always reach the router's loopback even if one path fails.

For the addressing, it's up to you. The IP used doesn't matter (except for summarization or other stuff like this). Usually /32 IP are used since there isn't a "real" network behind the loopback but it isn't mandatory.

If you plan to build a BGP peering between two loopback, be sure the two routers can reach the neighbor's loopback via static routes or other IGP.

Regards,

Christophe

[Richard Burts]

A loopback is a virtual interface. As such it is not subject to some restrictions common to physical interfaces (if you unplug a cable the physical interface will go down, but the loopback will not - unless the cable that you unplugged happened to be the power cable). So a loopback interface tends to be more stable than a physical interface.

Loopback interfaces can be especially helpful when there are multiple paths to/from the router. In this case we can get to the loopback over either path and we are not dependent on a particular physical interface and its addressing.

It can be especially helpful to use the loopback address to represent the router: it simplifies things like syslog so that the log message always has the same source address no matter which interface it went out. The same consideration might be important for aaa/TACACS and a lot of other things.

As far as addressing is concerned: the loopback only needs a single address. So the mask of 255.255.255.255 works just fine. But you could assign a subnet to the loopback if you wanted to.

HTH

Rick

[rhltechie]

Thanks to both of you. I think its clearer now than its ever been!

So in theory, i could use the address of 172.16.254.255 here and 172.16.253.255 at the remote side for loopbacks, correct? as i running bgp between my edge and providers edge, would i need to include this as a routed network in my bgp config?

thanks!