Loopback Interface

what is a loopback interface? can anyone give a detailed description? And in which conditions it is configured?

The loopback is just another interface on the router. You can use it to assign multiple IP addresses to one router. It has all sorts of uses. The loopback is often used in BGP for the routers peer address. If you use the interface address as your peering address and that interface goes down then the route is withdrawn from the route table and all your peering sessions will go down. If you use a loopback then your IGP will just advertise the loopback out another link and BGP will stay up. I've also used it when testing WAN CPE when the local interface has not been activated yet, i.e. you configure the loopback with the LAN subnet to make sure the dynamic routing is working ok. Also good for injecting routes into routing protocols for Lab scenerios.

HTH.

how many loopback interfaces can be configured on a given router?
the loopback interface is configured globally or at any given physical interface?
btw thanks for your reply!!

in a cisco router we can configure loopback addresses from 0-2147483647.These loopback addresses are mainly configured in a lab environment because we cannot buy many interface cards for experimenting.In real time we can use loopback addresses for example if we want a router to always be the DR then we can configure a loopback address on this router with an ip address which is highest than all the ip addresses in that segment.
I hope i have made it clear to you.

The loopback is just another interface on the router. You can use it to assign multiple IP addresses to one router. It has all sorts of uses. The loopback is often used in BGP for the routers peer address. If you use the interface address as your peering address and that interface goes down then the route is withdrawn from the route table and all your peering sessions will go down. If you use a loopback then your IGP will just advertise the loopback out another link and BGP will stay up. I've also used it when testing WAN CPE when the local interface has not been activated yet, i.e. you configure the loopback with the LAN subnet to make sure the dynamic routing is working ok. Also good for injecting routes into routing protocols for Lab scenerios.

HTH.

thanks for the info AAnjum!

as novembre has mentioned of using loopback int in WAN CPE testing, can we use loopback permanently for WAN links as they are quite crucial!

you can't use it as a WAN link - the loopback is it own interface completely seperate from any physical interface. Usually just a /32 host route. Configuration is done at the interface level for the loopback.

You use them to give the router a logical address which is what the most important internet routers (BGP speakers) do.