Loopback interface and address

Unanswered Question
Mar 2nd, 2007

When and why would you configure a loopback interface and address? What IP do you assign to it and do you have to?

I have this problem too.
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Danilo Dy Fri, 03/02/2007 - 04:06

If there is multiple path from source to destination, the loopback interface provides a stable address for protocols (i.e. OSPF, BGP, Telnet) to use so that they can avoid any impact if the physical interface is down. You can configure any IP Address in the loopback interface as long as they are routable but I recommend you use an IP Address depends on their location and security zone (Private or Public). You can also use any subnet mask but I recommend you use /32 subnet mask.

spremkumar Fri, 03/02/2007 - 04:14

Hi Brad

Loopback interfaces are usually created to establish session/neighborship with the remote peers when using routing protocols.

Loopback interfaces are logical interfaces assigned with ip address and doesn't go down untill unless you manually shut the interface.

It will not pass any active traffic as you normal physical or subinterface does..

You can also use loopback interface to have your tunnels established between your remote peers.

Also when using using NAT in your router you can effectively use the loopback interface.


bradlesliect Fri, 03/02/2007 - 05:28


I used a very basic config on a router placed at a client site. They provided the addressing for the interface into their network. Only created a ptp address created to bring up serial interface. I need to rdp to a server on their network.

I just need to route that IP down the interface on my side via static. Spoke to one of the other engineers and we dont need to amend any PIX policies to allow RDP sessions out.


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