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Loopback Interface in Router

Posted on 2011-10-10
Medium Priority
Last Modified: 2012-05-12
What is the importance of Loopback IP address configuration.
I have studued that if physical connectivity gets lost  then we can access Router using Loopback Address, what is the Logic behind that, how we can access with out physical ocnnectivity.


Question by:RAMU CH
LVL 18

Expert Comment

by:Garry Glendown
ID: 36947048
Well, not when there is NO physical connection anymore, but if you have _some_ connection through one of multiple redundant links, the IP is still available.
On normal physical interfaces, if a link goes down, the IP is disabled, so you can no longer reach the router through it. This does not happen with loopbacks, as they are always "up" (unless manually shut down).
LVL 14

Accepted Solution

Otto_N earned 1000 total points
ID: 36947964
To expand a bit on Garry's statement:  A loopback address is a virtual interface.  This virtual interface does not depend on physical line status to have the IP protocol stack initialised, and will therefore be up (pingable from the router) as long as the router is up (and the Loopback hasn't been shut).

This makes it easy for reaching a router with redundant connections:  Since the router is connected redundantly, you do not have a single interface that is guaranteed to be always available when the router is up - Any of the interfaces can go down, while the router is still reachable via others.  So, by introducing a Loopback address, you have a guaranteed interface, a router test-point:  If you can rech the Loopback address, you can reach the router.

Loopback addresses are not only useful for monitoring or managing the router, it also work for configurations like BGP, where you want to maintain a session between routers, even if one of the direct links fail.

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