Loopback on cisco switch/router

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Jun 25th, 2010
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Hi,

In my campus, i saw that there is loopback interface on the cisco switch/router. Actually I am not really understand what is loopback interface and why this interface configured on the switch/router. Can anyone help me on this?

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Ganesh Hariharan Fri, 06/25/2010 - 06:08
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Hi,

In my campus, i saw that there is loopback interface on the cisco switch/router. Actually I am not really understand what is loopback interface and why this interface configured on the switch/router. Can anyone help me on this?

If the loopback interface on a router is down, that means the router is unavailable as a whole.In contrast, a physical interface being down does not mean the router itself is out of commission. A router's ethernet port can go down, but the other physical interfaces on that router are still operational. Since a loopback interface is logical, there's nothing physical that can go wrong with it.


Hope to Help !!


Ganesh.H


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Lim Seow Hua Fri, 06/25/2010 - 07:17
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It is a must to configure the loopback interface on every switch/router?

joshiaroon Fri, 06/25/2010 - 07:24
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Its not mandatory to configure loopback on router and switches. For some protocol like Ospf and Bgp, we can assign router ip address by using loopback address.

So its optional for small deployment and can be used for testing in lab environment but when you go for BGP , it helps us for more control, like adding neighbour with loopback.


more http://tcpmag.com/qanda/article.asp?EditorialsID=295

Ganesh Hariharan Fri, 06/25/2010 - 07:28
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OK Generally we used to have loopback for various purpose in network and

see below exaplantaion on usage of loopback inetrface.


A loopback interface is also known as a virtual interface, in that it is

always up and is available after it has been configured. A loopback

interface is often used as a termination address for some routing

protocols, because it never goes down.There is another common use of

loopback address, that is to identify a router, for example ?If you wanted

to identify a particular router on your network to check if that router is

up. You know that the router has e0 interface with IP address 10.10.1.1

and you try to ping this IP but don?t get a response!!

So, is the router down? Well, it?s possible that the router is up and that

the ping reached the router on another interface, but you didn?t get a

response because e0 is down. To find out exactly if the router is up

you?ll have to ping another interface but that interface might be down too

and therefore causing the same scenario to happen.To get around the above

problem, you can create a loopback interface on the router with a unique

address, and then when you try telnet or ping to the configured loopback

interface?s IP you?ll always get a response no matter how the packets

reached the router.


There are other circumstances were the loopback interface might be handy

are the following:

-using ?unnumbered? cmd on serial links mixed with the loopback interface

to eliminate wasted IP address on serial links.

-Various routing protocols such as OSPF/BGP make use of ID, which should

be the link that is always up, therefore the loopback interface is greate

for this purpose.

-Loopback interface is also great to use with Management software, in

that, Management software will test whether the router is alive by pinging

the loopback interface IP address.


Hope to Help !!


Ganesh.H


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