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loopback?

what is a loopback interface, and what do we use it for?

thanks.

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Re: loopback?

Hello Ouarti -

A loopback interface is also known as a virtual interface, in that it is always up and is available after it has been configured. Remember that the loopback interface is not related/tied to the address 127.0.0.1 (ping loopback), it is an interface like any other, and can be assigned its own address. 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 addrs 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.

To assign IP address to a loopback interface, this is what you do:

interface loopback 0

ip address 10.10.1.2 255.255.255.255

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 the above explains Loopback Interface for you.

Hope this helps –

Jay.

1 REPLY
Gold

Re: loopback?

Hello Ouarti -

A loopback interface is also known as a virtual interface, in that it is always up and is available after it has been configured. Remember that the loopback interface is not related/tied to the address 127.0.0.1 (ping loopback), it is an interface like any other, and can be assigned its own address. 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 addrs 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.

To assign IP address to a loopback interface, this is what you do:

interface loopback 0

ip address 10.10.1.2 255.255.255.255

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 the above explains Loopback Interface for you.

Hope this helps –

Jay.

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