Loop back interface

walaboom

Hello guys, just have a question.
Loopback interface

Is this topic covered in the CCNA exam? It is not covered by the CCNA ICND book by Wendell Odom, however I do see quite a few questions regarding to loopback interface. Can someone briefly explain what is a loopback interface, what is its purpose? Thanks a lot.

Drakonblayde

It's an interface which doesn't physically exist. You can assign it an ip address, ping it, whatever. It's usually used in OSPF to set a static router ID (the ID is taken of the highest IP of the physical interfaces or the highest IP of the loopbacks, the loopbacks override the physical interfaces... the reason for this is it makes the RID stable since interfaces can go up and down, whereas a loopback interface will be up for as long as the router has power)

fusebox

The loopback address by default is 127.0.0.1 and is used for mainly diagnostic purposes eg: when testing if the TCP/IP stack is working on the machine/router.

Also when using OSPF, the router ID is determined by what loopback interface you have configured. For example, on a router the RID (Router ID) will chosen to be the one which has the highest IP address of all active physical interfaces, but if you configure the loopback address then this will become the RID. The loopback interface needs to be configured so that the RID is used to advertise the routes as well as elect the designated router (DR) and Backup Designated Route....

plus read what Drakonblayde said

Drakonblayde

fusebox wrote:

The loopback address by default is 127.0.0.1 and is used for mainly diagnostic purposes eg: when testing if the TCP/IP stack is working on the machine/router.

Well, be careful with that. The 127.0.0.1 doesn't apply to Cisco routers... that's more of a host based thing, and it represents the stack, not an interface. On a Cisco router (or at least, on the 1720 I just booted up to test it on) you cannot ping 127.0.0.1, you can't even assign that ip to a loopback interface...... so the loopback interface and the loopback address are two seperate ballgames.

Anyway, in a nutshell, yeah, a loopback interface is a fake interface that the router can emulate and treat like a legit physical interface. It has it's uses, which I'll leave you to discover in the course of your studies

fusebox

Drakonblayde wrote:

Well, be careful with that. The 127.0.0.1 doesn't apply to Cisco routers... that's more of a host based thing, and it represents the stack, not an interface. On a Cisco router (or at least, on the 1720 I just booted up to test it on) you cannot ping 127.0.0.1, you can't even assign that ip to a loopback interface...... so the loopback interface and the loopback address are two seperate ballgames.

Anyway, in a nutshell, yeah, a loopback interface is a fake interface that the router can emulate and treat like a legit physical interface. It has it's uses, which I'll leave you to discover in the course of your studies

Sorry yes you are right, I should have mentioned the differences between a loopback address of 127.0.0.1 and what the original question was ie: loopback interfaces.

walaboom

Thank you guys so much!!