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Pedro sent you a question.

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Thanks Pedro for the question.

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Pedro asked What is the purpose of the loopback interfaces.

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Now if you do a search in google you'll find all kinds of answers including that the loopback interface

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allows you to test connectivity to the 127 0 0 1 IP address.

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Now that may be true on the PC but on a Cisco device a loopback interface has an entirely different

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meaning in the apology.

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I've got three routers Router one wrote two and wrote a three configured with these IP addresses on

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rodded to show IP interface brief shows us that the router has IP addresses configured on the gigabit

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interfaces so gigabit 0 0 0 0 1 and 0 2 have IP addresses.

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Now in this example these are virtual routers but if they were physical routers and a cable got disconnected

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or got cut or broken in some way the interface would go down.

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The difference between a physical interface and a loop back interface is a loopback interface is a logical

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interface on a router and you can create many of these.

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So I notice as soon as I Karaite to that logical interface the interfaces come up.

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I can then give that interface an IP address such as this.

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Notice it's a Slusher 32 IP address.

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In other words this is the only IP address on that interface.

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A loopback interface is a logical interface on a router that won't go down unless you manually shut

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the interface down.

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It typically only has one IP address but you could configure multiple IP addresses on that interface

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if you wanted to.

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The first advantage of the loopback interface is the interface doesn't go down unless you explicitly

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shut it down.

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So it's not like there's a cable that can be unplugged and then interface goes down.

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Now why is that important when you telnet to a router so let's say telnet from wrote a one to Rodda

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to.

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I could telnet to one of the gigabit interfaces on the router so telnet tended wondered wonder to notice

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I can log in.

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So I'm not connected to rodded to go back to route of one.

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And then all telnet to the second IP address 10 1 to 2.

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Now that's fine.

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But firstly you need to remember which IP address is configured on which router.

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And secondly if for some reason an interface went down so let's say the cable was disconnected rather

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than me shutting it down manually here you would have a problem telnetting back to that IP address 10

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1 1 2 because the interface has gone down.

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However if you have the loopback IP address configured on the router and you advertise that loop back

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through a writing protocol such as always P.F. you can telnet to that loopback using this network or

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this network.

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So if one of the interfaces goes down it's not a problem because she can still reach the loopback interface.

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So at the moment on route one we can't ping the loopback of wrote it to because it's not advertised

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in a writing protocol.

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So what I'll do here is enable your GOP

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and I'll do something similar.

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Yes.

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Enable you GOP and all interfaces and let's do the same thing.

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I wrote a three.

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Now once the neighbor relationships have been established which they have done in this example so show

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IP GOP neighbors neighbor relationship rodder one can ping the loop back over it too.

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So now I can telnet to the loopback of Rodda too even though the interfaces don't notice gigabit 00

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is currently down.

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So by the same token if we brought up that gigabit 00 interface but then shut gigabit 0 to

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my telnet session still continues.

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So I notice that telnet session stayed up that would always happen in the real world.

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It depends how long the connection takes to establish but not a show IP interface brief.

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I'm still able to telnet to the loopback of route a two from Router one because the telnet traffic is

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using the alternate route so it doesn't matter if interface's go down or come up as long as I've got

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one route between wrote a one and wrote it.

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I can connect to the loopback and manage the router through the loopback interface.

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It's also a lot easier if you use a separate subnet for your management network.

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In this example I'm just going to keep it simple and create loop packs with IP addresses.

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Quotable one credible two and quadruple three.

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So I'll simply create an IP address on one with the loopback address and I'll do the same on rotas 3

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on one show IP route.

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We have two routes to get to the loop of route.

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Two and two routes to get to the Luebeck of rotas street.

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So paying quotables three telnet quotables three telnet is not working.

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Let's have a look.

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So show run pipe begin Vicci why.

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Notice I've got transport input none line Vicci y 0 for transport input and I'll just say all tell it

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again.

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We can log in to write a three.

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So from a management point of view it's a lot easier to use LOOP banks.

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You might not want to do it the way I've done it in the real world.

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You might have a separate network that you use for management.

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So in this example I'll just use one on two network.

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And actually what I'll do is create a separate loopback.

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So put this back to quadruple T and create a additional loop back for 1 and 2 1 6 8 1 to 2.

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So I could do something like this to make it consistent.

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So use interface loop back to it's always good practice to keep it simple.

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So I use the same loopback number.

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Don't do what I've done here.

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Try to keep them consistent.

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So let's change that.

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No interface.

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Loopback one interface loop back one line to notice however it's very easy to create loopback interfaces

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just create as many as you like

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there are normally some kind of limitation on the router.

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But notice the numbers that we could use here.

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You could use really long numbers if you wanted to but in this example I'll just keep it simple and

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use one on two.

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So show IP ront.

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We've now learnt the roots of the loop backs and now all I need to remember is that when I telnet to

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write it to it's 1 2 1 6 8 1 dot and the router which is door to when I want to tell me it derogate

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3 it's 1 9 2 1 6 8 1 dot Roda 3 so 10 1 and 2 1 6 8 1 2 3.

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And I can log in.

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So that's the first reason Pedros for using loopback interfaces cover a another reason in a separate

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blog.