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Pedro sent me 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 127 001 IP address.

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

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meaning. In this topology

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I've got three routers, router 1, router 2 and router 3 configured with this IP addresses.

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On router 2, show IP interface brief shows us that the router has IP addresses configured on the gigabit

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interfaces. So gigabit 00 01 and 02 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 loopback 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 notice as soon as I create 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 slash 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 a 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 I'll telnet from router 1 to router

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2.

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I could telnet to one of the gigabit interfaces on the router so telnet 10.1.1.2, notice

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I can log in.

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So I'm now connected to router 2, I'll go back to router 1

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and then I'll telnet to the second IP address 10 122.

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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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112 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 loopback

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through a routing protocol such as OSPF, 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 you can still reach the loopback interface.

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So at the moment on router 1, we can't ping the loopback of router 2 because it's not advertised

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in a routing protocol.

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So what I'll do here is enable EIGRP

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and I'll do something similar here

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so

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enable EIGRP in all interfaces and let's do the same thing

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on router 3.

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

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IP EIGRP neighbors, there's our neighbor relationship, router 1 can ping the loopback of router 2.

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So now I can telnet to the loopback of router 2 even though the WAN interface is down, 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 02

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my telnet session still continues.

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So I notice the telnet session stayed up that won't always happen in the real world.

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It depends how long the connection takes to establish but notice show IP interface brief.

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I'm still able to telnet to the loopback of router 2 from router 1 because the telnet traffic is

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using the alternate route. So it doesn't matter if interfaces go down or come up as long as I've got

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one route between router 1 and router 2.

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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 gonna keep it simple and create loopbacks with IP addresses,

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Quadruple 1, Quadruple 2 and Quadruple 3.

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So I'll simply create an IP address on router 1 with the loopback address and I'll do the same on router 3

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on router 1 show IP route.

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We have two routes to get to the loop of router

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2 and two routes to get to the loopback of router 3.

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So ping quadruple 3, telnet quadruple 3, telnet is not working.

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Let's have a look.

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So show run pipe begin VTY.

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Notice I've got transport input none, line VTY 04, transport input and I'll just say all, telnet

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again.

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We can log in to router 3.

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So from a management point of view, it's a lot easier to use loopbacks.

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Now you might not wanna do it the way I've done it here 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 192 network

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

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So I'll put this back to quadruple 2 and create a additional loopback for 192 168 1.2.

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

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So use interface loopback 192 it's always good practice to keep it simple.

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So use the same loopback number.

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Don't do what I've done here.

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Try and keep them consistent.

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So let's change that

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no interface.

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Loopback 1, interface loopback 192. 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 192.

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So show IP route, 

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we've now learnt the routes of the loopbacks and now all I need to remember is that when I telnet to

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router 2 it's 192 168 1 dot and the router which is dot 2. When I wanna telnet to router 3

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it's 192 168 1 dot router 3, so telnet 192 168 1.3

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and I can log in.

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So that's the first reason Pedro for using loopback interfaces. I'll cover another reason in a separate

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blog.