Cisco Lab: 01-41 Configuring The Loopback Interface
Before we jump in and configure this whole network our iBGP peers and eBGP peers, we’ve got to learn one more thing and that’s the loopback interface. Loopback interface is a logical interface on your router that can respond on all your interfaces. Let’s say we have a situation where we got our iBGP peer here, we’ve configure it to be neighbors on this IP address which is this interface. Well if this interface goes down then everything is broken so these peers can no longer talk to each other so we’ve got a single point of failure. What we can do is we can use the loopback interface to configure BGP peers. Most of the time were gonna use it to configure iBGP peers but we can also use to it configure eBGP peers if we work with the administrators and the other autonomous system. Let me show you, let’s say we have another link here so we’ve got, we done ways to get from this router to this router and go this way or I can go this way.
Now, if I use the loopback interface, let’s say for this router its 220.127.116.11 and for this router 18.104.22.168 and were actually gonna configure this. If I’m trying to get to 22.214.171.124, my routing protocol that I’m using in here, let’s say EIGRP is gonna tell me the best route to get there. It might say go this way so I gonna go this way and if this route goes down EIGRP is gonna recalculate the route and it’s gonna say ok, this is down, you go this way so now the traffic is gonna automatically be routed this way and since the loopback interface never goes down the TCP session isn’t turn down and you don’t have to rebuild it. So that’s the major _ loopback interface. Let’s go and configure it, here I am on router 1, config T, interface and its loopback and this is where we just give it a number. Now I gonna call loopback 0, hit enter, now we can give it an IP address, let’s give it 126.96.36.199 with a subnet mask of 255.255.255.255, hit enter, exit, exit. Now watch when I ping 188.8.131.52, it works so I’m pinging myself and if you wanna see a Show interfaces you can see exactly what it looks like, its internet 00000 and here’s loopback 0 and its UP and we can see the IP address. Next let’s configure EIGRP because one of the challenges is our routers have to know how to route traffic when were trying to talk to another routers loopback address. I’m gonna turn ON EIGRP, config T, router EIGRP, let’s give an autonomous system of 100 and I’ll type in network 184.108.40.206. Remember, were just telling which interfaces we want to run on and I’m gonna type in No Auto dash (-) summary and I also want to run on network 220.127.116.11. Let’s configure router 2, config T, router EIGRP 100, No Auto dash (-) summary, network 18.104.22.168, were gonna configure the loopback on this also, hit enter, network 22.214.171.124. EIGRP is good to go and as you can see we formed an adjacency there now were gonna go back in and configure a loopback interface. I can just type in loopback interface 0 (LO 0), IP address 126.96.36.199. subnet mask of 255.255.255.255, hit enter.