Cisco CCNA Exam Tutorial: Loopback Interfaces

As a CCNA candidate, you most likely have some background in PC hardware and workstation support. If so, you're already familiar with loopback interfaces, particularly 127.0.0.1, the loopback address assigned to a PC.

When you're learning all about the different physical interfaces for your CCNA exam - serial, ethernet, and BRI, among others - there's one logical interface you need to know about, and that is - you guessed it! - the loopback interface.

What isn't as immediately apparent is why we use loopback interfaces on routers and switches to begin with. Many of the Cisco router features that can use loopbacks are intermediate and advanced features that you'll learn about in your CCNP and CCIE studies, but these features all come back to one basic concept: If the loopback interface on a router is down, that means the router is unavailable as a whole.

In contrast, a physical interface being down does not mean the router itself is out of commission. A router's ethernet port can go down, but the other physical interfaces on that router are still operational. Since a loopback interface is logical, there's nothing physical that can go wrong with it.

As I mentioned, you'll learn different Cisco router and switch features that utilize loopback interfaces as you climb the Cisco certification ladder. There's one misconception about Cisco loopback interfaces that you want to get clear on now, though. You’re probably familiar with loopback interfaces on a PC, and may even know that the address range 127.0.0.0 is reserved for loopback addressing.

Note that this reserved address range does not apply to loopbacks on Cisco devices, however. If you attempt to assign an address from this range to a Cisco loopback interface, you get this result:

R1#conf t

Enter configuration commands, one per line. End with CNTL/Z.

R1(config)#interface loopback0

R1(config-if)#ip address 127.0.0.2 255.255.255.0

Not a valid host address - 127.0.0.2

R1(config-if)#ip address 127.1.1.1 255.255.255.0

Not a valid host address - 127.1.1.1

The range 127.0.0.0 is reserved for host loopbacks (such as PCs), not routers or switches. The most commonly used address from this range is 127.0.0.1 – if you can’t ping that on a workstation, that means you can’t ping yourself, which means there’s a problem with the TCP/IP install itself.

Keep these details in mind on the exam and in the workplace, and you’re on your way to CCNA exam success!

Cisco CCNA / CCNP Certification Exam Lab: Frame Relay Subinterfaces And Split Horizon

Earning your Cisco CCNA and CCNP is a tough proposition, and part of that is the fact that you quickly learn that there’s usually more than one way to do things with Cisco routers – and while that’s generally a good thing, you better know the ins and outs of all options when it comes to test day and working on production networks. Working with Frame Relay subinterfaces and split horizon is just one such situation.

One reason for the use of subinterfaces is to circumvent the rule of split horizon. You recall from your CCNA studies that split horizon dictates that a route cannot be advertised out the same interface upon which it was learned in the first place. In the following example, R1 is the hub and R2 and R3 are the spokes. All three routers are using their physical interfaces for frame relay connectivity, and they are also running RIPv2 172.12.123.0 /24. Each router is also advertising a loopback interface, using the router number for each octet.

R1(config)#int s0

R1(config-if)#ip address 172.12.123.1 255.255.255.0

R1(config-if)#no frame inverse

R1(config-if)#frame map ip 172.12.123.2 122 broadcast

R1(config-if)#frame map ip 172.12.123.3 123 broadcast

R1(config-if)#no shut

R2(config)#int s0

R2(config-if)#encap frame

R2(config-if)#no frame inver

R2(config-if)#frame map ip 172.12.123.1 221 broadcast

R2(config-if)#frame map ip 172.12.123.3 221 broadcast

R2(config-if)#ip address 172.12.123.2 255.255.255.0

R3(config)#int s0

R3(config-if)#encap frame

R3(config-if)#no frame inver

R3(config-if)#frame map ip 172.12.123.1 321 broadcast

R3(config-if)#frame map ip 172.12.123.2 321 broadcast

R3(config-if)#ip address 172.12.123.3 255.255.255.0


R1#show ip route rip

2.0.0.0/32 is subnetted, 1 subnets

R 2.2.2.2 [120/1] via 172.12.123.2, 00:00:20, Serial0

3.0.0.0/32 is subnetted, 1 subnets

R 3.3.3.3 [120/1] via 172.12.123.3, 00:00:22, Serial0

R2#show ip route rip

1.0.0.0/32 is subnetted, 1 subnets

R 1.1.1.1 [120/1] via 172.12.123.1, 00:00:06, Serial0

R3#show ip route rip

1.0.0.0/32 is subnetted, 1 subnets

R 1.1.1.1 [120/1] via 172.12.123.1, 00:00:04, Serial0

The hub router R1 has a route to both loopbacks, but neither spoke has a route to the other spoke's loopback. That's because split horizon prevents R1 from advertising a network via Serial0 if the route was learned on Serial0 to begin with.

We've got two options here, one of which is to disable spilt horizon on the interface. While doing so will have the desired effect in our little network, disabling split horizon is not a good idea and should be avoided whenever possible. We’re not going to do it in this lab, but here is the syntax to do so:

R1(config)#interface serial0

R1(config-if)#no ip split-horizon

A better solution is to configure subinterfaces on R1. The IP addressing will have to be revisited, but that's no problem here. R1 and R2 will use 172.12.123.0 /24 to communicate, while R1 and R3 will use 172.12.13.0 /24. R3's serial0 interface will need to be renumbered, so let's look at all three router configurations:

R1(config)#interface serial0

R1(config-if)#encap frame

R1(config-if)#no frame inverse-arp

R1(config-if)#no ip address


R1(config-if)#interface serial0.12 multipoint

R1(config-subif)#ip address 172.12.123.1 255.255.255.0

R1(config-subif)#frame map ip 172.12.123.2 122 broadcast


R1(config-subif)#interface serial0.31 point-to-point

R1(config-subif)#ip address 172.12.13.1 255.255.255.0

R1(config-subif)#frame interface-dlci 123


R2(config)#int s0

R2(config-if)#ip address 172.12.123.2 255.255.255.0

R2(config-if)#encap frame

R2(config-if)#frame map ip 172.12.13.3 221 broadcast

R2(config-if)#frame map ip 172.12.123.1 221 broadcast

R3(config)#int s0

R3(config-if)#ip address 172.12.13.3 255.255.255.0

R3(config-if)#encap frame

R3(config-if)#frame map ip 172.12.13.1 321 broadcast

R3(config-if)#frame map ip 172.12.123.2 321 broadcast

A frame map statement always names the REMOTE IP address and the LOCAL DLCI. Don't forget the broadcast option!

Show frame map shows us that all the static mappings on R1 are up and running. Note the "static" output, which indicates these mappings are a result of using the frame map command. Pings are not shown, but all three routers can ping each other at this point.

R1#show frame map

Serial0 (up): ip 172.12.123.2 dlci 122(0x7A,0x1CA0), static,

broadcast, CISCO, status defined, active

Serial0 (up): ip 172.12.13.3 dlci 123(0x7B,0x1CB0), static,

broadcast, CISCO, status defined, active

After the 172.12.13.0 /24 network is added to R1 and R3’s RIP configuration, R2 and R3 now have each other's loopback network in their RIP routing tables.

R2#show ip route rip

1.0.0.0/32 is subnetted, 1 subnets

R 1.1.1.1 [120/1] via 172.12.123.1, 00:00:20, Serial0


3.0.0.0/32 is subnetted, 1 subnets

R 3.3.3.3 [120/1] via 172.12.123.1, 00:00:22, Serial0

R3#show ip route rip

1.0.0.0/32 is subnetted, 1 subnets

R 1.1.1.1 [120/1] via 172.12.13.1, 00:00:20, Serial0

2.0.0.0/32 is subnetted, 1 subnets

R 2.2.2.2 [120/1] via 172.12.13.1, 00:00:22, Serial0

While turning split horizon off is one way to achieve total IP connectivity, doing so can have other unintended results. The use of subinterfaces is a more effective way of allowing the spokes to see the hub's loopback network.