Contents

Introduction
Prerequisites
      Requirements
      Components Used
      Conventions
What is an Unnumbered Interface?
IP and IP Unnumbered
Configuration Examples
      Same Major Net, Different Subnets
      Different Major Nets, No Subnets
      Major Net with Subnet, Major Net with No Subnet
      Two Different Major Nets and Their Respective Subnets

Introduction

This document explains the concept of IP unnumbered, and provides several configuration examples for reference. The ip unnumbered configuration command allows you to enable IP processing on a serial interface without assigning it an explicit IP address. The ip unnumbered interface can "borrow" the IP address of another interface already configured on the router, which conserves network and address space.

Prerequisites

Requirements

There are no specific requirements for this document.

Components Used

This document is not restricted to specific software and hardware versions.

Conventions

For more information on document conventions, refer to Cisco Technical Tips Conventions.

What is an Unnumbered Interface?

Consider the network shown below. Router A has a serial interface S0 and an Ethernet interface E0.

Router A's Ethernet 0 interface can be configured with an IP address as shown below:

interface Ethernet0
ip address 172.16.10.254 255.255.255.0

Logically, to enable IP on interface S0, you would need to configure a unique IP address on it. However, it is also possible to enable IP on the Serial interface and bring it up without assigning a unique IP address to it. This is done by borrowing an IP address already configured on one of the router's other interfaces. To do this, the ip unnumbered interface mode command is used as shown below.

interface Serial 0
ip unnumbered Ethernet 0

The ip unnumbered <type> <number> interface mode command borrows the IP address from the specified interface to the interface on which the command has been configured. Use of the ip unnumbered command results in the IP address being shared by two interfaces. Thus, in our example, the IP address which was configured on the Ethernet interface is also assigned to the Serial interface, and both interfaces involved function normally. This can be verified using the output of the show ip interface brief command, as shown below:

RouterA# show ip interface brief 

Interface    IP-Address      OK?    Method   Status   Protocol
Ethernet0    172.16.10.254   YES    manual   up       up
Serial0      172.16.10.254   YES    manual   up       up

As you can see from the output of the show ip interface brief command above, the serial interface has an IP address identical to that of the Ethernet interface, and both interfaces are fully functional. The interface that borrows its address from one of the router's other functional interfaces is called the "unnumbered interface". In our example, Serial 0 is the unnumbered interface.

The only real disadvantage that the unnumbered interface suffers from is that it is unavailable for remote testing and management. You should also remember that the unnumbered interface should borrow its address from an interface that is up and running. If the unnumbered interface is pointing to an interface that is not functional (that is, which does not show "Interface status UP", "Protocol UP"), the unnumbered interface does not work. This is precisely why it is recommended that the unnumbered interface point to a loopback interface since loopbacks do not fail. Finally, remember that the ip unnumbered command works on point-to-point interfaces only. When you configure the command on the Multiaccess interface (that is, Ethernet) or the loopback interface, the following messages are displayed:

RouterA(config)# int e0 
RouterA(config-if)# ip unnumbered serial 0 
Point-to-point (non-multi-access) interfaces only 
RouterA(config-if)# ip unnumbered loopback 0 
Point-to-point (non-multi-access) interfaces only  

IP and IP Unnumbered

On a Cisco router, every interface connecting to a network segment must belong to a unique subnet. Directly connected routers have interfaces connecting to the same network segment and are assigned IP addresses from the same subnet. If a router needs to send data to a network that is not directly connected, it looks in its routing table and forwards the packet to the next directly connected hop toward the destination. If there is no route in the routing table, the router forwards the packet to its gateway of last resort. When a router that is directly connected to the final destination receives the packet, it delivers the packet directly to the end host.

The IP routing table contains either subnet routes or major network routes. Each route has one or more directly attached next hop address(es). Subnet routes are aggregated or summarized by default at major network boundaries in order to reduce the size of the routing table.

Note: The aggregation scheme discussed above assumes a traditional distance-vector routing protocol such as Routing Information Protocol (RIP) or Interior Gateway Routing Protocol (IGRP).

Let's consider assigning IP addresses to the interfaces of a router using a class B network that has been subnetted using eight bits of subnetting. Every interface requires a unique subnet. Although each point-to-point serial connection has only two end points to address, if we assign an entire subnet to each serial interface, we use 254 available addresses for each interface where only two addresses are needed. If we use IP unnumbered on each serial interface, we save address space; the address of a LAN interface is "borrowed" and used as the source address for routing updates and packets sourced from the serial interface. In this way, address space is conserved. IP unnumbered only makes sense for point-to-point links.

A router receiving a routing update installs the source address of the update as the next hop in its routing table. Normally, the next hop is a directly-connected network node. This is no longer the case if we use IP unnumbered because each serial interface "borrows" their IP address from a different LAN interface, each in a different subnet and possibly in a different major network. When IP unnumbered is configured, routes learned through the IP unnumbered interface have the interface as the next hop instead of the source address of the routing update. Thus we avoid an invalid next hop address problem due to the source of the routing update coming from a next hop that is not directly connected.

Configuration Examples

Note: The information in these configuration examples is based on Cisco IOS^® Software version 12.2(10b) and was tested on Cisco 2500 series routers.

Let us look at four different sample configurations for IP unnumbered.

Note: We could have used loopback interfaces instead of ethernet interfaces.

Same Major Net, Different Subnets

Figure 1 shows that on either side of the serial link we have the same major net with different subnets.

Current configuration:

interface Ethernet0
 ip address 171.68.178.196 255.255.255.192 

interface Serial0
 ip unnumbered Ethernet0

router igrp 10
 network 171.68.0.0 

Current configuration:

interface Ethernet 0
 ip address 171.68.179.1 255.255.255.192
 
interface Serial 0
 ip unnumbered Ethernet0 

router igrp 10
 network 171.68.0.0 

Different Major Nets, No Subnets

Figure 2 shows that on either side of the serial link we have different major nets and no subnets.

Current configuration:

interface Ethernet0
 ip address 171.68.178.196 255.255.0.0 

interface Serial0
 ip unnumbered Ethernet0 
 
router igrp 10
 network 171.68.0.0 

Current configuration:

interface Ethernet 0
 ip address 172.68.1.1 255.255.0.0 

interface Serial 0
 ip unnumbered Ethernet0 
 
router igrp 10
 network 172.68.0.0

Major Net with Subnet, Major Net with No Subnet

Figure 3 shows that on one side of the serial link we have a major net with a subnet, and on the other side a major net with no subnet.

Current configuration:

interface Ethernet0
 ip address 171.68.178.196 255.255.255.192

interface Serial0
 ip unnumbered Ethernet0 
 
router igrp 10
 network 171.68.0.0 

Current configuration:

interface Ethernet 0
 ip address 172.68.1.1 255.255.0.0

interface Serial 0
 ip unnumbered Ethernet0 
 
router igrp 10
 network 172.68.0.0

Two Different Major Nets and Their Respective Subnets

Figure 4 shows that on both sides of the serial link we have two different major nets with respective subnets.

Current configuration:

interface Ethernet0
 ip address 171.68.178.196 255.255.255.192 

interface Serial0
 ip unnumbered Ethernet0 
 
router igrp 10
 network 171.68.0.0

Current configuration:

interface Ethernet 0
 ip address 172.68.1.1 255.255.255.192 
 
interface Serial 0
 ip unnumbered Ethernet0 
 
router igrp 10
 network 172.68.0.0