ip unnumbered

ip unnumbered

To enable IP processing on a serial interface without assigning an explicit IP address to the interface, use the ip unnumbered interface configuration command. To disable the IP processing on the interface, use the no form of this command.

ip unnumbered type number
no ip unnumbered type number

Syntax Description

type number

Type and number of another interface on which the router has an assigned IP address. It cannot be another unnumbered interface.

Default

Disabled

Command Mode

Interface configuration

Usage Guidelines

Whenever the unnumbered interface generates a packet (for example, for a routing update), it uses the address of the specified interface as the source address of the IP packet. It also uses the address of the specified interface in determining which routing processes are sending updates over the unnumbered interface. Restrictions include the following:

• Serial interfaces using HDLC, PPP, LAPB, and Frame Relay encapsulations, as well as SLIP and tunnel interfaces can be unnumbered. It is not possible to use this interface configuration command with X.25 or SMDS interfaces.

• You cannot use the ping EXEC command to determine whether the interface is up, because the interface has no address. Simple Network Management Protocol (SNMP) can be used to remotely monitor interface status.

• You cannot netboot a runnable image over an unnumbered serial interface.

• You cannot support IP security options on an unnumbered interface.

The interface you specify by the type and number arguments must be enabled (listed as "up" in the show interfaces command display).

If you are configuring IS-IS across a serial line, you should configure the serial interfaces as unnumbered. This allows you to conform with RFC 1195, which states that IP addresses are not required on each interface.

Note    Using an unnumbered serial line between different major networks (majornets) requires special care. If at each end of the link there are different majornets assigned to the interfaces you specified as unnumbered, then any routing protocol running across the serial line must not advertise subnet information.

Example

In the following example, the first serial interface is given Ethernet 0's address:

     interface ethernet 0

     ip address 131.108.6.6 255.255.255.0

     !

     interface serial 0

     ip unnumbered ethernet 0