The ip unnumbered configuration command allows you to enable IP processing on a serial interface without assigning it an explicit IP address. Using ip unnumbered interface, we can “borrow” the IP address of another interface already configured on the router, which conserves network and address space. In other ways, when serial or any other interface is configured as “ IP Unnumbered interface” , it doesn’t need to have an ip address configured on it.
It’s quite easy to configure :
- Just go to the interface and use the highlighted command. Here, Loopback0 shows ip configured on loopback is being used on serial1/0.
To configure ip ununumbered on multiple vlans ,
Switch(config)# interface range vlan 1 – 10
Switch(config-if)# ip unnumbered fastethernet 3/1
But remember it’s for Point to point interfaces only otherwise it will throw you an error.
Advantage and applications
- It’s a simple approach to maximize use of ip address. We can use multiple interface to be unnumbered interface by using same source ip.
- If you’re migrating ISP Core network from ATM or SDH to Ethernet carrier , you can use ip unnumbered point to point links.
- IP unnumbered interface can be used on OSPF virtual links.
Learn more : http://blog.ipexpert.com/2012/05/01/ospf-virtual-links-and-ip-unnumbered-interfaces/
- In some cases, the host IP address is assigned statically. The IP Unnumbered Interfaces feature can learn the static host IP address dynamically.
- For Fastethernet interfaces, you can use the ip unnumbered command only on Ethernet subinterfaces, not on the main interface. To use it on a non-VLAN link, you have to create a subinterface with dot1q native encapsulation.
- As earlier said, ip unnumbered interface can’t be used with multi-access media like Ethernet. It can be used with point to point links.
- If you’re using Ethernet or any other physical interface as source for ip unnumbered interface and that interface with ip address went down. You will find it difficult to troubleshoot the network. You won’t be able to telnet/ssh it as you will lose access. That’s why in the begging of the post, I’ve mentioned to use loopback as source of ip unnumbered interface.
- For IP unnumbered interfaces, the following features are not supported:
- Dynamic routing protocols
- Static arp
- Unnumbered interface and Numbered interface in different VRFs
Reference : http://www.cisco.com/c/en/us/td/docs/switches/lan/catalyst4500/12-2/37sg/configuration/guides/config/unnumber.html