2.7 Using IP Unnumbered
2.7.2 Configuring IP unnumbered on serial interfaces
The show interface commands from RouterA (see RouterA#show interface serial 1 command output) and RouterC (see RouterC#show interface serial 1 command output) show that IP unnumbered is now in use on both serial interfaces. 

RIP and IGRP Behave the Same

The rules governing the successful configuration of IP unnumbered that were given in the previous section apply to RIP V1 and IGRP equally. When properly configured, both protocols behave in the same manner. All of the material presented in the following scenario, which shows the proper configuration of IP unnumbered using RIP, applies to IGRP as well.

Improper configuration of IP unnumbered causes both RIP V1 and IGRP to lose IP connectivity for the misconfigured subnets. By applying the concepts presented in this scenario, you should be able to understand what is going wrong when you are faced with a network in which IGRP has been configured with an improper use of IP unnumbered.

This section is broken into the following three scenarios:

RIP with IP unnumbered configured properly.

This section shows IP unnumbered working as intended and provides insights into proving that it is working as intended.

RIP with IP unnumbered configured improperly --- a different subnet mask was used.

This section shows IP unnumbered not working as intended because an improper subnet mask has been applied. It also provides some insights into proving that it is not working as intended.

RIP with IP unnumbered configured improperly --- a different subnet mask and a different major net were used.

This section combines the use of different major networks with a misconfiguration caused by using a subnet mask on one of the networks. It shows how IP unnumbered works with different major nets as well as how the use of a subnet masked major net can cause problems in some configurations.