background image
Extending IP Addresses
Remember that you can avoid this situation by proper address planning
ahead of time. However, you may find yourself in a situation where you are
dealing with a legacy installation, and you need to overcome this issue of dis-
contiguous networks.
One solution is to turn off route summarization on the routers. To keep
routing protocols such as RIPv2 and EIGRP from automatically summariz-
ing routes, we can explicitly disable route summarization in the Cisco IOS.
Following are examples of IOS configurations, where we are disabling auto-
matic route summarization. As the OSPF chapters will show, OSPF does not
automatically summarize.
To turn off auto-summarization for RIP version 2 routed networks, use
the following router configuration:
router rip
version 2
no auto-summary
To turn off auto-summarization for EIGRP routed networks, use the fol-
lowing router configuration:
router eigrp 100
no auto-summary
Another way to allow discontiguous networks to be interconnected over
a serial link is to use Cisco's IOS feature called IP unnumbered. We'll look
at this next.
IP Unnumbered
With IP unnumbered, a serial interface is not on a separate network, as all
router interfaces tend to be. Instead, the serial port "borrows" an IP address
from another interface. In the following router configuration example, inter-
face Serial 0 is using a borrowed IP address from interface Ethernet 0:
interface serial 0
ip unnumbered ethernet 0
Copyright ©2001 SYBEX , Inc., Alameda, CA