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400 Chapter 11: Remote Access
Now look at Figure 11-10. IP unnumbered has been brought into the picture. Define the BRI
port with the command ip unnumbered Ethernet 0, which uses the IP address of the Ethernet
port for the BRI interface as the source. Because there are unnumbered interfaces, this will work
as soon as the ISDN link is brought up. The routers exchange routing updates, and Router A
puts in the routing table "to get to net 1.1.1.0, go out interface BRI0." Router B installs in its
routing table "to get to network 2.2.2.0, go out interface BRI0."
Figure 11-10
IP Unnumbered
This unnumbered feature adds a huge advantage to backup capabilities. It means that you can
configure the ISDN routers to call different sites. Normally, IP addresses would be assigned to
these interfaces, preventing the ISDN router from calling another router that might be in a
different subnet. The unnumbered feature removes that restriction. In Figure 11-11, Router
Nashville backs up the serial link with ISDN to reach headquarters in Washington, D.C., via the
Atlanta regional office router. If the Atlanta regional router becomes unavailable, when the
Nashville router dials the Atlanta office, there is no response. Using the unnumbered feature,
first configure Nashville to dial into Atlanta. If there is no response, you can configure a second
dial map statement to dial into Dallas. In this situation, it is a good idea to use a routing protocol
that supports discontiguous subnets--preferably EIGRP. OSPF supports discontiguous
subnets, but the configuration of areas could be rather difficult to design and maintain.
NOTE
AppleTalk and IPX do not use the unnumbered feature.
Host Nick
2.2.2.2
Network 2.2.2.0
Router A
2.2.2.1
e/0
S0
Host Erik
1.1.1.2
Network 1.1.1.0
Router B
1.1.1.1
e/0
S0
IP unnumbered E0
Int BRIO
Int BRIO
87200333.book Page 400 Wednesday, August 22, 2001 2:53 PM