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ISDN Overview 399

NOTE

The trade-off with this feature is that it is a little more difficult to troubleshoot network
problems with the unnumbered feature. You don't have a way to ping or monitor the interface
through SNMP because the interface basically uses another interface's IP address.

Loopback interfaces are popular with IP unnumbered. When using a real interface, such as
Ethernet, for your unnumbered configuration, if the Ethernet interface goes down, you lose the
whole connection between the routers. The loopback interface will always stay up (even when
your Ethernet is down), except when your router is powered off.

TIP

When designing addressing for networks, design your loopback interfaces from the start to fit
into summarization schemes and so on. Then, you can use them for peer connections,
unnumbered interfaces, router IDs, and so on.

IP Unnumbered and ISDN

Normally, when an ISDN call is placed, an address is associated to the BRI port. For example,
in Figure 11-8, Router A calls Router B with ISDN. Address 1.1.1.1 on Router A calls Router
B at address 1.1.1.2. This works fine because they are on the same subnet.

Figure 11-8

ISDN Call

Now take a look at Figure 11-9, where Router A has address 1.1.1.1.

Figure 11-9

ISDN Call: A Second Look

Calling Router B with address 2.2.2.1 on the ISDN interface will not work because the
addresses are on different subnets. Data will not be able to be exchanged between routers.

Router A

1.1.1.2

Router B

1.1.1.1

BRIO

Router A

2.2.2.1

1.1.1.1

BRIO

87200333.book Page 399 Wednesday, August 22, 2001 2:53 PM