
Dial Backup Implementation Notes
114062 Rev. B 6-3
Sample Network Using Unnumbered Interfaces
Figure 6-1 shows a sample network using IP unnumbered interfaces and dial
backup service. Router A, the remote router, connects to Router B, the regional
router, via a primary leased line. Router B connects to Router C, the central router,
via a regular leased line, that is, a leased line without dial backup service.
Figure 6-1. Dialing an Alternative Router Using IP Unnumbered Interfaces
If the primary connection between Routers A and B goes down, the backup circuit
takes over, adopting the configuration of the primary circuit. The backup circuit
tries to call Router B, but it cannot establish a connection.
Because you configure unnumbered IP addresses on the primary circuit, Router A
is not limited to calling Router B. Router A dials the next phone number in its
outgoing phone list, Router C’s phone number, and makes a connection.
0.0.0.0 192.32.10.2
Router B
Regional
0.0.0.0 192.32.10.1
0.0.0.0
0.0.0.0
Backup
Primary
4
12
Leased
Line 1 – Primary line between routers A and B
Line 2 – Leased line between routers B and C
Line 3 – First attempt backup line for router A
Demand line for router C
Demand
Router C
Central
Router A
Remote
DS0018A
3
Line 4 – Second attempt backup line for router A
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