
Configuring Dial Services
1-14 114062 Rev. A
How Backup Lines, Pools, and Circuits Work Together
For each leased circuit that needs a backup, you designate that circuit as a primary
circuit and assign it a backup pool ID. If the primary circuit fails, the router
activates the backup line to carry the backup circuit. If your network uses a
multiline or multilink primary, the backup line activates when the last line in the
multiline or multilink bundle fails. Note that you must first configure a leased
circuit and a backup pool before you can configure primary and backup circuits.
Figure 1-8 shows how lines, pools, and circuits work together. The router in San
Diego has two leased lines, one going to a router in Phoenix and the other to a
router in San Francisco. The circuits for each of these destinations are associated
with Backup Pool 1. If either of these leased lines fail, the San Diego router
activates Backup Line 1 from Backup Pool 1 to continue routing traffic to the
destination. Although there is only one line, both use this line as a backup.
Figure 1-8. Example of Backup Pools, Lines, and Circuits
You can assign the same backup pool ID to more than one primary circuit. If you
want only one backup line dedicated to a primary circuit, then you should
configure only one line in a pool and assign that pool exclusively to that circuit.
DS0028A
128.32.17.0
128.32.18.0
Circuit 1 - 128.32.17.1
(the local interface to Phoenix)
Circuit 2 - 128.32.18.1
(the local interface to San Francisco)
Configuration of the San Diego Router
San Diego
Phoenix
San Francisco
128.32.17.2
128.32.18.2
Backup Pool 1:
Line 1
ISDN
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