HP 200 Series Services And Applications page 207

Improving Network Availability
Circuit Group Considerations
A backup circuit is defined when the circuit is added to a circuit group as a
"backup circuit group member". Figure 5 shows a circuit group with a
primary and a backup circuit. A backup circuit is enabled when the primary
circuit(s) in a circuit group fails. The backup circuit is disabled when a
primary circuit is restored. Traffic is transmitted on either the primary
circuit or the backup circuit but not on both at once. Thus, backup circuits
cannot be used for bandwidth-on-demand applications. Additionally,
dynamic routing protocols are normally used on backup circuits since they
are operational only when the primary circuit has failed.
Primary and backup circuits are assigned to the same circuit group and thus
share network interface definition attributes such as addresses. Backup
circuits are recommended for use with all of the routable protocols. Since a
backup circuit is normally inactive, a switched (dial-up) circuit is the logical
choice to implement as a backup circuit, since it is the most cost-effective
alternative.
When a switch from primary to backup circuits must be performed quickly,
ISDN and Switched 56 are preferred to analog modems. ISDN and Switched
56 connections can typically be completed in 2 to 5 seconds, versus 25 to 30
seconds for analog modems.
Pool Circuits
Pool circuits (circuit group pool members) can be used as primary or
backup circuits. All pool members of a circuit group have the same attrib-
utes of the network interface definition, since they are members of the same
circuit group. Thus, they share the address defined for the circuit group. IP
pool circuits can be used only to transmit and receive IP traffic. Figure 6
shows a circuit group with several pool circuits.
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