HP 200 Series Services And Applications page 200

Improving Network Availability
Circuit Group Manager
If one circuit fails, traffic is carried on the remaining circuit(s). As long as
one circuit within a circuit group is functioning, the redirectors are not
informed of circuit failures and thus do not try to find an alternate route.
When the failed circuit is restored, it will automatically be used for sending
traffic to the remote router. Since a circuit group did not fail, an alternate
route is not selected. Instead, traffic is aggregated on the remaining cir-
cuit(s) within the group. Thus, convergence is not an issue with this type of
circuit failure. The time to detect and remove a failed circuit is typically 2 to
20 seconds, depending on the characteristics of the failure.
For bridging, must the Spanning Tree Protocol be enabled to eliminate the
loop created by the additional load-sharing circuit? No, the Spanning Tree
Protocol is not required since logically there is no loop. The bridging
redirector directs traffic to the single circuit group, rather than the two
circuits.
There are limitations on the use of coterminus circuits. To understand those
limitations you need to know how the circuit group manager works.
Circuit Group Manager
After a routing or bridging forwarder has received a packet and has
determined that the packet must be forwarded, it hands the packet to the
circuit group manager with instructions to send it on a particular circuit
group. When multiple circuits exist in a circuit group, the circuit group
manager decides which circuit to use based on its circuit assignment
algorithm. One of two basic algorithms is used, depending on the type of the
packet.
Circuit Assignment Algorithm #1: Random
The first algorithm assigns traffic on circuits randomly. That is, when a
packet is being transmitted on a circuit group with two circuits, the choice
of which circuit to use is made on a random basis. Table 1 shows the packet
types for which the the random circuit assignment method is used.
3-6