In group mode, the appliances that are part of the group each take ownership for a portion of the group's connections. If a given appliance is the owner of a connection, it makes all the acceleration decisions about that connection and is responsible for compression, flow control, packet retransmission, and so on.
If an appliance receives a packet for a connection for which it is not the owner, it forwards the packet to the appliance that is the owner. The owner examines the packet, makes the appropriate acceleration decisions, and forwards any output packets back to the non-owning appliance. This process preserves the link selection made by the router, while allowing all packets in the connection to be managed by the owning appliance. For the routers, the introduction of the appliances has no consequences. The routers do not need to be reconfigured in any way, and the appliances do not need to understand the routing mechanism. They simply accept the routers' forwarding decisions.
Group mode has two, user-selectable failure modes, which control how the group members interact with each other if one of them fails. The failure mode also determines whether the failed appliance's bypass card opens (blocking traffic through the appliance) or remains closed (allowing traffic to pass through). The failure modes are:
Continue to accelerate- If a group member fails, its bypass card is opened and no traffic passes through the failed appliance. The result is presumably a fail-over if redundant links are used. Otherwise, the link is simply inaccessible. The other appliances in the group continue to accelerate. The usual hashing algorithm handles the changed conditions. (That is, the old hashing algorithm is used, and if the failed unit is indicated as the owner, a hashing algorithm based on the new, smaller group is applied. This preserves as many older connections as possible.)
Do not accelerate- If a group member fails, its bypass card closes, allowing traffic to pass through without acceleration. Because an unaccelerated path introduces asymmetric routing, the other members of the group also go into pass-through mode when they detect the failure.