10.2 Queuing Options
10.2.7 Custom queuing operation
As shown in the Figure, CQ has two components:
  • Traffic filtering—The forwarding application—such as IP, IPX, or AppleTalk—applies a set of filters or access-list entries to each message that it forwards. The messages are placed in queues, based on the filtering.
  • Queued message forwarding—CQ uses a round-robin dispatching algorithm to forward traffic. Each queue continues to transmit packets until the configured byte limit is reached. When the threshold of this queue is reached or the queue is empty, the queuing software services the next queue in sequence.

When PQ, CQ, or WFQ is enabled, IP traffic is classified by fast-switching logic. All other traffic is classified as process switched. The differences are as follows:

Process Switching

In process switching, the first packet is copied to the system buffer. The router looks up the Layer 3 network address in the routing table and initializes the fast-switch cache. The frame is rewritten with the destination address and sent to the exit interface that services that destination. 

Subsequent packets for that destination are sent by the same switching path. The route processor computes the cyclical redundancy check (CRC).

Fast Switching

When packets are fast switched, the first packet is copied to packet memory and the destination network or host is found in the fast-switching cache. The frame is rewritten and sent to the exit interface that services the destination. Subsequent packets for the same destination use the same switching path. The interface processor computes the CRC.