Information Technology Reference
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is assigned to one of the four output queues. Packets with a higher priority are transmitted first.
If there is traffic in the high and normal queues, the high priority traffic is transmitted first and
then the normal queue traffic. Traffic in the medium, normal, and low queues is not transmitted
until all traffic in the high queue is sent. PQ gives mission-critical traffic the highest priority.
One drawback is the potential for queue starvation as lower priority traffic waits to be serviced.
PQs
Figure 11-1
HIGH
MEDIUM
NORMAL
LOW
Configuring PQ
PQ is configured by specifying the priority queue levels for different protocols by using the
priority-list command. A default priority level is also configured for those protocols or ports
that do not match access-list statements. The priority list is then applied to an interface with the
priority-group command. Example 11-2 shows a sample configuration of PQ.
Example 11-2 Configuration of PQ
interfaces serial 0
priority-group 1
!
priority-list 1 protocol dlsw high
priority-list 1 protocol ip medium list 99
priority-list 1 default normal
priority-list 1 protocol cdp low
!
access-list 99 permit 192.168.1.0 0.0.0.255
CQ
CQ uses a different approach than PQ for prioritizing traffic. Similar to PQ, traffic can be
assigned to various queues based on protocol, port number, or other criteria. However, CQ
allows traffic to be assigned to one of 16 queues that are dequeued in a round-robin fashion.
Priority is established by defining how many bytes can be transmitted from each queue in turn.
This is equivalent to allocating bandwidth to a protocol. Up to 16 queues can be configured. The
transmission size of each queue is specified in bytes. After the configured byte-count of a queue
is transmitted, the router sends the current packet and moves on to the next queue.
When the transmission window size is reached by transmitting the appropriate number of
frames from a queue, the next queue is dequeued, as shown in Figure 11-2. Traffic assigned to
queue 1 is sent (up to 2000 bytes), then traffic assigned to queue 2 (up to 700 bytes), then traffic
assigned to queue 3 (up to 1200 bytes), and then it round-robins to queue 1. CQ is fairer than
PQ, although PQ is more powerful for prioritizing mission-critical protocol.
 
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