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Quality of Service (QoS) Networking 305
Based on the assigned priority, packets can be classified and placed into one of the four output
queues. Packets that are not classified by priority are considered normal.
Design Trade-Offs
If the queue for voice traffic is always full, other data packets in any other queues will be
delayed or dropped. When priority queuing is used on a busy network, lower-priority traffic can
be denied bandwidth in favor of higher-priority traffic. Priority queuing could result in lower-
priority traffic never being sent. This is all because priority queuing introduces extra overhead.
The system takes longer to switch packets because the processor card classifies the packets.
Configuring priority queuing is a manual process. It increases administrative overhead in a
dynamic network, because priority queuing does not adapt to changing network conditions.
TIP
To avoid losing lower-priority traffic, use traffic shaping or committed access rate (CAR) to
rate-limit the higher-priority traffic.
CAR is available only on selected hardware platforms--primarily the 7000 series.
Routers-a-GoGo: An Example of Priority Queuing
We are at the world-famous Routers-a-GoGo club. What a crowd! It's SNA Night, and the
packets have really come out. My, would you just look at the crowd of packets lined up to get
in! We've got voice, video, data. We've got big packets, small packets. They all want to get in
to Routers-a-GoGo.
Dayta Link and the Fabulous Frames are playing tonight. They're doing a special salute to SNA,
so we promised the SNA packets that they would be able to get in no matter what. Whenever a
packet comes to the interface booth and shows the SNA header, it can get in first with no
waiting. As long as there is a line at the door, the other packets must wait because tonight is
SNA night. As soon as all the SNA packets are seated, the other packets will be seated. If any
SNA packet shows up, it gets the preferential red-carpet treatment.
Example of Priority Queuing
Packets can be classified by the following: protocol or subprotocol type, interface, packet size,
fragments, or access list. The following example assigns 1 as the arbitrary priority list number,
specifies IP as the protocol type, and assigns a high-priority level to the IP packets transmitted
on this interface:
priority-list 1 protocol ip high
87200333.book Page 305 Wednesday, August 22, 2001 2:53 PM