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532 Chapter 14: SNA Topologies

Configuring Traffic Priority

When designing large DLSw+ networks, you will probably need to specify traffic priority and
configure it. Three steps are required for configuring traffic priority:

Step 1

Configure the priority keyword on the appropriate DLSw+
remote-peer statements.

Step 2

Classify the packets on the incoming port and assign the traffic to
the appropriate TCP connection.

Step 3

Assign traffic to the appropriate queue based on protocol, TCP
port number, or message size, and define the queuing technique to
be used on the interface.

TIP

Queuing does not occur until the total number of packets exceeds the capacity of the
outbound link.

Queuing Algorithms

After priority traffic has been assigned to TCP ports, you can use a variety of Cisco IOS queuing
algorithms to improve QoS. Priority and custom queuing come to mind, as well as WFQ. These
subjects were discussed in Chapter 9, and they apply here too.

Priority and Custom Queuing for DLSw+

If you have many slow links or less than 56 K in your network, you might want to use priority
queuing. This queuing technique is somewhat drastic. It is easy to shoot yourself in the foot.
Priority queuing is drastic because traffic in the lower queues is not sent until the high queue is
empty, which can be unfair to the lower queues. If you absolutely must have DLSw traffic as
the first priority, this is the approach to take. If so much DLSw traffic goes through that your IP
traffic can't get through, you might lose your routing updates, which is not a good idea. A less-
drastic method is custom queuing. Priority queuing was designed to provide strict, absolute
priority to the most important traffic on the network.

WARNING

Be careful with priority queuing. If there is enough priority traffic, it might keep your routing
updates from getting through.

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