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IP RTP priority

Cisco's IOS command "ip rtp priority" is a simple way to help you prioritize your voice traffic.

In Cisco's IOS, and most of its clones, the "ip rtp priority" command is one of the easiest ways to give your VoIP (and video) traffic the priority it needs. Basically, after configuring this command, if there is a UDP packet in a queue that has a destination port in the range you specified, that packet will be dequeued and scheduled before any others in the queue. In other words, "ip rtp priority" provides a strict priority.

Configuring this feature is as simple as enabling Weighted Fair Queuing, and then typing in the "ip rtp priority <port range> <bandwidth>" command. Because it classifies traffic based on UDP ports instead of IP Precedence or DiffServ Code Points (DSCP), you don't have to worry about marking your traffic or understanding more complex arrangements like Expedited Forwarding and Assured Forwarding classes.

Therefore, "ip rtp priority" is good for simple networks. It has a little room for growth in complexity as well, as you can also use it with Class-Based Weighted Fair Queuing (CBWFQ) as well. This will allow you to continue to give strict priority to voice but also prefer some data applications over others, while still avoiding the task of marking traffic.

Finally, a caveat: According to Cisco's documentation, the maximum bandwidth you can reserve using this command is "2000" or 2 Mb/s. This is a pretty substantial amount, greater than a T1, but still it would not be too difficult to exceed this. For instance, a multi-point, 4-way videoconference and a dozen or so voice calls could be a problem, depending on the codecs you use. So, before you base your QoS strategy on this simple method, make sure you won't outgrow it any time soon.


Thomas Alexander Lancaster IV is a consultant and author with over 10 years experience in the networking industry, focused on Internet infrastructure.

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