CISCO IOS in a Nutshell

Managing Priorities with Queues

In most cases, you don’t need to worry about the details of packet processing. However, when dealing with various kinds of congestion, there are situations in which you need explicit control over how packets flow through the router. IOS gives you three techniques for prioritizing packet delivery: priority lists, custom queue lists, and Weighted Fair Queuing.

Priority Lists

Priority lists allow the router to sort and process incoming packets based on priorities you assign. These lists should be used only on serial WAN links that occasionally become congested; during congested periods, we want to give mission-critical packets top priority. If the serial link is always congested, don’t even try using priority lists; more bandwidth is the only solution.

With priority lists, we assign different queue levels to entire protocols or to particular ports. Packets are handled depending on the queue to which they are assigned: low, medium, normal, or high. For example, in Figure 7-3 the users on Network 2 access the web server on Network 1 across the WAN link. The web server is the primary application for our users, and it is the interface to the company database. During times of congestion, web traffic should have number-one priority. We give telnet medium priority, which allows our telnet sessions to get through, and give all other traffic normal priority.

Figure 7-3. Using ...