IP Precedence Caveats

IP Precedence has no built-in mechanism for refusing incorrect IP Precedence settings. The network administrator needs to take precautions to ensure that the IP Precedence settings in the network remain as they were originally planned. The following example shows the problems that can occur when IP Precedence is not carefully configured.

Company B uses WFQ with VoIP on all its WAN links and uses IP Precedence to prioritize traffic on the network. Company B uses a precedence setting of 5 for VoIP and a precedence setting of 4 for Systems Network Architecture (SNA) traffic. All other traffic is assumed to have a precedence setting of 0 (the lowest precedence).

Although in most applications the precedence is 0, some applications might be modified to request a higher precedence. In this example, a software engineer modifies his gaming application to request a precedence of 7 (the highest setting) so that when he and a co-worker in another office play, they get first priority on the WAN link. This is just an example, but it is possible. Because the gaming application requires a large amount of traffic, the company's VoIP and SNA traffic are not passed.

Creating the workaround for this is easy. You can use Cisco IOS to change to 0 any precedence bits arriving from non-approved hosts, while leaving all other traffic intact. This is discussed further in the "Policy Routing" section later in this chapter.

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