Use differentiated WRED to prevent congestion in all three classes

This example of CBWFQ with WRED focuses on a network that provides these three different service levels for three traffic classes:

■ Mission-critical class: Marked with IP Precedence values 3 and 4 (3 is used for high-drop service, and 4 is used for low-drop service within the service class), and should get 30 percent of an interface bandwidth

■ Bulk class: Marked with IP Precedence values 1 and 2 (1 is used for high-drop service, and 2 is used for low-drop service), and should get 20 percent of the interface bandwidth

■ Best-effort class: Should get the remaining bandwidth share, and should be fair-queued

To enforce this service policy, a router will use CBWFQ to perform bandwidth sharing and WRED within service classes to perform differentiated drop.

Drop Probability


Drop Probability


22 24 26 28



Size class-map Mission-critical match ip precedence 3 4

class Bulk bandwidth percent 20

random-detect random-detect precedence 1 22 36 10 random-detect precedence 2 24 36 10 class class-default fair-queue ra n dom-de tec t class-map Bulk match ip precedence 1 2

policy-map Policy1

class Mission-critical bandwidth percent 30

random-detect random-detect precedence 3 26 40 10 random-detect precedence 4 28 40 10

The figure shows the WRED traffic profile representing the QoS service policy, and the configuration that is used to implement the example service policy. The traffic is classified based on the precedence bits, and all noncontract traffic is classified into the default class, as follows:

■ The mission-critical class is guaranteed at least 30 percent of bandwidth with a custom WRED profile that establishes a low-drop and a high-drop per-hop behavior.

■ The bulk class is guaranteed at least 20 percent of bandwidth, is configured with somewhat lower WRED drop thresholds, and is therefore more likely to be dropped than the mission-critical class in the event of interface congestion.

■ All other traffic is part of the default class and is fair-queued with default WRED parameters.

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