10.2 Backup Interface Operations
10.2.3 Configuring weighted fair queuing
The fair-queue command enables fair queuing on an interface.

Router(config-if)#fair-queue {congestive-discard-threshold}

The congestive-discard-threshold is the number of messages to queue for high-volume traffic. In other words, the maximum packets in a conversation held in a queue before they are discarded. Valid values are 1 to 512, inclusive. The default is 64 messages. The fair-queue 128 command sets the congestive discard threshold number to 128.

The congestive-discard-threshold policy applies only to high-volume conversations that have more than one message in the queue. The discard policy tries to control conversations that could monopolize the link. If an individual conversation queue contains more messages than the congestive-discard threshold, that conversation will not have any new messages queued until the content of that queue drops below one-fourth of the congestive-discard value. In the example of fair-queue 128, the queue must contain fewer than 32 entries (1/4 of 128).

In the Figure, interface serial 1 is attached to a Frame Relay network and is configured to operate at a 56-Kbps link speed.

WFQ is used by default on serial interfaces at E1 speeds (2.048 Mbps) and below. WFQ is disabled on serial interfaces that use X.25 or compressed PPP. LAN interfaces and serial lines, operating at E3 or T3 speeds, are not available for WFQ.

The fair-queue command enables fair queuing on an interface. In the Figure, interface serial 1 is attached to a Frame Relay network and is configured to operate at a 56-Kbps link speed. The fair-queue 128 command sets the congestive discard threshold number to 128.

Because conversations may not have any new messages queued until the content of that queue drops below one-fourth of the congestive-discard value, a queue must contain fewer than 32 entries (1/4 of 128).

Lab Activity    
  Our company has given us the task of connecting our east coast office with the west coast office via ISDN. There will be some large file transfers over this link but we do not want them to dominate the link. Since we want low bandwidth applications such as telnet to be able to get through at any time, we have decide to use weighted fair queuing to solve this problem. It is our responsibility to connect these two routers together and configure weighted fair queuing correctly.