WFQ Drop Policy Number of Queues and Queue Lengths

WFQ uses a slightly modified tail-drop policy for choosing when to drop packets. The decision is based on several factors, one being the SN of the packet.

WFQ places an absolute limit on the number of packets enqueued among all queues; this value is called the hold-queue limit. If a new packet arrives, and the hold-queue limit has been reached, the packet is discarded. That part of the decision is based not on a single queue, but on the whole WFQ queuing system for the interface.

The next decision is based on an individual queue. If a packet needs to be placed into a queue, and that queue's congestive discard threshold (CDT) has been reached, the packet may be thrown away. CDT is a little like a maximum queue length for each flow's queue, but WFQ puts a little twist on how the concept is used (hence the use of another term, instead ofjust calling it the maximum queue length). To appreciate how the CDT is used, examine Figure 4-17.

Figure 4-17 WFQ Modified Tail Drop and Congestive Discard Threshold

Figure 4-17 WFQ Modified Tail Drop and Congestive Discard Threshold

The hold-queue size limits the total number of packets in all of the flow or conversation queues. However, CDT limits the number of packets in each individual queue. If CDT packets are already in the queue into which a packet should be placed, WFQ considers discarding the new packet. Normally, the new packet is discarded. If a packet with a larger SN has already been enqueued in a different queue, however, WFQ instead discards the packet with the larger SN! It's like going to Disneyland, getting in line, and then being told that a bunch of VIPs showed up, so you cannot ride the ride, and you will have to come back later. (Hopefully Disney would not take you out of the line and send you to the bit bucket, though!) In short, WFQ can discard a packet in another flow when the queue for a different flow has exceeded CDT but still has lower sequence numbers. You can configure the CDT to a value between 1 and 4096, inclusive.

Finally, WFQ can be configured for a maximum of 4096 queues, but interestingly, the actual value can only be a power of 2 between 16 and 4096, inclusive. The IOS restricts the values because WFQ performs a hash algorithm to classify traffic, and the hash algorithm only works when the number of queues is one of these valid values.

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