Class-Based Weighted Fair Queuing – CB-WFQ

The WFQ mechanism made sure that no traffic was starved out. However, WFQ did not make a specific amount of bandwidth available for defined traffic types.

You can, however, specify a minimum amount of bandwidth to make available for various traffic types using the CB-WFQ mechanism.

CB-WFQ is configured through the tree-step MQC process. Using MQC, you can create up to 63 class-maps and assign a minimum amount of bandwidth for each one.

Why not 64 ?

Because the default class-map is already configured.

Traffic for each class-map goes into a separate queue. Therefore, one queue can be overflowing , while other queues are still accepting packets.

Bandwidth for class-maps can be specified following 3 ways:

  • Bandwidth
  • Percentage of bandwidth
  • Percentage of remaining bandwidth

By default , each queue that is used by CB-WFQ has a capacity of 64 packets. Also , only 75 percent of an interface’s bandwidth can be allocated by default.

The remaining 25 percent is reserved for nonclassified overhead traffic ( CDP,LMI,Routing,..)

But you can always overcome this limitation with the command max-reserved-bandwidth <<percentage>>.

CB-WFQ is therefore an attractive queuing mechanism thanks to MQC configuration and the ability to assign a minimum bandwidth allocation.

The only major drawback to CB_WFQ is its inability to give priority treatment to any class. To overcome this drawback, Low Latency Queuing (LLQ) was created tosupport traffic prioritization.

Weighted Fair Queuing – WFQ

WFQ is enabled by default on slow-speed interfaces (< 2,048 Mbps).
WFQ allocates a queue for each flow, for as many as 256 flows by default.
WFQ uses IP Precedence values to provide a weighting to Fair Queuing(FQ).

How FQ is different from WFQ ?


Hardware >< Software Queuing

A router has 2 types of queues:

  • A hardware queue
  • A software queue

The hardware queue , which is most of the time the transmit queues (TxQ) , uses FIFO queuing and it is only when the hardware queue is full that the software queue will handle these packets.

So remember that it is only valid when you are experiencing interface congestion (overflow of the hardware queue).

FIFO is also the default mechanism for software queue for higher speed interfaces ( > 2048 Mbps)

Weighted Round Robin Queuing

This queuing mechanism is the same as the Round Robin Queuing but this time ,we are placing a weight on the various queues which will service a different number of bytes or packets from the queues during a round robin cycle .

CQ ( Custom Queuing) is an example of  Weighted Round Robin.

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