Enterasys Networks X-PeditionTM Queuing and Services, Describing Class-Based Weight Fair Queuing

Mechanisms Providing QoS

•The priority command assigns traffic from this class a Priority Queue (PQ) and sets the parameter for the queue. Priority queues provide guaranteed bandwidth - they always receive the bandwidth requested. Priority class is not allowed to send more than its guaranteed bandwidth and excess traffic is discarded. Unused priority bandwidth is picked up by the class-defaultclass.

For classes that are assigned to CBWFQ you can control the maximum rate of traffic sent or received on a port as follows:

•The police command controls traffic received by a queue by defining the action taken for packets that conform or exceed the specified rate. You may drop the packet, change its IP precedence or DSCP setting, or forward it without modification.

Both CBWFQ and Priority Queues can control queue size and the type of congestion avoidance mechanism, as well as mark packets as follows:

•The set ip precedence, set ip dscp commands mark a packet by setting the IP precedence or DSCP field. The Differentiated Services Field is defined in RFCs-2474 and 2475.

•The queue-limitcommand specifies or modifies the maximum number of packets the queue can hold before tail drop for TCP/IP traffic for a class policy configured in a policy map.

•The random-detectcommand sets Random Early Detect (RED), a congestion avoidance mechanism that slows traffic by randomly dropping packets when congestion exists.

Traffic not assigned to a class in the policy map is assigned to class-defaultwhich is present by default and assigned as a CBWFQ. Bandwidth for the class-defaultcomprises whatever remains after all other classes are served. You can configure class-defaultas any other CBWFQ, except that you cannot assign bandwidth to it.

Queuing and Services

Once traffic has been classified, it is dropped into different queues so that each class of traffic can be treated differently (priority, bandwidth etc.). The following describes two queue types used in the XSR: Class Based Weight Fair Queuing and Priority Queuing. They are mutually exclusive - only one type of queue may be applied to one class. But, they may be mixed in a policy-map when applied to different classes.

Describing Class-Based Weight Fair Queuing

The configured bandwidth of a class is the bandwidth delivered to the class during congestion. The higher the bandwidth, the more likely the packet is being transmitted under congested conditions. If there is no data on a particular queue, then its share of the bandwidth will be divided and shared among the active queues in proportion to their specified bandwidth.

CBWFQ specifies the exact amount of bandwidth to allocate for a specific class, or queue, of traffic. Taking into account available bandwidth on the interface, you can configure up to 64 classes and control distribution among them. If excess bandwidth is available, it is divided among other CBWFQs in proportion to their configured bandwidths.

When bandwidth is specified as an absolute number, it is used to calculate the weight of the class. In such a case, the sum of bandwidth for all classes, including priority classes, should not exceed the link bandwidth otherwise the bandwidth for the default class will be zero causing a traffic blockage and packet pileup in the queue.

Note: For each policy-map, only one type of bandwidth, percentage or absolute bandwidth, can be used for all the CBWFQ classes inside the policy-map.

12-4 Configuring Quality of Service