Cisco 10000 Series Configuration Manual page 386

Class-Based Weighted Fair Queuing
After defining a class according to its match criteria, you can assign it characteristics by assigning it
bandwidth, weight, and maximum packet limit. The bandwidth you assign to a class is the guaranteed
bandwidth delivered to the class during congestion. You can also specify the maximum number of
packets allowed to accumulate in the queue for a class, referred to as the queue limit for the class. Packets
belonging to a class are subject to the bandwidth and queue limits that characterize the class.
After a queue reaches its configured queue limit, enqueueing of additional packets to the class causes
packet drop to occur. The router drops packets using one of the following methods, depending on how
you configured the traffic class:
Tail drop—The default congestion avoidance mechanism for Layer 3 queues. Tail drop activates
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when a queue becomes full. After being activated, no packets make it to the queue. Tail drop treats
all traffic equally and does not differentiate between classes of service.
• Weighted Random Early Detection (WRED)—A mechanism for avoiding congestion of Layer 3
queues. WRED combines the capabilities of the random early detection (RED) mechanism with IP
precedence, differential services code point (DSCP), and discard-class to provide preferential
handling of higher priority packets. WRED attempts to anticipate and avoid congestion. WRED
implements a proactive queuing strategy that manages congestion before a queue reaches its queue
depth. By selectively dropping packets, WRED prevents packets from enqueuing to the Layer 3
queue.
Note If you use WRED packet drop instead of tail drop for one or more traffic classes in a policy map, the
interface to which you attach that policy map cannot have WRED configured.
If you configure the class-default class using the bandwidth command, the router places all unclassified
traffic into a single FIFO queue and allocates bandwidth according to the configured bandwidth. If you
do not configure the class-default class, then by default the router gives best-effort treatment to the traffic
that does not match any of the configured classes. After the router classifies a packet, all of the standard
mechanisms that you can use to differentiate service among the classes apply.
For CBWFQ, the weight specified for the class becomes the weight of each packet that meets the match
criteria of the class. The router classifies packets that arrive at the output interface according to the match
criteria filters you define. The router then assigns each one the appropriate weight. The router derives
the weight for a packet belonging to a specific class from the bandwidth you assign to the class when
you configure it. In this sense the weight for a class is user-configurable.
After the router assigns the weight for a packet, the router enqueues the packet in the appropriate class
queue. CBWFQ uses the weights assigned to the queued packets to ensure that the router services the
class queue fairly.
You can configure CBWFQ on a physical interface only if the interface is in the default queuing mode.
Serial interfaces at E1 (2.048 Mbps) and below use WFQ by default; other interfaces use FIFO by
default. Enabling CBWFQ on a physical interface overrides the default interface queuing method.
Enabling CBWFQ on an ATM permanent virtual circuit (PVC) does not override the default queuing
method.
Cisco 10000 Series Router Quality of Service Configuration Guide
12-2
Chapter 12 Sharing Bandwidth Fairly During Congestion
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