Configuring Weights And Ecn For Wred - Dell Force10 Z9000 Configuration Manual

NOTE: The show cam-usage command provides much of the same information as the test cam-usage command,
but whether a policy-map can be successfully applied to an interface cannot be determined without first measuring how
many CAM entries the policy-map would consume; the test cam-usage command is useful because it provides this
measurement.
• Verify that there are enough available CAM entries.
test cam-usage
Example of the test cam-usage Command
Dell# test cam-usage service-policy input pmap_l2
Port-pipe | CAM Partition | Available CAM | Estimated CAM | Status
=====================================================================
0 L2ACL

Configuring Weights and ECN for WRED

The feature to configure a weight for WRED and ECN functionality for backplane ports is supported on the Z9000 platform.
The WRED congestion avoidance functionality drops packets to prevent buffering resources from being consumed. Traffic is a
mixture of various kinds of packets. The rate at which some types of packets arrive might be greater than others. In this case, the
space on the buffer and traffic manager (BTM) (ingress or egress) can be consumed by only one or few types of traffic, leaving no
space for other types. You can apply a WRED profile to a policy-map so that the specified traffic can be prevented from consuming
too much of the BTM resources.
WRED drops packets when the average queue length exceeds the configured threshold value to signify congestion. ECN is a
capability that enhances WRED by marking the packets instead of causing WRED to drop them when the threshold value is
exceeded. If you configure ECN for WRED, devices employ ECN to mark the packets and reduce the rate of sending packets in a
congested network.
In a best-effort network topology, data packets are transmitted in a manner in which latency or throughput is not maintained to be at
an effective level. Packets are dropped when the network experiences a large traffic load. This best-effort network deployment is not
suitable for applications that are time-sensitive, such as video on demand (VoD) or voice over IP (VoIP) applications. In such cases,
you can use ECN in conjunction with WRED to resolve the dropping of packets under congested conditions.
Using ECN, the packets are marked for transmission at a later time after the network recovers from the heavy traffic state to an
optimal load. In this manner, enhanced performance and throughput are achieved. Also, the devices can respond to congestion
before a queue overflows and packets are dropped, enabling improved queue management.
When a packet reaches the device with ECN enabled for WRED, the average queue size is computed. To measure the average
queue size, a weight factor is used. This weight factor is user-configurable. You can use the wred weight number command to
configure the weight for the WRED average queue size. The mark probability value is the number of packets dropped when the
average queue size reaches the maximum threshold value.
The weight factor is set to zero by default, which causes the same behavior as dropping of packets by WRED during network loads
or also called instantaneous ECN marking. In a topology in which congestion of the network varies over time, you can specify a
weight to enable a smooth, seamless averaging of packets to handle the sudden overload of packets based on the previous time
sampling performed. You can specify the weight parameter for front-end and backplane ports separately in the range of 0 through
15.
You can enable WRED and ECN capabilities per queue for granularity. You can disable these functionality per queue, and you can also
specify the minimum and maximum buffer thresholds for each color-coding of the packets. You can configure maximum drop rate
percentage of yellow and green profiles. You can set up these parameters for both front-end and backplane ports.
500 200
port-set 0
Allowed(2)
Quality of Service (QoS)
595