Users Guide

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-set 0 | port pipe
Port-pipe | CAM Partition | Available CAM | Estimated CAM | Status
=====================================================================
0 L2ACL 500 200 Allowed(2)
Configuring Weights and ECN for
WRED
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
Quality of Service (QoS) 878