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Pre-Calculating Available QoS CAM Space
Before Dell EMC Networking OS version 7.3.1, there was no way to measure the number of CAM entries a policy-map would
consume (the number of CAM entries that a rule uses is not predictable; from 1 to 16 entries might be used per rule depending
upon its complexity). Therefore, it was possible to apply to an interface a policy-map that requires more entries than are
available. In this case, the system writes as many entries as possible, and then generates an CAM-full error message (shown in
the following example). The partial policy-map configuration might cause unintentional system behavior.
%EX2YD:12 %DIFFSERV-2-DSA_QOS_CAM_INSTALL_FAILED: Not enough space in L3
Cam(PolicyQos) for class 2 (TeGi 12/20) entries on portpipe 1
The test cam-usage command allows you to verify that there are enough available CAM entries before applying a policy-map
to an interface so that you avoid exceeding the QoS CAM space and partial configurations. This command measures the size of
the specified policy-map and compares it to the available CAM space in a partition for a specified port-pipe.
Test the policy-map size against the CAM space for a specific port-pipe or all port-pipes using these commands:
● test cam-usage service-policy input policy-map {stack-unit } number port-set number
● test cam-usage service-policy input policy-map {stack-unit } all
The output of this command, shown in the following example, displays:
● The estimated number of CAM entries the policy-map will consume.
● Whether or not the policy-map can be applied.
● The number of interfaces in a port-pipe to which the policy-map can be applied.
Specifically:
● Available CAM — the available number of CAM entries in the specified CAM partition for the specified line card or
stack-unit port-pipe.
● Estimated CAM — the estimated number of CAM entries that the policy will consume when it is applied to an interface.
● Status — indicates whether the specified policy-map can be completely applied to an interface in the port-pipe.
○ Allowed — indicates that the policy-map can be applied because the estimated number of CAM entries is less or equal
to the available number of CAM entries. The number of interfaces in the port-pipe to which the policy-map can be
applied is given in parentheses.
○ Exception — indicates that the number of CAM entries required to write the policy-map to the CAM is greater than
the number of available CAM entries, and therefore the policy-map cannot be applied to an interface in the specified
port-pipe.
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
DellEMC# 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
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Quality of Service (QoS)