The Cisco NCS 520 Router supports tail drop queuing on egress interfaces using the queue-limit command. The following limitations apply to egress queuing:

• Egress QOS can be applied to a total of 91 EFPs at a system level.

• If you configure a queue size that the router cannot achieve within 1% accuracy, the configuration is rejected. The command output presents recommendations for the closest possible lower and higher configuration value.

• Egress policy-map with queuing action is not supported on port-channel interface(LAG). The policy must be applied to the policy-maps on the member links.

• The maximum bytes value of the queue-limit number-of-packets [bytes | ms | packets] command is 200 KB.

• The show policy-map interface command displays the default queue-limit.

• The queue-limit percent command is supported.

The following additional queuing usage guidelines:

• The router supports QoS policies that allow for low-latency queuing (LLQ) across multiple EFPs. For more information about this feature, see http://www.cisco.com/en/US/docs/ios-xml/ios/qos_plcshp/configuration/xe-3s/qos-plcshp-ehqos-pshape.html.

• CBWFQ is supported only on third level class.

• Queue-limit is supported only in leaf-level (per-hop behavior) classes.

• Queue-limit can not be configured without first configuring a scheduling action (bandwidth, shape average, or priority).

• Queue-limit can not co-exist with queue-limit percent.

• Queue-limit policy can be applied only on egress interface.

• Queue-limit can be configured in bytes or microseconds, or percent per class in the egress-policy.

• Default queue-limits for 1 and 10 G are 80 and 120 KB, repectively.

• Maximum queue-limit that can be configured in bytes is 200 KB.

• Ensure that you configure the queue-limit to a value greater than the default allocation value.

  When a minimum value is configured for queue-limit, for example, lesser than 11000 bytes, then the frame-size of outgoing traffic should be lesser than that of the configured queue-limit value.