Concept Guide

qos-rate-adjust overhead-bytes
For example, to include the Preamble and SFD, type qos-rate-adjust 8. For variable length overhead elds, know the number of
bytes you want to include.
The default is disabled.
Enabling Strict-Priority Queueing
In strict-priority queuing, the system de-queues all packets from the assigned queue before servicing any other queues. You can assign
strict-priority to one unicast queue, using the strict-priority command.
• Policy-based per-queue rate shaping is not supported on the queue congured for strict-priority queuing. To use queue-based rate-
shaping as well as strict-priority queuing at the same time on a queue, use the Scheduler Strict feature as described in Scheduler
Strict .
• The strict-priority supersedes bandwidth-percentage conguration.
• A queue with strict priority can starve other queues in the same port-pipe.
• Assign strict priority to one unicast queue.
INTERFACE mode
service-policy output policy-map-name
Enter the name for the policy map in character format (32 characters maximum).
Queue Classication Requirements for PFC
Functionality
Queue classication requirements for PFC functionality are mentioned below:
• On untagged ports, Queue classication must be based on DSCP.
• On tagged ports, Queue classication must be based on Dot1p. Layer 3 classication congurations should not be present on the port.
• On hybrid ports, Queue classication can be based on either Dot1p (for tagged packets) or DSCP (for untagged packets) but not both.
Example Case:
PFC does not work for tagged trac, when DSCP based class map is applied on a hybrid port or on a tagged port. Assume two switches A
and B are connected back to back.
Consider the case where untagged packets arrive on switch A, if you want to generate PFC for priority 2 for DSCP range 0-7, then you
have to match the interested trac. You must use the class map and associate to queue 1 using the policy map. The same class map needs
to be applied in switch B as well and when queue 1 gets congested, PFC would be generated for priority 2. Switch A on receiving PFC
frames with priority 2 would stop scheduling queue 1.
If a tagged packet with VLAN dot1p as 5 ingresses on switch A. Consider that tagged packet also has DSCP in range of 0-7.These packets
will match the class map and get queued on queue 1 on both the switches.
But when queue 1 gets congested on switch B, PFC frames for tagged packets will not be generated as PFC is not enabled on dot1p
priority 5.
Support for marking dot1p value in L3 Input Qos
Policy
In case the incoming packet is untagged and the packet which goes out to the peer is tagged, then the dot1p should be marked
appropriately using L3 Input Qos Policy. This is required because in the peer switch PFC will be generated based on the dot1p value.
Quality of Service (QoS)
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