Users Guide

• Ethernet Type/Length: 2 bytes
• Payload: (variable)
• Cyclic redundancy check (CRC): 4 bytes
• Inter-frame gap (IFG): (variable)
You can optionally include overhead elds in rate metering calculations by enabling QoS rate adjustment.
QoS rate adjustment is disabled by default.
• Specify the number of bytes of packet overhead to include in rate limiting, policing, and shaping calculations.
CONFIGURATION mode
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.
CONFIGURATION mode
strict-priority
The range is from 1 to 3.
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.
Quality of Service (QoS)
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