Service Manual

● Cyclic redundancy check (CRC): 4 bytes
● Inter-frame gap (IFG): (variable)
You can optionally include overhead fields 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 fields, 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 configured 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 configuration.
● 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 Classification Requirements for PFC
Functionality
Queue classification requirements for PFC functionality are mentioned below:
● On untagged ports, Queue classification must be based on DSCP.
● On tagged ports, Queue classification must be based on Dot1p. Layer 3 classification configurations should not be present on
the port.
● On hybrid ports, Queue classification 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 traffic, 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 traffic. 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.
720
Quality of Service (QoS)