Users Guide

• 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.
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. Currently if the ingress is untagged and egress is tagged, then
dot1p priority 0(default) will be added as part of the tag header and from the next hop PFC will be based on
that dot1p priority. Support is added to mark the dot1p value in the L3 Input Qos Policy in this feature. Hence
it is possible to mark both DSCP and Dot1p simultaneously in the L3 Input Qos Policy. You are expected to
mark the Dot1p priority when the ingress packets are untagged but go out to the peer as tagged
NOTE
: L2 qos-policy behavior will be retained and would not be changed, that is we would not allow to
set both DSCP and Dot1p in the L2 Input Qos Policy.
Example case:
Consider that two switches A and B are connected back to back via a tagged interface. 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 must need to match the interested traffic using the class map. You should create an L3 Input Qos
Policy and mark vlan dot1p as 2. You have to associate both the L3 class map and L3 Input Qos Policy to
queue 1 using the policy map.
In switch B, global dot1p honoring should be enabled, this will queue the packets on queue 1 as the dot1p will
be 2 and PFC should be enabled for priority 2. The policy map applied on switch A need not be enabled in
switch B. When queue 1 in switch B gets congested, PFC will be generated for priority 2 which will be
honored in switch A.
You will not get the below CLI errors after adding this support:
Dell(conf)#qos-policy-input qos-input
Dell(conf-qos-policy-in)#set mac-dot1p 5
Quality of Service (QoS) 787