Administrator 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:
DellEMC(conf)#qos-policy-input qos-input
DellEMC(conf-qos-policy-in)#set mac-dot1p 5
% Error: Dot1p marking is not allowed on L3 Input Qos Policy.
DellEMC(conf-qos-policy-in)#
You will also be able to mark both DSCP and Dot1p in the L3 Input Qos Policy:
DellEMC(conf)#qos-policy-input qos-input
DellEMC(conf-qos-policy-in)#set mac-dot1p 2
DellEMC(conf-qos-policy-in)#set ip-dscp 5
DellEMC(conf-qos-policy-in)#
Weighted Random Early Detection
Weighted random early detection (WRED) is a congestion avoidance mechanism that drops packets to prevent buffering resources from
being consumed.
The WRED congestion avoidance mechanism drops packets to prevent buffering resources from being consumed.
Traffic is a mixture of various kinds of packets. The rate at which some types of packets arrive might be greater than others. In this case,
the space on the buffer and traffic manager (BTM) (ingress or egress) can be consumed by only one or a few types of traffic, leaving no
Quality of Service (QoS)
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