Concept Guide

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 trac 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 buering resources from
being consumed.
The WRED congestion avoidance mechanism drops packets to prevent buering resources from being consumed.
Trac 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 buer and trac manager (BTM) (ingress or egress) can be consumed by only one or a few types of trac, leaving no
space for other types. You can apply a WRED prole to a policy-map so that specied trac can be prevented from consuming too much
of the BTM resources.
WRED uses a prole to specify minimum and maximum threshold values. The minimum threshold is the allotted buer space for specied
trac, for example, 1000KB on egress. If the 1000KB is consumed, packets are dropped randomly at an exponential rate until the maximum
threshold is reached (as shown in the following illustration); this procedure is the “early detection” part of WRED. If the maximum threshold,
for example, 2000KB, is reached, all incoming packets are dropped until the buer space consumes less than 2000KB of the specied
trac.
Quality of Service (QoS)
763