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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 space for other types. You can apply a WRED profile to a policy-map so that specified traffic can be
prevented from consuming too much of the BTM resources.
WRED uses a profile to specify minimum and maximum threshold values. The minimum threshold is the allotted buffer space for
specified traffic, 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 buffer space
consumes less than 2000KB of the specified traffic.
Figure 110. Packet Drop Rate for WRED
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Quality of Service (QoS)