Dell EMC Networking OS Configuration Guide for the S5048F–ON System 9.14.2.4

• 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.