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A networking course that I am following mentions that layer 2 marking can be performed through the use of PCP in the dot1q tag.

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So to help me understand, once a switch receives a frame with a dot1q tag, does it classify it as one of the traffic types mentioned in the slide above and then change the PCP value to mark it?

Also, does this happen automatically or is configuration required?

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    Notice that is in a 801.1Q tag, so it is only in frame headers for trunks, and that the native VLAN on a trunk is not tagged, so it cannot be marked. A frame must be classified before it is marked, so that happens outbound on a trunk, usually based on an ACL.
    – Ron Maupin ♦
    Nov 11, 2022 at 13:50
  • If doing 802.1p only, an otherwise "access" port will send a fully tagged frame with a VID of 0. This has been actively used in the real world for many decades. (and documented in various standards docs.)
    – Ricky
    Nov 11, 2022 at 22:08
  • @RonMaupin I worked with devices that would classify on inbound (to determine internal QoS priority) and mark on outbound
    – user253751
    Nov 11, 2022 at 22:54
  • @user253751, that is what I explained. It must first be classified, then it is marked outbound., but only for a trunk. The internal switching needs no classification because it has no congestion in the switching matrix, it is only on an outbound interface where there may be congestion.
    – Ron Maupin ♦
    Nov 11, 2022 at 23:01

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What a switch does with the PCP field depends on its configuration. By default, (most) switches don't do anything with those values.

Also, the values and traffic types in the table are just recommendations. It's up to the network admin to adapt and implement them.

Additionally, IP differentiated services have mostly taken over QoS signalling but there's often a mapping between PCP and DSCP.

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  • 802.1p only really applies "on-link". Unless the switch is configured to do something with it, the tag usually gets removed, so nothing else ever sees it.
    – Ricky
    Nov 11, 2022 at 22:11

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