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What is meaning of "Enabling diffserv examination will cause dot1p replacement of 802.1q tagged packets."

What is the meaning of the below Warning?

Warning: Enabling diffserv examination will cause dot1p replacement of 802.1q tagged packets.[/code]
It seems to imply that when using tagged packets, using 802.1p, when enabling diffserv examination those tagged packets will no longer be tagged and instead replaced with dot1p marked packets.

I've read a number of posts that indicate that ONLY tagged, 802.1p, packets support dot1p so this seems a little confusing to me. In my situation I only need to use DCSP 46 marked packet QoS and I'd like to keep it that way, but when I use the following configuration I always get this warning:

create qosprofile qp6
configure qosprofile qp6 minbw 10 maxbw 100 ports all configure qosprofile qp6 use-strict-priority configure diffserv examination code-point 46 qosprofile qp6 enable diffserv examination port all disable dot1p examination port all
When the command "enable diffserv examination port all" or any subsequent enabling of it, such as "enable diffserv examination port 1:10" I will get this warning.

I see a lot of people ignoring this warning, but for me my telephony device DOES use tagged packets. Each port uses an untagged VLAN for the computer pass-through, and then a tagged VLAN for the voice traffic. Do I need to be concerned, or should I also be ignoring this warning for reasons I have yet to understand.

8 replies

What model of switch...

When DiffServ examination is enabled on a Summit X670, or X770, the following warning message does not apply:

"Warning: Enabling diffserv examination will cause dot1p replacement of 802.1q tagged packets."

Summit x440-48p, also using XOS patch1-14 if it makes any difference.
Thanks, I see that it says:

When DiffServ examination is enabled on 1 Gigabit Ethernet ports for BlackDiamond 8800 series switches, SummitStack, and Summit family switches, 802.1p replacement is enabled and cannot be disabled. The ingress 802.1p value is replaced with the 802.1p value assigned to the egress QoS profile.

However, it just says that 802.1p replacement is enabled, which I believe implies the 802.1p priority field is updated in packets when enabled. I guess that makes sense, however it states replacement. Does that mean that regardless of "what" is in the dot1p field in a packet it's NOW replaced even though you are only asking for DSCP packet marking?

Just want to make sure that it isn't messing with the tagged packets in any other way.
Userlevel 6
Nathan, did that answer your question? Enabling diffserv examination will not effect 802.1q tagging.
Thanks for clarifying Stephen. What is the scenario that you'd need to look out for in the case of this warning?
Userlevel 7
You need to understand several things:

- precedence of the various QoS grouping: 802.1p examination has the highest priority, and is enabled by default. Meaning only 802.1p will be used for tagged packet if you don't disable .1p examination. Please note some modification of this precedence behavior starting with 16.1.

- for untagged packet, or tagged packet if .1p examination is disabled, you can group with several methods, one of them is diffserv examination.

- as the message implies, when using diffserv examination (assuming 802.1p examination is off), remarking of the .1p field (for tagged packets) happens based on the replacement table (show dot1p replacement).

Yes, 802.1p field is modified (replaced) by diffserv examination. As you may have .1p examination for tagged traffic and diffserv examination for untagged packets (if both are enabled at the same time), make sure both tables correspond to one another. Of course, the 802.1q tag is not modified, nor removed.

That explanation, additional info, helps out immensely...thank you.