prio-tagged frames *towards* a station?

Folks,

maybe someone can share insight with me as 802.1Q left me not much wiser... I *do* understand (I think) about *sending* priority-tagged into a network. I am, however, puzzled what frames shall look like, per 802.1, in the reverse direction, that is, sent from a switch to a station.

Would the switch transmit frames always untagged, tagged, prio-tagged, or would it just depend on the VLAN setting (tagged, untagged) on that port? I'd not be interested in what *could* be done, or is done in some implementation but would be desperate for what would be an 802.1-compliant behavior of such switch.

I tend believing that *sending out* prio-tagged frames off a switch won't be allowable while I could imagine both untagged or tagged be permitted per 802.1, just depending on the VLAN settings for the PVID of the port in question.

Anyone willing to shed little more light on me? ;-)

Thanks, /Thomas.

Reply to
Thomas Bahls
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the format is the same and the rules are the same

if a station is using VLAN tags, then the switch needs to be consistent, or the frames may not get delivered to the right VLAN within the station.

as a practical matter the switch uses 802.1p bits the same whether the target is a switch or end station - often it doesnt have any way to tell the difference.

FWIW on a cisco switch if you send DSCP packets out a tagged interface, then yo either get a default 802.1p set of bits, or a map derived from the DSCP value, or you can configure in other behaviour.

the switch must use the same format per vlan as the other device on the port - or packets may get mis-identified.

remember that with transparent bridging the switch cannot tell whether the downstream device is a switch or end station, so trying to behave in different ways for each is not going to work :).

then there are all those wierd cases - like an in line IP phone between a laptop and a PoE switch.....

Reply to
Stephen

Going by strict word of spec (802.1Q), a bridge never transmits priority-tagged frames. The whole point of priority tagged frames is for a device that doesn't know about VLANs, but wants to indicate priority, be able to do so. A switch never has a problem knowing which VLAN a frame belongs to. If an end station doesn't know about VLANs but knows about priority it can always ignore the VLAN information in the frame.

Anoop

Reply to
anoop

That sounds plausible to me and is merely what I suspected to be the case. Would you know, however, where exactly (clause, section) in 802.1Q to find such a clear statement? I had searched but didn't find that much clarity there. :-/

That sounds like a good point of argument to me. I hadn't looked at the problem this way.

Thanks, /Thomas.

Reply to
Thomas Bahls

AFAIK, there is nothing in the spec that specifically prohibits the bridge from emitting a priority tagged frame. However, there is no control to actually ever make that happen. A VLAN is either tagged or untagged on a port, and there is no control that says "send untagged VLAN(s) as priority tagged".

Anoop

Reply to
anoop

Aaah, that might be a good reason for not having found something specific. I shall have another read with this in mind. ;-)

Thanks, /Thomas.

Reply to
Thomas Bahls

If I read it correctly, this is discussed in clause 6.7.2 where it states that frames on an untagged port should not have a tag header inserted. I guess this implicitly means that you can't send priority tagged frames.

Table 5.1 in 802.1Q-2003 show a nice little matrix named "Support requirements for insertion, removal, and modification of tag headers". This table states that for priority tagged frames that are received and are to be transmitted on an untagged port, the bridge "Shall support removal of tag headers". ...when I think about it, that doesn't really say that you MUST remove the tag either now does it?

Geir

Reply to
Geir G.

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