[IEEE 802.1q] terminology: access, trunk & hybrid ports


I was studying 802.1q standard (two editions were on my desk, 1998 and 2005 year). There was some confusion with terminology used in the standards: the

1998 edition has a small chapter on port types, it dpecifically defines access, trunk and hybrid modes of operation, while in 2005 edition of 802.1q standard that was removed.

It seems that the standard borrows these terms from Cisco (I see the same in Cisco documents ). But the behavior of the modes isn't completely clear to me, so as I have understood so far:

1) Trunk port has only tagged frames passing in and out (802.1q-1998), so obviously before forwarding a frame out the trunk port, the switch must insert a proper tag in the frame, if the latter doesn't contain one. Is that correct? However, Cisco defines 'trunk' it means that the trunk port just belongs to a multiple VLAN.

2) The switch believes that frames entering the 'access' port as untagged (as per 802.1.q-1998). I don't understand - is it illegal to have a tagged frame in 'access' network? What is the switch's behavior when it meets a tagged frame from the access port?

3) 'Hybrid' port - may have both tagegd and untagged. (This is only defined in the standard, Cisco doesn't have it)

PS. I also checked "The Swicth book", but Mr.Seifert doesn't mention about access or trunk modes of ports.

Would appreciate a lot for helping me to understand !


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not quite.

a cisco trunk on a switch is tagged - but that could be with just 1 vlan.

in some cases vlan 1 will appear anyway as that is where some cisco control protocols do their magic.

you are mixing the same term in 2 different contexts. "access" here just means the switch is set up with untagged port.

What is the switch's behavior when it meets a

in theory it should be dropped, as tagged prts arriving at an untagged port means misconfigured network, or even deliberate attenpt to "push" a frame to a different vlan.

in practice the hardware may enforce dropping of tagged frames, but if it is documented at all it will be buried somewhere - easiest way to find out is to test it.

Cisco has the idea of a "native vlan" - which is where untagged frames arriving at a tagged port go to.

i think this was invented for stuff like adding a phone with simple embedded switch with a PC on a daughter port.

PC untagged traffic goes thru without any complex processing, phone traffic is tagged and the main switch can treat the port as having 2 different vlans.

good luck

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