topic Re: ports, vlans and tagging in Switches, Hubs, and Modems

ports, vlans and tagging

grants — Tue, 26 Oct 2010 15:26:54 GMT

Switch - ProCurve J9280A

I have a bunch of different VLANs configured on my switch containing Tagged and Untagged ports.

If we are talking about a VLAN other than the default VLAN, how does tagging the port affect it as opposed to untagging the port? I just cannot wrap my head around this tagging concept, I understand that a VLAN is simply an area that contains broadcasts but the tagging thing has me confused.

Thanks for any help on this.

Re: ports, vlans and tagging

Richard Brodie_1 — Tue, 26 Oct 2010 15:57:27 GMT

Tagging the port adds some extra bytes to the Ethernet header, containing the VLAN number and a few other things. See e.g. http://en.wikipedia.org/wiki/IEEE_802.1Q

You can only have one (or zero) untagged VLANs on an individual link.

It makes more sense when you have multiple switches, because it allows you to carry multiple tagged VLANs between switches using a single physical link.

Normally, end equipment (except maybe phones), is going to expect untagged.

Re: ports, vlans and tagging

grants — Wed, 27 Oct 2010 07:27:22 GMT

Thanks for the link thats a pretty low level explanation though - Im looking for lamens terms on how to deny traffic from one port to another or alternatively, allow traffic between particular ports.

Let me ask you this: On my switch VLAN1 there are some ports 'tagged' and some 'no untagged' from a terminal session.
When I log into the web interface I get options "auto, tagged, untagged, forbid" I dont see an option for "no untagged". So what is this no untagged option and how do I apply it to a port?

Re: ports, vlans and tagging

Jaap Laaij — Sun, 31 Oct 2010 08:04:54 GMT

Hi,

Vlan 1 is the default vlan.
From scratch all port will be untagged in this Vlan.
If yo add a Vlan, you can put a port untagged in the new vlan. Since a port can be placed just once as untagged in a Vlan this port will be automatically 'no untagged' in Vlan 1.
It's also possible you want a port just to be tagged in a certain Vlan, e.g. uplinks. This port will still be untagged in Vlan 1 after you've configured it tagged. If you want to prevent Vlan 1 to pass on that port you give the 'no untag 'port' command in Vlan 1.
You cannot put a port in 'no untagged' if the port isn't yet configured in any other vlan.

Re: ports, vlans and tagging

grants — Tue, 02 Nov 2010 11:12:04 GMT

Many thanks for the explanation Jaap. That clarifies things a little better for me.