Vlan tagging

[jpduminy]

Hello All,

I just want a clarity.

Wanted to know when does the switch exactly do a tagging.

while traffic enters into access port does switch do any vlan tags or just when the traffic exit from trunk port switch adds a vlan tag for that traffic.

[Joseph W. Doherty]

Excluding trunk native VLANs, tags are needed on egress as other side needs then on ingres.

When such tags are added to the frame to be transmitted would depend on the switch architecture.

[jpduminy]

Hi @Joseph W. Doherty 

Does switch do vlan tags when the traffic enters from access port ?

[Joseph W. Doherty]

It might process them, but it's up to the switch how to track/process VLANs internally, to the switch.

[jpduminy]

how does the switch process the traffic of access ports as there is no vlan id  tags to that frame.

[Joseph W. Doherty]

A VLAN is assigned to the port, implicitly VLAN 1.

[Richard Burts]

Perhaps I understand the question a bit differently than Joseph. And perhaps we might want to clarify what you mean when you mention a switch do a tagging. A switch will apply a tag to a frame when it transmits that frame over a trunk. A switch will read and process a tag when it receives a frame on a trunk. There is no tag when a switch transmits a frame on an access port and the switch does not look for a tag on a frame received on an access port.

[Joseph W. Doherty]

There is no tag when a switch transmits a frame on an access port and the switch does not look for a tag on a frame received on an access port.

BTW, although probably pretty unusual (I've never seen it in practice), an access port can send/receive frames with .1Q tags, with a zero VLAN ID, for CoS purposes.

Also, "access" ports, with a voice VLAN assignment, send those frames with .1Q tags, with the voice VLAN ID.

Lastly, on Cisco trunk ports, the "native" VLAN frames are usually set untagged.

Anyway, my understanding of OP, which can easily be incorrect, OP was wondering when, exactly, frames get their tag, and, perhaps, whether the VLAN tags are also "carried" internally.

For the latter, likely tags are only added to frame, upon egress, when actually needed/expected by device(s) on the other end of the port's connection.  But, internal VLAN tracking/processing, is up to the switch itself (which I suspect, but don't know, doesn't rely on the frame tag, which isn't always required externally, and often isn't expected, externally, e.g. access ports).