VLAN: Enable Tx Force untag (SLM-2008 smart switch)

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Feb 23rd, 2011
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Could you please help me with following question. I've done my best to convey everything clearly, but if there are any doubts, please ask me to clarify.

The manual says:

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When this option is enabled, all egress frames from this port become untagged. The default value is Disable. When this function is disabled, only frames with the VLAN ID equal to the PVID will become untagged, otherwise, frames are sent with a VLAN tag.

What does this mean extactly? I put egress in bold as that word really leads to confusion.

Special notes:

- Take notice of the direction of the arrow

- -------- (dotted line = an ethernet frame)

A) the option is enabled

Switch ----------> VLAN UNAWARE DEVICE.

Explanation: it does not matter what VLAN ID the frame has, the tag will be stripped. The options strips tags because VLAN unaware devices do not understand them.

B) the option is disabled

Switch <------- VLAN AWARE DEVICE

Explanation: If a VLAN aware device attempts to send a tagged frame, this tag will be stripped first before it is allowed to enter the switch.

I'm logically assuming A is the correct answer.  Suppose the VLAN unaware device is in VLAN 5. Why would a switch untag a frame with VLAN ID 8 and allow this frame to enter VLAN 5? I'm just saying, what is the purpose of this option? I can't seem to come up with a use for it.

many thanks,


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