802.1Q trunk and native vlan command.

So let me see if I understand this correctly.

Say I have a trunk link between Switch A and Switch B that carries
VLAN 10 & 11. Normally when I hook a client up to a port on switch A
I assign it to a vlan (either 10 or 11).

Now if I didnt put that port in either VLAN 10 or 11 and that users
packet crosses the trunk link would it be assigned to the "native
vlan". Although, if I didnt assign the port a vlan wouldn't that port
still be in its native vlan of the switch which would be vlan 1.

I am having a hard time seeing the use for this command, because every
switch port on these switches would be in some vlan either deafult or
assigned manually.

Thanks
Amy.

 

It would think it useful in cases where there is a differnet administrative
authority on each side of the trunk. If you don't want their default VLAN
polluting your default VLAN, so you can shunt untagged (default) packets to
a VLAN other than your own default of 1. Or you might want to make it so
that any default-default VLAN ("1") is explicitly enumerated on the trunk.
If you have strict control over your VLANs and never expect to see a VLAN 1,
this may help find rogue VLAN members.

But I am really just guessing

 

The command decides what will happen to an untagged frame that arrives
on a trunk port.

In your example, that will (or at least should) never happen, as you
have two trunk ports interconnected. But if you have a client directly
connected to a trunk port, and that client is sending both untagged and
tagged frames (say, a Cisco IP phone tagging VoIP traffic with 802.1p),
the command makes sense.

 

If VLAN 1 is "native for that port - you can change this on various devices.

But one of the issue you can get with a large VLAN structure is that the tag
numbers are not used consistantly - this is one way to alter tag numbers as
they cross the system since native vlan tag doesnt get included in the
frame.

Having said that the most common use seems to be in IP telephony, where a
non tag aware device sites on a switch embedded in an IP phone and uses a
single connection shared between the 2. The phone uses tags for its
traffic - hey presto logically distinct voice and data traffic in different
vlans.