Trunk Links (802.1q) And Native VLAN

For 802.1Q trunk links I noticed the following command can be used

Switch(config-if)# switchport trunk native vlan vlan_num

Can someone explain exactly what that command is for?

I normally configure a trunk link as follows

interface GigabitEthernet2/1
description Trunk Connection To 3550-STACKA gig0/1
switchport access vlan 42
switchport trunk encapsulation dot1q
switchport trunk pruning vlan none
switchport mode trunk
no logging event link-status

Thanks
Amy.

 

If a frame arrives on that trunk from an attatched device and it has no
802.1q tag embeded in the frame, the switch puts the frame into the native
VLAN which is configured on a port by port basis. Often used on IP phones
wgere the grames fro the phone itself are put into the voice VLAN but the
data frames (untagged) from the attatched PC are out into the native VLAN.

BSOF

 

It changes the VLAN that you are going to use as the native VLAN.

<SNIP from Cisco web page>
In 802.1q trunking, all VLAN packets are tagged on the trunk link,
except the native VLAN. The native VLAN packets are sent untagged on
the trunk link. Therefore, the native VLAN should be the same on both
switches configured for trunking. This way, you can deduce to which
VLAN a frame belongs when you receive a frame with no tag. By default,
VLAN 1 is the native VLAN on all switches.

In CatOS, the native VLAN can be changed by issuing the set vlan
<vlan-id><mod/port> command, where <mod/port > is the trunk port.

In native IOS, the native VLAN can be changed by issuing the
switchport trunk native vlan <vlan-id> interface command which is
configured on the trunk port.

Pete

 

Pete,

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.

 

I must say that I've never actually used this command myself - like
you I couldn't see a situation that required it to be used, although
I'm sure some of the other guys (like Walter Roberson or Hansang Bae)
would be able to describe a situation where it was necessary. (Guys?)

I think your assumptions are correct though. The port would be in the
native VLAN if you didn't assign it. AFAIK the command would change
the native VLAN from VLAN 1 to another VLAN of your choice (which must
be the same at either end of the trunk of course).

I can't really shed any more light on it than that, but I hope that
helps.

Pete

 

Program ended abnormally on 05/12/2003 03:46, Due to a catastrophic Pete
Mainwaring error:

The "native vlan" is not tagged, so it would allow you to send packets to a
non-802.1q device on that link. You could do the following:


vlan11 -- switch a --- switch b ---- switch c -- vlan11
vlan10 --- ' '---- vlan10

Where switch B doesn't understand 802.1q tagging, if you put the ports facing B
of A and C in native vlan 11, the devices on switch B would be would be able to
talk to devices on vlan 11 on all 3 switches, and the devices on A and C on vlan
10 would be able to talk to one another.