Are native vlan packets really untagged?

From: Hiro Eiji (hpnkpn103@yahoo.co.jp)
Date: Sat Feb 18 2006 - 15:09:51 GMT-3

Hi,

I have a question about how native vlan works on cisco catalyst switches.

My understanding about native vlan was that the native vlan packets are
always sent untagged on the 802.1Q trunk link.

I had a chance to capture native vlan packets on a 802.1Q trunk link the
other day.
To my surprise, the captured data showed the result otherwise.
The native vlan packets were in fact tagged with the native vlan ID.

Here is the topology that I used.

Cat3550 - Damn Hub - Cat3550
                |
             My PC

When I changed the native vlan ID to 50 at both ends, the native vlan
packets were tagged with vlan ID 50.

Those native tagged vlan packets included the STP BPDUs, CDP frames
and ICMP packets between native vlan segments.

Has anyone experienced similar phenomena or have any idea why those native
vlan packets are tagged?
Or if someone could possibly run a quick test on the equipment and see if
native vlan packets are tagged,
that would be highly appreciated. ( and if you do, pls make sure you use
non-intel NIC to capture the packets.
Otherwise you cannot see the vlan tags ).

Thanks,

Hiro Eiji

This archive was generated by hypermail 2.1.4 : Wed Mar 01 2006 - 11:28:18 GMT-3