[c-nsp] Untagged packets on trunk interfaces

[Phil Mayers]

Brandon Price wrote:
> Thanks for the reply!!
> 

Please don't remove the list from the Cc: - the replies in the archived 
may help others

>>From the link you sent:
> 
> "The vlan dot1q tag native command is a global command that configures
> the switch to tag
> native VLAN traffic, and admit only 802.1Q tagged frames on 802.1Q
> trunks, dropping any
> untagged traffic, including untagged traffic in the native VLAN" 
> 
> Which tag is being applied to this formally "native" traffic?

I've never used this feature - because it's a chassis global it's 
useless - so I'm not certain, but I think it's fair to assume the native 
vlans tag number.

int gX/Y
  switchport mode trunk
  switchport trunk native vlan 123
  switchport trunk allowed vlan 123,456

...vlans 123 & 456 will come out tagged. I guess in this case, the only 
difference between a native and allowed vlan is... erm... the name?

FYI, you can also try this:

int gX/Y
  switchport mode trunk
  switchport trunk native vlan 999
  switchport trunk allowed vlan 123,456

...that is - 999 is a dummy vlan BUT is not in the allowed vlan list; I 
believe this stops it forwarding traffic.

Note that certain untagged packets will always come out of a Cisco if 
their functions are enabled e.g. CDP, 802.1d STP, MST (I think?)