[c-nsp] Untagged packets on trunk interfaces

[Kristian Larsson]

On Tue, Feb 05, 2008 at 10:53:49PM +0000, Phil Mayers wrote:
> 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?

no, 123 vill be untagged while 456 will carry a
tag.

> 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.

I believe you are right.

  -K

-- 
Kristian Larsson                                        KLL-RIPE
Network Engineer & Peering Coordinator      SpriteLink [AS39525]
+46 704 910401			              kll at spritelink.net