From: Brian McGahan (brian@cyscoexpert.com)
Date: Sun Oct 06 2002 - 16:50:54 GMT-3
Chris,
You're overcomplicating the issue. Let's assume that your
native vlan is vlan 10. This means that all traffic received on a .1q
trunk link that does not have a tag, belongs to vlan 10. Remember that
tagging only happens on the trunk line, the tag values are not carried
over access links (non trunk links).
If traffic is generated by a host in vlan 10, and this traffic
must traverse the .1q trunk, the packet will not be tagged. When the
switch on the other end of the trunk receives the frame, it knows that
this frame belongs to vlan 10, since the packet is untagged, and the
native vlan is 10. Your native vlan must match between all switches,
otherwise you will have traffic leaking between vlans. That case is as
follows.
Take the same situation, a host in vlan 10 generates a packet
that traverses a .1q trunk. The switch which this host is attached has
vlan 10 designated as the native vlan, however the switch on the other
side has vlan 20 designated as the native vlan. When the switch on the
remote side receives this packet, it assumes that the packet belongs to
vlan 20, and forwards it appropriately. This results in incorrect
forwarding, since the packet should actually be destined for vlan 10.
HTH
Brian McGahan, CCIE #8593
Director of Design and Implementation
brian@cyscoexpert.com
CyscoExpert Corporation
Internetwork Consulting & Training
http://www.cyscoexpert.com
Voice: 847.674.3392
Fax: 847.674.2625
> -----Original Message-----
> From: nobody@groupstudy.com [mailto:nobody@groupstudy.com] On Behalf
Of
> Chris
> Sent: Sunday, October 06, 2002 2:13 PM
> To: P729; chenyan; ccielab
> Subject: Re: 802.1q native vlan
>
> I have been looking through the Docs and indeed it does say that
native
> vlan traffic is not tagged. I guess I have missed that when reading
the
> switching docs previously, and was always taught that all traffic is
> tagged.
>
> Thanks for the clarification.
>
> This would also mean that it is restricted to the native vlan then
right?
> Without a tag it could not be forwarded to any other vlan.
>
>
>
> ----- Original Message -----
> From: "P729" <p729@cox.net>
> To: "Chris" <clarson52@comcast.net>; "chenyan"
<chenyan@deeptht.com.cn>;
> "ccielab" <ccielab@groupstudy.com>
> Sent: Sunday, October 06, 2002 2:05 PM
> Subject: Re: 802.1q native vlan
>
>
> > "Any untagged frames will get tagged..."
> >
> > Mmmm...sounds kinda contradictory doesn't it? Actually, frames
assigned
> to
> > the native VLAN of the trunk are sent untagged across the trunk,
period.
> But
> > one might ask, "how would the switches on each end know when there's
a
> > native VLAN mismatch?" The answer for Cisco switches is through CDP.
If
> CDP
> > is disabled or not available, then they wouldn't know and you can
pretty
> > much bridge the two VLANs together and maybe not know it...
> >
> > Regards,
> >
> > Mas Kato
> > https://ecardfile.com/id/mkato
> > ----- Original Message -----
> > From: "Chris" <clarson52@comcast.net>
> > To: "chenyan" <chenyan@deeptht.com.cn>; "ccielab"
> <ccielab@groupstudy.com>
> > Sent: Sunday, October 06, 2002 9:48 AM
> > Subject: Re: 802.1q native vlan
> >
> >
> > Any untagged frames will get tagged to the native vlan and travel
the
> native
> > vlan.
> >
> >
> > ----- Original Message -----
> > From: "chenyan" <chenyan@deeptht.com.cn>
> > To: "ccielab" <ccielab@groupstudy.com>
> > Sent: Sunday, October 06, 2002 11:13 AM
> > Subject: 802.1q native vlan
> >
> >
> > > hi,guys
> > >
> > > I want to know why there is native vlan for 802.1q and what is
that
> for?
> > >
> > > Thanks
This archive was generated by hypermail 2.1.4 : Tue Nov 05 2002 - 08:35:42 GMT-3