[c-nsp] VTP and Vlan 1

Tue Aug 26 08:56:49 EDT 2008

Hi Paul
Dont worry, I did the same at the beginning :-)

So UDLD on dot1Q uses the native VLAN while with ISL it stays within VLAN 1
no matter if we changed the native vlan or not?
This means that with ISL trunks UDLD behaves similar to CDP/VTP ?

Thanks

best regards

Michel

On 26/08/2008, Paul Cosgrove <paul.cosgrove at heanet.ie> wrote:
>
> Hi Michel,
>
> Appologies for confusing the issue. You are of course correct about VTP,
> which does use vlan 1.
>
> UDLD is not sent with a dot1q tag, but is associated with vlan 1 on ISL
> trunks.  Changing the (dot1q) native vlan on the trunk has no effect on
> how UDLD is sent over ISL, it is still sent on vlan 1.
>
> Paul.
>
> Michel Grossenbacher wrote:
> > Paul, indeed DTP is sent over the native VLAN, but VTP is pretty sure
> still
> > over VLAN 1. I did a trace and mixed VTP with DTP, hence I said its using
> > the native VLAN. But after I did some more traces the VTP packets did not
> > show any VLAN informations "anymore" (actually they never did I only hit
> the
> > wrong line within wireshark ;) ).
> > So Im quite sure VTP and CDP are not sent via the native VLAN, after I
> > changed it from VLAN 1 to VLAN 10. Probably have to have a look with ISL
> > too.
> >
> > Mike, I think I know what you mean, per definition (AFAIK) all VLANs get
> > encapsulated by ISL, while with dot1Q all but the native one get a Tag.
> But
> > within an ISL trunk Cisco defines a native VLAN (default is VLAN 1, same
> as
> > dot1Q) and you can configure it the same way as for a dot1Q one so I'd
> say
> > UDLD will use that one. I guess it will still be encapsulated but I did
> > never check that.
> > Do a *show interface trunk* if you configured an ISL trunk and you'll see
> it
> > at the top.
> >
> > Michel
> >
> > 2008/8/25 Paul Cosgrove <paul.cosgrove at heanet.ie>
> >
> >> Hi Michel,
> >>
> >> You may have been right the first time there.  I think VTP does indeed
> >> use the native vlan, not necessarily vlan 1.  DTP is also sent on the
> >> native vlan, untagged and tagged in its case.
> >>
> >> Paul.
> >>
> >> Michel Grossenbacher wrote:
> >>> A little correction on my answer, VTP does not use the Native VLAN :-)
> >>>
> >>> Here is what I found regarding the use of VTP and VLAN1:
> >>> The Case of VLAN 1
> >>>
> >>> You cannot apply VTP pruning to VLANs that need to exist everywhere and
> >> that
> >>> need to be allowed on all switches in the campus, in order to be able
> to
> >>> carry VTP, Cisco Discovery Protocol [CDP] traffic, and other control
> >>> traffic. However, there is a way to limit the extent of VLAN 1. The
> >> feature
> >>> is called VLAN 1 disable on trunk. The feature is available on Catalyst
> >>> 4500/4000, 5500/5000, and 6500/6000 series switches in CatOS software
> >>> release 5.4(x) and later. The feature allows you to prune VLAN 1 from a
> >>> trunk, as you do for any other VLAN. This pruning does not include all
> >> the
> >>> control protocol traffic that is still allowed on the trunk (DTP, PAgP,
> >> CDP,
> >>> VTP, and others). However, the pruning does block all user traffic on
> >> that
> >>> trunk. With this feature, you can keep the VLAN from spanning the
> entire
> >>> campus. STP loops are limited in extent, even in VLAN 1. Configure VLAN
> 1
> >> to
> >>> be disabled, as you would configure other VLANs to be cleared from the
> >>> trunk:
> >>>
> >>> UDLD uses native VLAN in order to talk to the neighbor. So, in a trunk
> >> port,
> >>> the native VLAN must not be pruned in order for UDLD to work properly.
> >>>
> >>
> http://www.cisco.com/en/US/tech/tk389/tk689/technologies_tech_note09186a0080890613.shtml
> >>> Sorry for the confusion.
> >>>
> >>> best regards
> >>>
> >>> Michel
> >>>
> >>>
> >>> On 25/08/2008, Michel Grossenbacher <pashtuk at gmail.com> wrote:
> >>>> Hi Mike
> >>>> Actually VLAN 1 is not pruning-eligible so you can not prune VLAN 1
> from
> >> a
> >>>> trunk. However you can remove it from the trunk.
> >>>> If you remove it from the trunk and change the native VLAN for the
> >> trunk,
> >>>> VTP will then use the new native VLAN for updates.
> >>>> best regards
> >>>>
> >>>> Michel
> >>>>
> >>>>
> >>>>  On 25/08/2008, Mike Louis <MLouis at nwnit.com> wrote:
> >>>>> List,
> >>>>>
> >>>>> I just read in a practice test for an upcoming cert that Vlan 1 is
> used
> >> to
> >>>>> carry VTP advertisements. However, it is possible to prune vlan 1
> from
> >> trunk
> >>>>> links. Will VTP continue to function without Vlan 1 being enabled on
> >> the
> >>>>> link? Has this changed in more recent IOS releases?
> >>>>>
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