From: shizzle wizzle kizzle (mcslikealbofbacon@yahoo.com)
Date: Sat Apr 30 2005 - 10:08:34 GMT-3
Another key concept to keep in mind with MST is what happens at the boundary of an MST region and a PVST region(if you have a switch running MST connected to a switch that doesn't).
1) One of the most important things to remember at the MST/PVST boundary is that either one of the switches must be the root for ALL vlans otherwise the link will be go into "Broken" state and shut down. So basically, no root bridge load balancing at a MST/PVST boundary. For example, if the switch running MST is the root for vlans 2-1000 but then you configure the PVST switch on the other end of the link to be root for vlan 500, the spanning tree instance for vlan 500 will shut down that link due to "root inconsistency."
2) Another thing to keep in mind is that MST wants to appear to the PVST region as if it is running PVST as well so it uses MST instance 0 to represent the entire MST domain as a CST(common spanning tree) to the PVST domain. This is important because you'll notice that the 3550s in PVST add the vlan # to the bridge priority. However, when you're running MST, it adds the MST instance number instead of the vlan #. So if you had vlan 100 before, your priority was 32768+100. Now it will be 32768.
3) Like Brian said below, it's pretty straightforward and MST is very useful when you have a large number of vlans which have pretty much the same L2 path since it reduces load and BPDUs. However, the problems come at the MST/PVST boundary if you ever do a phased migration.
Shinwoo
kwasi-ccie@comcast.net wrote:
Thanks, Brian.
Your comment "Just make sure that everyone in the same MST domain agrees on which VLANs are in which instance or you can create black holes or loops." answers my question.
-------------- Original message --------------
> The advantage of MST is that the STP instances are
> user-definable, where with CST there is only one instance, and with PVST
> there is one instance per VLAN. When you ask "do they have to be mapped
> to only one instance such as instance 1?", that is how CST works. With
> MST you can say for example VLANs 1-10 are in instance 1, VLANs 11-20
> are in instance 2, VLANs 21 - 30 are in instance 3... etc. Just make
> sure that everyone in the same MST domain agrees on which VLANs are in
> which instance or you can create black holes or loops.
>
> Design-wise what you should do is look for VLANs that you want
> to follow the same layer 2 forwarding path and put them in the same
> instance. This eliminates the unnecessary overhead of PVST, but gives
> you more control that just a common spanning-tree does.
>
>
> HTH,
>
> Brian McGahan, CCIE #8593
> bmcgahan@internetworkexpert.com
>
> Internetwork Expert, Inc.
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>
> > -----Original Message-----
> > From: nobody@groupstudy.com [mailto:nobody@groupstudy.com] On Behalf
> Of
> > kwasi-ccie@comcast.net
> > Sent: Friday, April 29, 2005 9:40 AM
> > To: ccielab@groupstudy.com
> > Subject: Help understanding MST
> >
> > Hi:
> >
> > I am trying to understand how MST works. One aspect that I am not
> clear
> > on is the fact that for any switch to be part of an MST Region it must
> > have the same VLAN-to-Instance mapping, beside the same MST name and
> > version #.
> >
> > Does this mean all VLANs on a switch in an MST region can be mapped to
> > only one MST instance? For instance can I have VLAN 10 and VLAN 20
> (on an
> > MST switch in an MST region) mapped to say instance s 1 and 2,
> > respectively? Or do they have to be mapped to only one instance such
> as
> > instance 1?
> >
> > If I can only map multiple VLANs to only one instance, then what is
> the
> > need for having instances 1 - 15 for internal MST switches?
> >
> > I have read CCO but the examples are not clear enough for me on this
> > issue.
> >
> > Your comments are welcome.
> > Thanks.
> >
> >
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