From: Brian McGahan (bmcgahan@internetworkexpert.com)
Date: Fri Apr 29 2005 - 12:56:19 GMT-3
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.
http://www.InternetworkExpert.com
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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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