[c-nsp] Maximum spannig tree instances

[Jon Lewis]

On Fri, 17 Jul 2009, Saku Ytti wrote:

> On (2009-07-17 09:09 -0500), Geoffrey Pendery wrote:
>
>> I'm not trying to say "MST is never useful and always terrible", but rather:
>> "MST doesn't fit all scenarios.  For many scenarios, RPVST is much
>> better, and it's a shame that we've only got an open standard MST,
>> rather than two open standards to cover both scenarios."
>
> Not arguing against, but would you happen to have example where
> MST does not fit? All my respect to the person who decidedly
> engineers L2 network with more then 65 planned and documented
> topologies[0], and succeeds to deliver higher SLA than what is possible
> with fewer.

What about a setup where you have a dual router/switch (6500s) core and 
lots of redundantly uplinked smaller switches with customer servers 
connected.  This seems like a perfect case for MST.  But, suppose you add 
a few trunks (perhaps even redundant) to the core switches from metro 
ethernet providers who bring you metro ethernet customer connections, each 
as their own vlan.  Now you have switches participating in STP that you 
don't control.

In cisco's PVST -> MST migration document, they say MST can interact with 
PVST, but that you should make sure the MST bridge is the root for all 
VLANs allowed on the trunk to a PVST bridge.  They don't say what happens 
if the PVST bridge becomes the root for the CST, but they make it sound 
like you don't want to find out.

http://l.pr/a4183/PVST-MST-Migration

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  Jon Lewis                   |  I route
  Senior Network Engineer     |  therefore you are
  Atlantic Net                |
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