[c-nsp] Maximum spannig tree instances

[Phil Mayers]

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.

"Does not fit" need not be limited solely to the number of available 
topologies.

Personally I find the (lack of) graceful change control the big killer. 
Our network is simply *NOT* capable of "defining the mappings ahead of 
time and never changing them" because of inherited legacy. I cannot 
tolerate the outages we'd need to incur every time a VLAN was added or 
removed, and I'm certainly not prepared to spend the many man-months 
re-numbering every vlan tag on campus into some arbitrary grouping just 
so I can run MST, when PVST works *now*.

I probably *could* run MST, after a lot of work, but why would I want to?

In addition, I have serious concerns about the scope of instance 0, 
particularly in the topology we run (a collapsed core/distribution 
triangle).

When I tried it on the bench, I could not come up with an MST setup that 
worked.