[c-nsp] pvst and mst

[Andrew Fort]

David J. Hughes wrote:
>  From our experience, dodge MST.  Rapid PVST can happily co-exist with 
> "standard" PVST so your legacy Cisco switches will be well supported.  
> Migration from PVST to RPVST is a much more pleasant experience (and a 
> lot easier to revert if it all goes pear shaped).
> 
> 
> David
> ...

I'm dragging up old threads - must be overworked :)

We have a reasonable (2 year+) experience with a MSTP implementation (on 
sup720 pfc3a) across a reasonable sized metro ethernet network.

Here are my recommendations/war scars:

  1 - avoid mstp.  use rstp if you have a requirement for stp, and just 
deal with blocked links for all traffic.  really.
  2 - if you must use mstp, see 1.  if you reckon you know better, 
remember to pre-allocate _every_ vlan to an instance.  if your 
allocation policy (you're a metro ethernet provider, say) doesn't allow 
for this, and you want to play traffic tweaking and 'optimise' paths 
through your rings, see 1.  if you won't pre-allocate, you're going to 
have to cause boundary ports temporarily as your hashes don't match on 
your switches.  I haven't found a real solution to use other than using 
something like Cisco CNS; it is still one of those "see you on the other 
side, hope it all goes well" types of configuration changes that are 
best designed around.
  3 - keep it simple.  you must be able to describe the rules about how 
it all works in about two steps, hence rstp is better: "block everything 
here".  consider your support staff (many instances = head hurts).
  4 - filter BPDUs carefully.  i mean, consider things other than your 
customer facing ports.  Be careful of bridging functions from various 
media back to ethernet - double check that the BPDU filtering works, and 
really does what you want.  last thing you want is the switch processing 
some .1d BPDU it gets from a random ATM port on a CWAN card and decides 
to eat it and report all other ports as boundary ports since it has an 
STP domain coming from somewhere it doesn't consider a valid port :).
  5 - if you are still reading, re-read 1.

If you're not a metro ethernet provider, and your network is static, you 
may enjoy MSTP just fine.  If you have a dynamic network, especially one 
that requires some configuration of the MSTP elements, my advice is 
avoid MSTP.

Basically, the prevailing wisdom of the 'old days' still stands true:

  - Limit the size of your layer 2 domains.

Some sort of VPLS looks like it may well be the way to go to manage the 
size of the pure ethernet domains.  At least this year... :)

-andrew