[c-nsp] backup with STP

[sthaug at nethelp.no]

> The only words I would say about MST are - DON'T GO THERE.
> 
> If your environment is capable of running Rapid PVST then it is 
> certainly what you want to be running.  RPVST+ extends the PVST+ that 
> you know and love to include rapid response to topology changes.  MST 
> gives you a totally different set of semantics (and issues/problems) to 
> get your head around.  Remember you no longer have an STP instance per 
> vlan.  The interaction with anything in your network that doesn't speak 
> MST can be troublesome and "interesting" to say the least.  RPVST+ is 
> the end-game and MST was a bump in the road on the way.

Unfortunately life isn't that simple if you want to scale to something
like 4K VLANs. RPVST+ is extremely simple to configure, but absolutely
not scalable precisely because it runs a Spanning Tree instance per VLAN.

In any bigger setting you need some form way of running a Spanning Tree
instance for a *group* of VLANs. MST is one of these, Extreme EAPS is
another. Problems here - EAPS is proprietary, MST supposedly not, but I
wouldn't trust MST interoperability today.

So, for some of us, RPVST+ is absolutely not the end-game.

Steinar Haug, Nethelp consulting, sthaug at nethelp.no