[c-nsp] backup with STP

[Andris Zarins]

Completely agree with Steinar about RSTP and MST. MST is far more
scalable, especially in SP networks. If there are 5 VLANs in your
network without any chances to increase - run RSTP and things will be
fine. If there are hundreds or thousands of VLANs - with RTSP there is
an STP instance per VLAN, and even more - they are all the same - same
root bridges, same links blocked... only resources wasted to do all
computations for all instances each and every time something changed in
your network ;)

If you intend to run RSTP to load balance between some links, no
problems - create two (or more, if you need to) instances of MST - and
load balance between them. Don't be afraid of MST :)

Second - completely agree with Aivars - don't run any kind of STP with
devices NOT under your control. Interoperability is only one of problems
you might face, perhaps even not the most painful one. Imagine that
under some conditions, customer starts telling your network, that his
device is the root bridge, and if your network believes it, I guess that
might mean somewhat like 'dead end'... 

If you need redundancy - think about some layer-3 solutions. 

Andris


-----Original Message-----
From: cisco-nsp-bounces at puck.nether.net
[mailto:cisco-nsp-bounces at puck.nether.net] On Behalf Of
sthaug at nethelp.no
Sent: Wednesday, September 28, 2005 3:45 PM
To: bambi at hughes.com.au
Cc: cisco-nsp at puck.nether.net
Subject: Re: [c-nsp] backup with STP

> 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
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