[c-nsp] backup with STP

[lists at hojmark.org]

> In the context of the original question, one of the driving
> points for the posters decision making was support for legacy
> dot1D devices. 

I'm sure interoperability with 802.1D and 802.1Q CST has also
been a major factor for IEEE when they made 802.1s and 802.1w...

>From a SP point of view I would think that being standards-based
(when at all possible) would be a requirement. MSTP does fulfill
that requirement, while Rapid-PVST doesn't. The option to fall
back to (also standards-based) 802.1D or 802.1Q CST doesn't like
an extremely attractive option.

> From my memory, all dot1D devices were held in a "virtual
> region" outside MST's IST instance.

The MSTP region looks to the 802.1D device like a single large
bridge. You could say that from the MSTP point of view it looks
like another region, but from a 802.1D point of view of cause
there is no such thing as regions.

> In this situation is a dot1D BPDU forwarded into the IST and
> as such broadcasted to all STP instances or is there another
> STP instance outside the IST for the dot1D devices?

It is correct that the IST (instance 0) runs on all switches
in the region. So does any other instance that you define for
the region. Actually that's the whole point.

It is also correct that the IST is used for providing backwards
compatibility with 802.1D, but also with 802.1Q CST and PVST+.
And yes, BPDUs are relayed with legacy STP (as opposed to the
hello-based communication with Rapid), but the same is true for
PVST+.

> Like I said, it's been a while but I thought legacy dot1D
> support was a weak point in MST.

Well, I can't say I have extensive real-world experience with
interoperating MSTP with 802.1D. (Actually, it's hard to find
large installations where they run 802.1D, which should not be
too surprising).

The only experience I have with mixing the two are lab tests,
but I must say it's worked very well for me.

-A
PS: Again, I'm not saying MSTP is 'better' or 'worse' than
Rapid-PVST. They're just different. I use both, but for different
things.