[j-nsp] interoperation between MSTP and old STP

[Victor Sudakov]

Ben Dale wrote:
> > 
> > I have several EX4200 switches with redundant links, all are running
> > MSTP with a couple of MSTIs.
> > 
> > If I include an older switch which can only run old-fashioned STP,
> > will they interoperate and still keep the topology loop-free? What if
> > it can run STP and RSTP, but not MSTP?
> 
> They will interoperate, however there are key differences to be aware of:

> - STP/RSTP forms topology with STP bridges regardless of whether
> correct VLANs are trunked on ports - this can mean that topologies
> may form that isolate VLAN segments if you haven't configured
> trunked ports correctly on all links

> - STP/RSTP bridges will treat an MSTP network (regardless of how
> many switches it contains) as single contiguous bridge, so you may
> find that the ports that block in your RSTP network aren't quite
> where you expected them to be when simply counting the radius from
> the root bridge.  

Ben, thank you for your explanation. 

> 
> > 
> > A link to some good documentation is also appreciated.
> > 
> > TIA for any input.
> 
> I highly recommend Petr Lapukhov's work here:
> 
> http://blog.internetworkexpert.com/2010/02/22/understanding-mstp/

I remember reading it when I set up MSTP for the first time, but I 
skipped the multi-region part of if. And the legacy STP link should
look to MSTP switches as a foreign region, from what I was told.


-- 
Victor Sudakov,  VAS4-RIPE, VAS47-RIPN
sip:sudakov at sibptus.tomsk.ru