[j-nsp] interoperation between MSTP and old STP

[Ben Dale]

Hi Victor,

On 15 May 2015, at 6:06 pm, Victor Sudakov <vas at mpeks.tomsk.su> wrote:

> Colleagues,
> 
> 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.  

> 
> 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/

The configuration and outputs are IOS-based and there are a few PVST+ interop discussions, but the supporting protocol description and break-down is truly excellent.

Cheers,

Ben