[c-nsp] Maximum spannig tree instances

[A.L.M.Buxey at lboro.ac.uk]

Hi,

> Rapid per VLAN spanning tree has scaling limitations in many environments.
> Which is why some people go with MST instead.

we hit the PVST limits so moved to RPVST..once we
hit those limits we're sure to be going to MST ;-)

alan