only a doubt about STP (802.1d).
Suppose to have a simple loop between three switches (in purely daisy chain).
What if one of them (say SWA) doesn't support STP?
Can I use STP nevertheless ?
The other two switch are able to manage the port block/forwarding algorithm, so mantaining connectivity without looping ?
Thank you in advance
It depends on whether the third-party switch passes the BPDUs through transparently. If it does so, the STP will block a port to prevent the loop. It would be as if you had a crossed-cable instead of the third-party switch.
If it doesn't, then the network will meltdown.