after having examined all the BPDUs transferred between the various switches by means of tcpdump and wireshark, I think I found the bug here which is really serious: a non-root SLM2008 sends BPDU frames where it steals the root's bridge ID and claims it as its own! This means the SLM2008 sends the root's bridge ID in both the source and root ID fields which is not allowed.
Either it should not send back BPDUs to the root at all, or it must use its own ID in the source field and mention the root ID only in the root's ID field.
If someone from Cisco reads this here: this bug should be fixed by the next firmware release as it can render a whole network unusable if STP is activated on the SLM2008 and it is used together with other switches.
Thanks and best regards,
PS: The examination also answered one of my questions - the protocol used is RSTP, at least by type and BPDU length.
--- old message follows ---
Now I own three of the little SLM2008 which seem to be designed quite well and mostly behave as expected, but one annoying problem bites me: the spanning tree protocol behaves strange as soon as I connect switches from other manufacturers to it.
A larger D-Link switch with RSTP and fallback STP enabled causes all connected and STP-enabled (without portfast) SLM2008s to go into blocking mode at the HOST (!) ports, the D-Link in turn blocks all ports where an SLM2008 is attached.
As the SLM2008 has almost no options except for portfast and enabling/disabling STP, I tried to fiddle around with the D-Link, logically as well as by random trial-error attempts, but there is no chance at the moment. It is always the same and very unsatisfying.
Using a cheaper Netgear (in the range of the SLM2008) instead of the D-Link works fine, but also the D-Link works perfectly with the Netgear. Why does the SLM2008 not fit in here?
To me it's also quite confusing that the manual states "STP" as the feature's name, but the open source note talks about RSTP... which variant is really implemented in the SLM2008?
Thanks in advance for any help!