I have 2x SLM2008, with a three VLANs defined, and the link between the switches being a trunk. Works like a charm, generally.
Now, when I switch on STP, it appears STP only runs on one of the VLANs. Root bridge is elected properly in that single VLAN, but in the other two VLANs nothing happens. Did a network sniff, and no BDPUs go over those "other" VLANs. Consequently, both switches think they're root for those two VLANs.
Well, that's part of the issue: there doesn't appear to be any option to set-up per-VLAN spanning tree, but the switches DO compute the spanning tree for each VLAN individually (but incorrectly for all VLANs but one).
There is one trunk connecting this switches, all VLANs are present on that trunk, all in tagged form. The same VLANs are defined in both switches. VLAN 1 is the native VLAN of the trunk, but VLAN 1 is otherwise unused (not assigned to any access port). When I enable STP, it only runs on VLAN 9, and elects a root bridge correctly there. On the other VLANs, both switches think they are root bridge for that VLAN (no BDPUs on those VLANs). Weird, eh?