As we have seen, the transparent bridging algorithm floods unknown and broadcast frames out of all the ports that are in the same VLAN as the received frame. This causes a potential problem. If the network devices that run this algorithm are connected together in a physical loop, flooded frames (like broadcasts) are passed from switch to switch, around and around the loop, forever. Dependent upon the physical connections involved, the frames can actually multiply exponentially due to the flooding algorithm, which can cause serious network problems.
There is a benefit to a physical loop in your network: it can provide redundancy. If one link fails, there is still another way for the traffic to reach its destination. In order to allow the benefits derived from redundancy, without breaking the network because of flooding, a protocol called spanning tree was created. Spanning tree was standardized in the IEEE 802.1d specification.
The purpose of the spanning tree protocol (STP) is to identify and temporarily block the loops in a network segment or VLAN. The switches run the STP, which involves electing a root bridge or switch. The other switches measure their distance from the root switch. If there is more than one way to get to the root switch, there is a loop. The switches follow the algorithm to determine which ports must be blocked in order to break the loop. STP is dynamic; if a link in the segment fails, ports that were originally blocking can possibly be changed to forwarding mod
http://www.cisco.com/en/US/tech/tk389/tk214/technologies_tech_note09186a00801767 20.shtml#spans
Slow DHCP for Macs...
http://www.macwindows.com/slow-dhcp-switch.html
Disabling...
http://techjournal.318.com/scripts/disabling-spanning-tree-on-cisco-switches/
On Appletalk...
http://support.apple.com/kb/TA25258?viewlocale=en_US