AKA STP or 802.1d

Spanning tree protocol is used for switches in large organisations to prevent loops, and allow for redundancy.

Switches with the lowest priority "wins" the election process.

Fastlink (sometimes called portfast) enables forwarding on a port by default and disables it if it's detected as being part of a loop later. This makes convergance times much faster, at the expense that if a loop is formed then you end up with traffic looping around until the switches determine whats going on. Fastlink needs to be enabled on machines that require their ports to be up quickly (such as PXE booting machines).

See also: