802.1D - Media Access Control (MAC) Bridges (Spanning Tree Protocol STP)

802.1D-1993 describes amongst other topics the STP, which was later replaced by RSTP 802.1w (later integrated into 802.1D-2004)

STP avoids loops in Layer 2 networks while allowing redundant links.

Switches communicate amongst each other by BPDUs, Root-bridge is elected and a 'spanning tree' is calculated according to a set of rules.

Disadvantages: relatively old protocol, convergence time of approx. 30 seconds are often far to slow for modern networks.

Links:

http://www.javvin.com/protocolSTP.html

http://en.wikipedia.org/wiki/IEEE_802.1D

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