Switches flood frames for unknown destinations until they learn the MAC addresses of the devices. Broadcasts and multicasts are also flooded.

A redundant switched topology may cause broadcast storms, multiple frame copies, and MAC address table instability problems.

Links that will cause a loop are put into a blocking state.

Switches send messages called the bridge protocol data units (BPDUs) to allow the formation of a loop free logical topology. BPDUs continue to be received on blocked ports. This ensures that if an active path or device fails, a new spanning-tree can be calculated.

BPDUs contain information that allow switches to perform specific actions:

    * Select a single switch that will act as the root of the spanning-tree.
    * Calculate the shortest path from itself to the root switch.
    * Designate one of the switches as the closest one to the root, for each LAN segment. This switch is called the designated switch. The designated switch handles all communication from that LAN segment towards the root bridge.
    * Choose one of its ports as its root port, for each non-root switch. This is the interface that gives the best path to the root switch.
    * Select ports that are part of the spanning-tree. These ports are called designated ports. Non-designated ports are blocked.

for every switched network the following elements exist:

    * One root bridge per network
    * One root port per non-root bridge
    * One designated port per segment
    * Unused, or non-designated ports

Root ports and designated ports are used for forwarding (F) data traffic.

Non-designated ports discard data traffic. These ports are called blocking (B) or discarding ports.

All switches receive the BPDUs and determine that the switch with the lowest root BID value will be the root bridge.

In the listening state, data is not forwarded and MAC addresses are not learned. BPDUs are still processed.

In this state, data is not forwarded, but MAC addresses are learned from traffic that is received. The learning state lasts for 15 seconds and is also called the forward delay. BPDUs are still processed.

A switched internetwork has converged when all the switch and bridge ports are in either the forwarding or blocking state. Forwarding ports send and receive data traffic and BPDUs. Blocking ports only receive BPDUs.

Convergence on a new spanning-tree topology that uses the IEEE 802.1d standard can take up to 50 seconds. This convergence is made up of the max-age of 20 seconds, plus the listening forward delay of 15 seconds, and the learning forward delay of 15 seconds.

The Rapid Spanning-Tree Protocol is defined in the IEEE 802.1w LAN standard. The standard and protocol introduce new features:

    * Clarification of port states and roles
    * Definition of a set of link types that can go to forwarding state rapidly
    * Concept of allowing switches in a converged network to generate BPDUs rather than relaying root bridge BPDUs