Spanning Tree Protocol

The purpose of the spanning tree protocol (STP) is to allow switches to automatically create a loop-free set of paths, even in a complex network with multiple paths connecting multiple switches. It provides the ability to dynamically create a tree topology in a network by blocking any packet forwarding on certain ports, and ensures that a set of Ethernet switches can automatically configure themselves to produce loop-free paths.

Operation of the spanning tree algorithm is based on configuration messages sent by each switch in packets called Bridge Protocol Data Units, or BPDUs. Each BPDU packet is sent to a destination multicast address that has been assigned to spanning tree operation.

The process of creating a spanning tree begins by using the information in the BPDU configuration messages to automatically elect a root bridge. The election is based on a bridge ID (BID) which, in turn, is based on the combination of a configurable bridge priority value (32,768 by default) and the unique Ethernet MAC address assigned on each bridge for use by the spanning tree process, called the system MAC. Bridges send BPDUs to one another, and the bridge with the lowest BID is automatically elected to be the root bridge.

Once a root bridge is chosen, each non-root bridge uses that information to determine which of its ports has the least-cost path to the root bridge, then assigns that port to be

the root port (RP). All other bridges determine which of their ports connected to other links has the least-cost path to the root bridge. The bridge with the least-cost path is assigned the role of designated bridge (DB), and the ports on the DB are assigned as designated ports (DP).

The bridge multicast group MAC address is 01-80-C2-00-00-00. Vendor-specific spanning tree enhancements may also use other addresses. For example, Cisco per-VLAN spanning tree (PVST) sends BPDUs to address 01-00-0C-CC-CC-CD.

The path cost is based on the speed at which the ports operate, with higher speeds resulting in lower costs. As BPDU packets travel through the system, they accumulate information about the number of ports they travel through and the speed of each port. Paths with slower speed ports will have higher costs. The total cost of a given path through multiple switches is the sum of the costs of all the ports on that path.

While blocked ports do not forward packets, they continue to receive BPDUs.

The Rapid Spanning Tree Protocol (RSTP) sends BPDU packets every two seconds to monitor the state of the network, and a blocked port may become unblocked when a path change is detected.

Spanning Tree Port States

Disabled

A port in this state has been intentionally shut down by an administrator, or has automatically shut down because the link was disconnected. This also could be a port that has failed, and is no longer operational.

Blocking

A port that is enabled, but is not a root port or designated port could cause a switching loop if it were active. To avoid that, the port is placed in the blocking state. No station data is sent or received over a blocking port.  A blocking port may also transition to the forwarding state if other links fail. BPDU data is still received while a port is in the blocking state.

Listening

In this state, the port discards traffic but continues to process BPDUs received on the port, and acts on any new information that would cause the port to return to the blocked state. Based on information received in BPDUs, the port may transition to the learning state. The listening state allows the spanning tree algorithm to decide whether the attributes of this port, such as port cost, would cause the port to become part of the spanning tree or return to the blocking state.

Learning

In this state, the port does not yet forward frames, but it does learn source addresses from any frames received and adds them to the filtering database. The switch will  populate the MAC address table with packets heard on the port (until the timer expires), before moving to the forwarding state.

Forwarding

This is the operational state in which a port sends and receives station data. Incoming BPDUs are also monitored to allow the bridge to detect if it needs to move the port into the blocking state to prevent a loop.