Key Concepts in Spanning Tree Protocols

This section outlines key concepts important when using STPs.

Bridge Protocol Data Units

Bridges use special frames known as bridge protocol data units (BPDUs) to transmit STP information between bridges. BPDUs carry information about bridge IDs and root path costs. STP uses BPDU packets to exchange information with other bridges. STP uses the information provided by the BPDUs to elect a root bridge, identify root ports for each bridge, identify designated ports for each physical LAN segment, and prune specific redundant links to create a loop-free tree topology.

There are two types of BPDUs:

• Configuration BPDUs—Under normal circumstances, once the root bridge is selected, configuration BPDUs are originated by the root bridge and flow outward along active paths radiating away from the root bridge.
• Topology change notification BPDUs—If there is any change in the topology, for example, a link is disconnected or a bridge fails, the bridge sends topology change notification (TCN) BPDUs to advertise the change. The TCN BPDUs flow upstream toward the root bridge to alert it that the active topology has changed.

Root Bridge

The STP root bridge is the base of the spanning tree topology, much like roots are the base of a tree. All redundant paths to the root bridge within the spanning-tree network are disabled by putting them into a blocked mode. The root bridge is chosen by all of the bridges, based on the results of the BPDU exchange process.

Bridge ID

A bridge ID (BID) is a single, 8-byte field that identifies a bridge through two subfields: the bridge priority and the MAC address of the bridge.

Path Cost

Bridges use the concept of path cost to evaluate how close they are to other bridges. The path cost depends on the bandwidth of the link. STP uses algorithms to evaluate least-cost paths as most attractive.

Port Roles

Bridges determine the root bridge and then compute the port roles based on the BPDU information, for any spanning-tree version.

Related Topics

• Decision Sequence for a Loop-Free STP Topology
• Multiple Spanning Tree Protocol
• Overview of Spanning Tree Protocol on Juniper Networks MX Series 3D Universal Edge Routers
• Port Roles in STP
• Rapid Spanning Tree Protocol Port States and Port Roles
• Restrictions and Cautions for Implementing STP
• Spanning Tree Protocol Operation
• Spanning Tree Protocol States
• STP Scaling and Performance on Juniper Networks MX Series 3D Universal Edge Routers
• VLAN Spanning Tree Protocol