Port Roles in STP

This section describes the function of the port roles in STP. The use and implementation of port roles differs slightly between different versions of STP.

Port roles include:

• Designated port—Responsible for forwarding the traffic coming from the spanning-tree root bridge onto this segment.
• Backup port—A backup for the designated port.
• Root port—The root port of a bridge is the port closest to the root bridge. It is responsible for carrying the traffic from this segment toward the root bridge, using the shortest path to reach the root.
• Alternate port—A backup for the root port.
• Master port (MSTP only)—The best possible path to reach the common internal spanning-tree (CIST) root bridge from any MSTP region.

Related Topics

• Decision Sequence for a Loop-Free STP Topology
• Key Concepts in Spanning Tree Protocols
• Multiple Spanning Tree Protocol
• Overview of Spanning Tree Protocol on Juniper Networks MX Series 3D Universal Edge Routers
• 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