BigIron RX Series Configuration Guide 35753-1002253-01Chapter13Configuring Rapid Spanning Tree ProtocolOverview of Rapid Spanning Tree ProtocolRSTP provides rapid convergence and takes advantage of point-to point wiring of the spanning tree.Failure in one forwarding path does not affect other forwarding paths. RSTP improves the operationof the spanning tree while maintaining backward compatibility.NOTEThe total number of supported STP, RSTP, or MSTP indices is 128.Bridges and bridge port rolesA bridge in an RSTP rapid spanning tree topology is assigned as the root bridge if it has the highestpriority (lowest bridge identifier) in the topology. Other bridges are referred to as non-root bridges.Unique roles are assigned to ports on the root and non-root bridges. Role assignments are basedon the following information contained in the BPDU (RSTp packet):• Root bridge ID• Path cost value• Transmitting bridge ID• Designated port IDRSTP algorithm uses this information to determine if the RST BPDU received by a port is superior tothe RST BPDU that the port transmits. The two values are compared in the order as given above,starting with the Root bridge ID. The RST BPDU with a lower value is considered superior. Thesuperiority and inferiority of the RST BPDU is used to assign a role to a port.If the value of the received RST BPDU is the same as that of the transmitted RST BPDU, then theport ID in the RST BPDUs are compared. The RST BPDU with the lower port ID is superior. Port rolesare then calculated appropriately.The port’s role is included in the BPDU that it transmits. The BPDU transmitted by an RSTP port isreferred to as an RST BPDU, while it is operating in RSTP mode.Ports can have one of the following roles:• Root – Provides the lowest cost path to the root bridge from a specific bridge• Designated – Provides the lowest cost path to the root bridge from a LAN to which it isconnected• Alternate – Provides an alternate path to the root bridge when the root port goes down• Backup – Provides a backup to the LAN when the Designated port goes down• Disabled – Has no role in the topology
