348 BigIron RX Series Configuration Guide53-1001810-01Overview of Rapid Spanning Tree Protocol13RSTP 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 topologyAssignment of port rolesAt system start-up, all RSTP-enabled bridge ports assume a Designated role. Once start-up iscomplete, RSTP algorithm calculates the superiority or inferiority of the RST BPDU that is receivedand transmitted on a port.On a root bridge, each port is assigned a Designated port role, except for ports on the same bridgethat are physically connected together. In these type of ports, the port that receives the superiorRST BPDU becomes the Backup port, while the other port becomes the Designated port.On non-root bridges, ports are assigned as follows:• The port that receives the RST BPDU with the lowest path cost from the root bridge becomesthe Root port.• If two ports on the same bridge are physically connected, the port that receives the superiorRST BPDU becomes the Backup port, while the other port becomes the Designated port.• If a non-root bridge already has a Root port, then the port that receives an RST BPDU that issuperior to those it can transmit becomes the Alternate port.• If the RST BPDU that a port receives is inferior to the RST BPDUs it transmits, then the portbecomes a Designated port.• If the port is down or if RSTP is disabled on the port, that port is given the role of Disabled port.Disabled ports have no role in the topology. However, if RSTP is enabled on a port with a linkdown and the link of that port comes up, then that port assumes one of the following portroles: Root, Designated, Alternate, or Backup.The following example (Figure 40) explains role assignments in a simple RSTP topology.NOTEAll examples in this document assume that all ports in the illustrated topologies are point-to-pointlinks and are homogeneous (they have the same path cost value) unless otherwise specified.
