354 BigIron RX Series Configuration Guide53-1001986-01Overview of Rapid Spanning Tree Protocol13Assignment 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.The topology in Figure 40 contains four bridges. Switch 1 is the root bridge since it has the lowestbridge priority. Switch 2 through Switch 4 are non-root bridges.
