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In addition to port states, RSTP assigns and maintains port roles for all ports in a Spanning Tree domain. One of five possible roles can be assigned to a port: root, designated, alternate, backup, and disabled.
Root and designated ports are the only ports that actively participate in the spanning tree (by forwarding frames). Alternate and backup ports are blocked, but if a failure occurs in the network, they will rapidly change to root or designated if necessary.
Alternate and backup ports are essential to RSTP's rapid recovery from port failures. An alternate or backup port can forward traffic immediately. If a port fails in an STP network, however, a 30-second loss of traffic occurs while STP recalculates the Spanning Tree topology.
Root ports provide the lowest cost path to the root bridge. Each bridge in the Spanning Tree domain has a root port that forwards frames to the root bridge. The root bridge does not have a root port.
Designated ports provide the lowest cost path from a network segment to the root bridge. Each network segment has one designated switch on which one port is designated. All traffic sent to and from the network segment passes through the designated port.
Alternate ports provide an alternate path in the direction of the root bridge. If the root port on the bridge fails, one of the alternate ports quickly changes to forwarding state.
Backup ports provide a backup path in the direction of the leaves of the spanning tree. If a designated port on a LAN segment fails, then one of the backup ports on that LAN segment quickly assumes the role of designated port for the segment.
Because RSTP maintains this information, it is able to more quickly activate a redundant path.
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