Spanning Tree Protocol: RSTP Port Roles

RSTP includes the some normal port roles from 802.1d STP but imposes two new roles:

  • Root Port
    • Has same meaning as 802.1d, this is the port where the received BPDU is superior to all other BPDUs received on other ports
  • Designated Port
    • Has the same meaning as 802.1d, this is the port that is forwarding the least cost superior BPDU onto other LAN segments
  • Alternate Port (New)
    • This is a port that is considered an alternate replacement for the switch’s Root Port
  • Backup Port (New)
    • This is a port that is considered an alternate replacement for the switch’s Designated port into a shared LAN segment

The Alternate Port is new and in the event a Root Port goes down, these Alternate ports can immediately takeover converging the network in less than one second.

prstpportroles

Newly negotiated Designated Ports aren’t free to immediately move to a forwarding state using Backup Ports, instead in the event of a link failure on a Designated Port, Backup Ports must go through the Learning phase before it’s put into a Forwarding State.

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