The Spanning Tree Protocol ensures a loop-free topology for a bridged LAN. It allows a network design to include spare (redundant) links to provide automatic backup paths if an active link fails without the danger of bridge loops flooding the network or the need for manual enabling/disabling of these backup links.

RSTP significantly reduces the convergence time after a topology change occurs in the network. While STP can take 30 to 50 seconds to transit from a blocking state to a forwarding state, RSTP is typically able to respond less than 10 seconds of a physical link failure.

RSTP works by adding an alternative port and a backup port compared to STP. These ports are allowed to immediately enter the forwarding state rather than passively wait for the network to converge.

RSTP bridge port roles:

  • Root port— A forwarding port that is the closest to the root bridge in terms of path cost.
  • Designated port— A forwarding port for every LAN segment.
  • Alternate port— A best alternate path to the root bridge. This path is different than using the root port. The alternative port moves to the forwarding state if there is a failure on the designated port for the segment.
  • Backup port— A backup/redundant path to a segment where another bridge port already connects. The backup port applies only when a single switch has two links to the same segment (collision domain). To have two links to the same collision domain, the switch must be attached to a hub.
  • Disabled port— Not strictly part of STP, a network administrator can manually disable a port.
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