RSTP Facts

RSTP Facts

Rapid Spanning Tree Protocol (RSTP) is based on the 802.1w standard and provides faster spanning tree convergence after a topology change. Enhancements added to RSTP to improve convergence are similar to the Port Fast and Uplink Fast features introduced by Cisco. RSTP operates much like STP with Cisco's enhancements. RSTP uses the following port states:

RSTP Port State	STP Port State*	Description
Discarding	Disabled	A port in discarding state: Discards frames received on the interface Discards frames switched from another interface for forwarding Does not learn MAC addresses Listens for BPDUs
	Blocking
	Listening
Learning	Learning	A port in the learning state: Discards frames received on the interface Discards frames switched from another interface for forwarding Learns MAC addresses Listens for BPDUs
Forwarding	Forwarding	A port in the forwarding state: Receives and forwards frames received on the interface Forwards frames switched from another interface Learns MAC addresses Listens for BPDUs

RSTP uses bridge and port roles similarly to STP:

• There is a single root bridge.
• Each segment has a single designated bridge. The port on the designated bridge is identified as the designated port. All ports on the root bridge are designated ports.
• Each designated bridge has a single port identified as the root port. The root port is the best path back to the root bridge. The root bridge is the only bridge that does not have a root port.
• Instead of having blocking ports, RSTP splits this role into two roles:
  • An alternate port is the switch's best alternative to its current root port. An alternate port can be used to replace the root port if the root port fails.
  • A backup port is the switch's alternative port connected to the same network segment as the designated port. A backup port provides an alternate path to the same segment, but not an alternate path back to the root bridge.

  Both port roles are in the blocking state.

In addition to the port roles, RSTP uses the port type to determine whether to use advanced features that provide rapid convergence. These port types are:

Port Type	Description
Point-to-point	A point-to-point link is a port that connects only to another switch. The presence of full-duplex communication indicates a point-to-point link. Because the link has only a single connected switch, it can take advantage of RSTP improvements that help it recover quickly. A point-to-point link is similar to Cisco's Uplink Fast feature for STP.
Shared	A shared link is a link with more than a single attached device. The presence of half-duplex communication indicates a shared link. Ports connected to shared links cannot use RSTP improvements.
Edge	An edge port is a port that is not connected to another switch. Because the edge port does not have a switch, the possibility of a loop is eliminated. Edge ports can be put into the forwarding state immediately. An edge port is like Cisco's Port Fast feature for STP. If the port receives a BPDU, it treats the port as a point-to-point or shared link.

Note: When any RSTP port receives legacy 802.1d BPDU, it falls back to legacy STP and the inherent fast convergence benefits of 802.1w are lost when it interacts with legacy bridges. However, this allows you to mix RSTP and STP in the same topology during a staged migration without any problems.