Basic concepts in RSTP
Port roles
In addition to root port and designated port, RSTP also uses the following port roles:
Alternate port—Acts as the backup port for a root port. When the root port is blocked, the alternate port takes over.
Backup port—Acts as the backup port of a designated port. When the designated port is invalid, the backup port becomes the new designated port. A loop occurs when two ports of the same spanning tree device are connected, so the device blocks one of the ports. The blocked port is the backup port.
Edge port—Directly connects to a user host rather than a network device or network segment.
Port states
RSTP uses the discarding state to replace the disabled, blocking, and listening states in STP. Table 10 shows the differences between the port states in RSTP and STP.
Table 10: Port state differences between RSTP and STP
STP port state | RSTP port state | Sends BPDU | Learns MAC addresses | Forwards user data |
|---|---|---|---|---|
Disabled | Discarding | No | No | No |
Blocking | Discarding | No | No | No |
Listening | Discarding | Yes | No | No |
Learning | Learning | Yes | Yes | No |
Forwarding | Forwarding | Yes | Yes | Yes |