RSTP Port Roles and Port States

RSTP Port Roles

Figure 1 shows the four port roles defined in RSTP: root port, designated port, alternate port, and backup port. Compared with STP, RSTP adds two port roles.

Figure 1 Diagram of port roles

The functions of the root port and designated port are the same as those defined in STP. Table 1 shows the alternate port and backup port.

**Table 1** Alternate port and backup port
Category	Alternate Port	Backup Port
From the perspective of configuration BPDU transmission	An alternate port is blocked after learning a configuration BPDU sent from another bridge.	An alternate port is blocked after learning a configuration BPDU sent from itself.
From the perspective of user traffic	An alternate port acts as a backup of the root port and provides an alternate path from the designated bridge to the root bridge.	A backup port acts as a backup of the designated port and provides a backup path from the root bridge to the related network segment.

After RSTP determines all port roles, the topology convergence is completed.

Due to changes in port roles compared with those in STP, RSTP uses a different configuration BPDU format. To describe port roles, RSTP uses the Flags field.

The value of the Type field is changed from 0 to 2. Devices running STP will discard configuration BPDUs sent from devices running RSTP.
The Flags field uses the six bits reserved in STP. This configuration BPDU is called a Rapid Spanning Tree Bridge Protocol Data Unit (RST BPDU), as shown in Figure 2.

Figure 2 Format of the Flags field in an RST BPDU

RSTP Port States

RSTP reduces the number of port states to three. Depending on whether a port can forward user traffic and learn MAC addresses, the port will be in one of the following states:

Discarding: The port does not forward user traffic or learn MAC addresses.

Learning: The port does not forward user traffic but learns MAC addresses.

Forwarding: The port forwards user traffic and learns MAC addresses.

Table 2 compares the port states defined in STP and RSTP and port roles. Port states are not necessarily related to port roles.

**Table 2** Comparison between port states defined in STP and RSTP
STP Port State	RSTP Port State	Port Role
Forwarding	Forwarding	Root port or designated port
Learning	Learning	Root port or designated port
Listening	Discarding	Root port or designated port
Blocking	Discarding	Alternate port or backup port
Disabled	Discarding	Disabled port