The features discussed in the previous section – PortFast, UplinkFast and BackboneFast were added by Cisco and because of this they worked only on Cisco switches. IEEE added these features in a new STP protocol called Rapid Spanning Tree Protocol (RSTP) under the 802.1w standard.
One big different between 802.1D STP and 802.1w RSTP is that there are lesser number of port states. As you know, there are five states in 802.1D. RSTP only has 3 states. The disabled, blocking and learning states have been combined into a new discarding state in RSTP. Table 6-2 shows a comparison of the port states.
Table 6-2 STP and RSTP port states comparison
|802.1D Port states||802.1w Port states||Is port active?||MAC addresses learned?|
Similar to “traditional” spanning tree, RSTP will also elect a root bridge using the same parameters as STP and ports will be elected as root and designated ports. In addition to the standard root and designated ports, RSTP ports can have one of the following roles:
- Alternate Port – This is a port that provides an alternative path to the root bridge. This path is less desirable that the path provided by the root port but will be used if the path from the root port goes down.
- Backup Port – This is a port that provides a redundant path to a network segment but this path is less desirable than the one provided by the designated port. This path will be used if the path provided by the designated port goes down.
Figure 6-12 shows an example of a network with all port roles in RSTP.
Figure 6-12 RSTP Port Roles
RSTP is backward compatible with 802.1D STP. If a switch with STP is discovered, the new features such as UplinkFast and BackboneFast will not be used.
Changing from 8021.D to 802.1w RSTP requires a single command on the switch – spanning-tree mode rapid-pvst. This is a global configuration mode command and will cause the switch to change to RSTP. Remember that this can cause the network to be temporarily unavailable. An example is shown below:
The change can be verified with the show spanning-tree command as shown below:
Switch#show spanning-tree vlan 5
Spanning tree enabled protocol rstp