Spanning Tree Protocol (STP) (IEEE 802.1D standard) configures a meshed network for robustness by eliminating loops within the network and calculating and storing alternate paths to provide fault tolerance.
As the port comes up and STP calculation takes place, the port is set to Blocked state. In this state, no traffic can pass through the port. Since STP calculations take up to a minute to complete, the port is not operational thereby effecting the network behind the port. When the STP calculation is complete, the port‘s state is changed to Forwarding and traffic is allowed.
Rapid Spanning Tree Protocol (RSTP) (IEEE 802.1w standard) is an evolution over the standard STP. The primary aim is to reduce the time taken to respond to topology changes while being backward compatible with STP. PortFast allows the port to bypass the listening and learning states, thereby rapidly changing the state of a port from Blocked to Forwarding. The port allows traffic while the STP calculation is in progress.
Multiple Spanning Tree Protocol (MSTP) provides an extension to RSTP to optimize the usefulness of VLANs. MSTOP allows for a separate spanning tree for each VLAN group, and blocks all but one of the possible alternate paths within each spanning tree topology.
If there is only one VLAN in the access point managed network, a single spanning tree works fine. However, if the network contains more than one VLAN, the network topology defined by single STP would work, but it is possible to make better use of the alternate paths available by using an alternate spanning tree for different VLANs or groups of VLANs.
An MSTP supported deployment uses multiple MST regions with multiple MST instances (MSTIs). Multiple regions and other STP bridges are interconnected using one single common spanning tree (CST). MSTP includes all of its spanning tree information in a single Bridge Protocol Data Unit (BPDU) format. BPDUs are used to exchange information bridge IDs and root path costs. Not only does this reduce the number of BPDUs required to communicate spanning tree information for each VLAN, but it also ensures backward compatibility with RSTP.
MSTP encodes additional region information after the standard RSTP BPDU as well as a number of MSTI messages. Each MSTI message conveys spanning tree information for each instance. Each instance can be assigned a number of configured VLANs. The frames assigned to these VLANs operate in this spanning tree instance whenever they are inside the MST region. To avoid conveying their entire VLAN to spanning tree mapping in each BPDU, the access point encodes an MD5 digest of their VLAN to an instance table in the MSTP BPDU. This digest is used by other MSTP supported devices to determine if the neighboring device is in the same MST region as itself.