If all the legacy switches on the link are RSTP devices, they can process MSTP BPDUs as if they are RSTP BPDUs. Therefore, MSTP devices send either a Version 0 configuration and TCN BPDUs or Version 3 MSTP BPDUs on a boundary port. A boundary port connects to a LAN, the designated device of which is either a single spanning-tree switch or a switch with a different MST configuration. Rapid Spanning Tree Protocol Overview The RSTP takes advantage of point-to-point wiring and provides rapid convergence of the spanning tree. Reconfiguration of the spanning tree can occur in less than 1 second (in contrast to 50 seconds with the default settings in the IEEE 802.1D spanning tree).
Port Roles and the Active Topology The RSTP provides rapid convergence of the spanning tree by assigning port roles and by learning the active topology. The RSTP builds upon the IEEE 802.1D STP to select the device with the highest device priority (lowest numerical priority value) as the root device. The RSTP then assigns one of these port roles to individual ports: • Root port—Provides the best path (lowest cost) when the device forwards packets to the root switch. • Designated port—Connects to the designated device, which incurs the lowest path cost when forwarding packets from that LAN to the root switch.
The port through which the designated device is attached to the LAN is called the designated port. • Alternate port—Offers an alternate path toward the root switch to that provided by the current root port. • Backup port—Acts as a backup for the path that is provided by a designated port toward the leaves of the spanning tree. A backup port can exist only when two ports are connected in a loopback by a point-to-point link or when a device has two or more connections to a shared LAN segment. • Disabled port—Has no role within the operation of the spanning tree. A port with the root or a designated port role is included in the active topology.
A port with the alternate or backup port role is excluded from the active topology. In a stable topology with consistent port roles throughout the network, the RSTP ensures that every root port and designated port immediately transition to the forwarding state while all alternate and backup ports are always in the discarding state (equivalent to blocking in IEEE 802.1D). The port state controls the operation of the forwarding and learning processes. Table 1: Port State Comparison Configuring Multiple Spanning-Tree Protocol Rapid Spanning Tree Protocol Overview Operational Status STP Port State (IEEE 802.1D) RSTP Port State Is Port Included in the Active Topology?
Enabled Blocking Discarding No Enabled Listening Discarding No Enabled Learning Learning Yes Enabled Forwarding Forwarding Yes Disabled Disabled Discarding No Configuring Multiple Spanning-Tree Protocol 10