91• Master port—Serves as a port on the shortest path from the local MST region to the common rootbridge. The master port is not always located on the regional root. It is a root port on the IST or CISTand still a master port on the other MSTIs.• Boundary port—Connects an MST region to another MST region or to an STP/RSTP-running device.In MSTP calculation, a boundary port's role on an MSTI is consistent with its role on the CIST.However, that is not true with master ports. A master port on MSTIs is a root port on the CIST.Port statesIn MSTP, a port can be in one of the following states:• Forwarding—The port receives and sends BPDUs, learns MAC addresses, and forwards usertraffic.• Learning—The port receives and sends BPDUs, learns MAC addresses, but does not forward usertraffic. Learning is an intermediate port state.• Discarding—The port receives and sends BPDUs, but does not learn MAC addresses or forwarduser traffic.NOTE:When in different MSTIs, a port can be in different states.A port state is not exclusively associated with a port role. Table 8 lists the port states that each port rolesupports. (A check mark [√] indicates that the port supports this state, while a dash [—] indicates that theport does not support this state.)Table 8 Port states that different port roles supportPort role (right)Port state (below)Root port/masterport Designated port Alternate port Backup portForwarding √ √ — —Learning √ √ — —Discarding √ √ √ √How MSTP worksMSTP divides an entire Layer 2 network into multiple MST regions, which are connected by a calculatedCST. Inside an MST region, multiple spanning trees, called MSTIs, are calculated. Among these MSTIs,MSTI 0 is the IST.Like STP, MSTP uses configuration BPDUs to calculate spanning trees. An important difference is that anMSTP BPDU carries the MSTP configuration of the bridge from which the BPDU is sent.CIST calculationDuring the CIST calculation, the following process takes place:• The device with the highest priority is elected as the root bridge of the CIST.• MSTP generates an IST within each MST region through calculation.• MSTP regards each MST region as a single device and generates a CST among these MST regionsthrough calculation.The CST and ISTs constitute the CIST of the entire network.
