150 CHAPTER 14: MSTP CONFIGURATIONRegion rootA region root is the root of the IST or an MSTI in an MST region. Differentspanning trees in an MST region may have different topologies and thus havedifferent region roots.In region D0 shown in Figure 49, the region root of MSTI 1 is switch B, and theregion root of MSTI 2 is switch C.Common root bridgeThe common root bridge is the root of the CIST. The common root bridge of thenetwork shown in Figure 49 is a switch in region A0.Port roleDuring MSTP calculation, the following port roles exist: root port, designated port,master port, region edge port, alternate port, and backup port.■ A root port is used to forward packets to the root.■ A designated port is used to forward packets to a downstream networksegment or switch.■ A master port connects an MST region to the common root. The path from themaster port to the common root is the shortest path between the MST regionand the common root.■ A region edge port is located on the edge of an MST region and is used toconnect one MST region to another MST region, an STP-enabled region or anRSTP-enabled region■ An alternate port is a secondary port of a root port or master port and is usedfor rapid transition. With the root port or master port being blocked, thealternate port becomes the new root port or master port.■ A backup port is the secondary port of a designated port and is used for rapidtransition. With the designated port being blocked, the backup port becomesthe new designated port fast and begins to forward data seamlessly. When twoports of an MSTP-enabled switch are interconnected, the switch blocks one ofthe two ports to eliminate the loop that occurs. The blocked port is the backupport.In Figure 50, switch A, switch B, switch C, and switch D form an MST region. Port1 and port 2 on switch A connect upstream to the common root. Port 5 and port6 on switch C form a loop. Port 3 and port 4 on switch D connect downstream toother MST regions. This figure shows the roles these ports play.n ■ A port can play different roles in different MSTIs.■ The role a region edge port plays is consistent with the role it plays in the CIST.For example, port 1 on switch A in Figure 50 is a region edge port, and it is amaster port in the CIST. So it is a master port in all MSTIs in the region.
