3Com 5500 SI - Switch - Stackable, Configuration Manual

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MSTP Overview 165
IST
An internal spanning tree (IST) is a spanning tree in an MST region.
ISTs, along with the common spanning tree (CST), form the common and internal
spanning tree (CIST) of the entire switched network. An IST is a branch of CIST and is
a special MSTI. In Figure 42, CIST has a branch in each MST region, which is the IST in
the region.
CST
A CST is the spanning tree connecting all the MST regions in a switched network. If
you consider each MST region a “switch”, the CST is the spanning tree calculated by
STP or RSTP with these “switches” as the nodes. In Figure 42 , the lines in red depict
the CST.
CIST
A common and internal spanning tree (CIST) is the spanning tree in a switched
network that connects all switches in the network. It comprises the ISTs and the CST.
In Figure 42, the IST of each MST region and the CST form the CIST.
Region root
A region root is the root of an IST or MSTI in a MST region. Since the spanning trees in
a MST region have different topologies, they may hold different region roots. For
region D0 in Figure 42, the region root of MSTI 1 is switch B, and the region root of
MSTI 2 is switch C.
Common root bridge
A common root bridge is the root of a CIST. The common root bridge of the network
shown in Figure 42 is a switch in region A0.
Port role
In MSTP, the following port roles exist: root port, designated port, master port, region
edge port, alternate port, and backup port.
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A root port is in charge of forwarding packets in the direction of the root.
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A designated port is in charge of forwarding packets to downstream network
segments or switches.
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A master port connects a MST region to the common root bridge. It is located
along the shortest path from the MST region to the common root bridge.
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A region edge port is located on the edge of an MST region and is used to connect
the MST region to another MST region, a region running STP, or a region running
RSTP.
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An alternate port serves as the backup of a mater port. Once the master port is
blocked, it becomes the new master port.
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A loop occurs when two ports of a switch are connected. In this case, the switch
blocks one of the two ports. The blocked one is the backup port.
In Figure 43, switches A, B, C, and D form a MST region. Port 1 and port 2 on switch
A connect upstream to the common root bridge. Port 5 and port 6 on switch C form
a loop. Port 3 and port 4 on switch D connect downstream to other MST regions.
Figure 43 shows the roles of the ports.
A port can play different roles in different MSTIs.