3Com 7757 Configuration Manual page 243

IST
An internal spanning tree (IST) is a spanning tree in an MST region.
ISTs together with the common spanning tree (CST) form the common and
internal spanning tree (CIST) of the entire switched network. An IST is a special
MSTI; it belongs to an MST region and is a branch of CIST. In Figure 60, each MST
region has an IST, which is a branch of the CIST.
CST
A CST is the spanning tree in a switched network that connects all MST regions in
the network. If you regard each MST region in the network as a switch, then the
CST is the spanning tree generated by STP or RSTP running on the "switches". In
Figure 60, the lines in red depict the CST.
CIST
A 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 60, the ISTs in the MST
regions and the CST connecting the MST regions form the CIST.
Region root
A region root is the root of the IST or an MSTI in a MST region. Different spanning
trees in an MST region may have different topologies and thus have different
region roots. In region D0 shown in Figure 60, the region root of MSTI 1 is switch
B, and the region root of MSTI 2 is switch C.
Common root bridge
The common root bridge is the root of the CIST. The common root bridge of the
network shown in Figure 60 is a switch in region A0.
Port roles
In MSTP, 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.
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A designated port is used to forward packets to a downstream network
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segment or switch.
A master port connects a MST region to the common root. The path from the
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master port to the common root is the shortest path between the MST region
and the common root.
A region edge port is located on the edge of an MST region and is used to
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connect the MST region to another MST region, an STP-enabled region or an
RSTP-enabled region
An alternate port is a backup port of a master port. It becomes the master port
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if the existing master port is blocked.
A loop occurs when two ports of a switch are connected to each other. In this
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case, the switch blocks one of the two ports. The blocked port is a backup
port.
In Figure 61, switch A, B, C, and D form an MST region. Port 1 and port 2 on
switch A connect upstream to the common root. Port 5 and port 6 on switch C
MSTP Overview 243