3Com 4200G 12-Port Configuration Manual page 178

region A0 contains these mappings: VLAN 1 to MSTI 1; VLAN 2 to MSTI 2, and other VLANs to CIST. In
an MST region, load balancing is implemented according to the VLAN-to-MSTI mapping table.
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 is a branch of CIST in the MST region.
In Figure 1-4, each MST region has an IST, which is a branch of the CIST.
CST
A CST is a single 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".
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 1-4, 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 an MST region. Different spanning trees in an MST
region may have different topologies and thus have different region roots.
In region D0 shown in
switch C.
Common root bridge
The common root bridge is the root of the CIST. The common root bridge of the network shown in
1-4 is a switch in region A0.
Port role
MSTP calculation involves the following port roles: root port, designated port, master port, region
boundary 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 network segment or switch.
A master port connects an MST region to the common root. The path from the master port to the
common root is the shortest path between the MST region and the common root. In the CST, the
master port is the root port of the region, which is considered as a node. The master port is a
special boundary port. It is a root port in the IST/CIST while a master port in the other MSTIs.
A region boundary port is located on the boundary of an MST region and is used to connect one
MST region to another MST region, an STP-enabled region or an RSTP-enabled region.
An alternate port is a secondary port of a root port or master port and is used for rapid transition.
With the root port or master port being blocked, the alternate port becomes the new root port or
master port.
A backup port is the secondary port of a designated port and is used for rapid transition. With the
designated port being blocked, the backup port becomes the new designated port fast and begins
to forward data seamlessly. When two ports of an MSTP-enabled switch are interconnected, the
Figure 1-4, the region root of MSTI 1 is switch B, and the region root of MSTI 2 is
1-11
Figure