3Com 8807 Configuration Manual page 123

CST
Common Spanning Tree (CST): a LAN has only one CST. CST connects the
spanning trees of all MST regions. Regard every MST region as a "switch", and the
CST is generated by the computing of "switches" through STP/RSTP. For example,
the red line in Figure 23 indicates the CST.
CIST
Common and Internal Spanning Tree (CIST): A single spanning tree made up of
ISTs and CST. It connects all switches in a switching network. CIST of Figure 23 is
composed of ISTs in all MST regions and the CST.
MSTI
Multiple Spanning Tree Instance (MSTI): multiple spanning trees can be generated
with MSTP in an MST region and independent of one another. Such a spanning
tree is called an MSTI. As shown is Figure 23, every MST region have many STIs.
Each STI corresponds to a VLAN and is called a MSTI.
Region root
The region root refers to the root of the IST and MSTI of the MST region. The
spanning trees in an MST region have different topology and their region roots
may also be different. For example, the region root of the STI 1 is the switch B and
that of the STI 2 is the switch C, as shown in Figure 23.
Common Root Bridge
The Common Root Bridge refers to the root bridge of CIST. For example, the
common root bridge is a certain switch in A0, as shown in Figure 23.
Edge port
The edge port refers to the port located at the MST region edge, connecting
different MST regions, MST region and STP region, or MST region and RSTP
region. For MSTP calculation, the edge port shall take the same role on MSTI and
CIST instance. For example, as shown in Figure 23, if a switch in region A0
connects to the first port on a switch in region D0, and the common root bridge of
the whole switching network is in A0, then this first port is an edge port of region
D0.
Port role
In the process of MSTP calculation, a port can serve as a designated port, root
port, master port, alternate port, or backup port.
The root port is the one through which the data are forwarded to the root.
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The designated port is the one through which the data are forwarded to the
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downstream network segment or switch.
Master port is the port connecting the entire region to the Common Root
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Bridge and located on the shortest path between them.
An alternate port is a backup of the mater port, and also a backup port of a
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root port in the region. As a backup of the mater port, an alternate port will
become a new master port after a master port is blocked.
If two ports of a switch are connected, there must be a loop. In this case, the
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switch blocks one of them. The blocked one is called a backup port.
Introduction to MSTP 123