3Com Switch 8800 Configuration Guide
Chapter 38 MSTP Region-configuration
38-3
IV. 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 38-1 indicates the CST.
V. 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 38-1 is
composed of ISTs in all MST regions and the CST.
VI. 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 38-1, every MST region have many STIs. Each STI
corresponds to a VLAN and is called a MSTI.
VII. 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 38-1.
VIII. 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 38-1.
IX. 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 38-1, 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.
X. 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.
z
The root port is the one through which the data are forwarded to the root.
z
The designated port is the one through which the data are forwarded to the
downstream network segment or switch.
