3Com 7700 Configuration Manual page 231

MSTP 223
group several switches into a MST region, using MSTP configuration commands.
For example, in Figure 5, in MST region A0, the 4 switches are configured with the
same region name, vlan mapping table (VLAN1 map to instance 1, VLAN 2 map to
instance 2, other VLAN map to instance 0), and revision level (not indicated in
figure2-1).
VLAN Mapping Table
A VLAN mapping table is an attribute of an MST region and is used for describing
the mapping relationship of VLAN and STI. For example, the VLAN mapping table
of MST region A0 in Figure 5 is VLAN1 map to instance 1, VLAN 2 map to
instance 2, other VLAN map to instance 0.
Internal Spanning Tree (IST)
The entire switching network has a Common and Internal Spanning Tree (CIST).
An MSTP region has an Internal Spanning Tree (IST), which is a fragment of CIST.
For example, every MST region in Figure 5 has an IST.
Common Spanning Tree (CST)
CST connects the spanning trees of the MST region. Taking every MST region as a
"switch", the CST can be regarded as their spanning tree generated with
STP/RSTP. For example, the red line indicates the CST in Figure 5.
Common and Internal Spanning Tree (CIST)
A single spanning tree made of IST and CST. The CIST in Figure 5 is composed of
each IST in every MST region and the CST.
Multiple Spanning Tree Instance (MSTI)
Multiple spanning trees can be generated with MSTP in an MSTI and independent
of one another. Such a spanning tree is called an MSTI. Every MST region can have
many STIs called MSTI. These STIs are related to the corresponding VLAN.
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. In each MST region in Figure 5, every STI has its region root.
Common Root Bridge
The common root bridge refers to the root bridge of the CIST. There is only one
common root bridge in the network.
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 takes the same role on MSTI and CIST
instance. For example, the edge port as a master port on a CIST instance should
serve as a master port on every MSTI in the region.
Port role
In the process of MSTP calculation, a port can serve as a designated port, root
port, master port, alternate port, or BACKUP.
The root port is the port through which the data is forwarded to the root.
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