MSTP Overview 149
A switched network can contain multiple MST regions. You can group multiple
switches into one MST region by using the corresponding MSTP configuration
commands.
As shown in Figure 49, all the switches in region A0 are of the same MST
region-related configuration, including:
■ Region name
■ VLAN-to-MSTI mapping (that is, VLAN 1 is mapped to MSTI 1, VLAN 2 is
mapped to instance 2, and the other VLANs are mapped to CIST.)
■ MSTP revision level (not shown in Figure 49)
MSTI
A multiple spanning tree instance (MSTI) refers to a spanning tree in an MST
region.
Multiple spanning trees can be established in one MST region. These spanning
trees are independent of each other. For example, each region in
Figure 49
contains multiple spanning trees known as MSTIs. Each of these spanning trees
corresponds to a VLAN.
VLAN mapping table
A VLAN mapping table is a property of an MST region. It contains information
about how VLANs are mapped to MSTIs. For example, in
Figure 49, the VLAN
mapping table of region A0 is: VLAN 1 is mapped to MSTI 1; VLAN 2 is mapped to
MSTI 2; and other VLANs are mapped to CIST. In an MST region, load balancing is
implemented according to the VLAN 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 49, 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 49, the ISTs in the MST regions and the CST connecting the MST regions
form the CIST.
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