3Com Switch 4800G 24-Port Configuration Manual page 208

208 C 23: MSTP C
HAPTER ONFIGURATION
The same region name,
■
The same VLAN-to-instance mapping (VLAN 1 is mapped to MST instance 1,
■
VLAN 2 to MST instance 2, and the rest to the command and internal spanning
tree (CIST). CIST refers to MST instance 0), and
The same MSTP revision level (not shown in the figure).
■
Multiple MST regions can exist in a switched network. You can use an MSTP
command to group multiple devices to the same MST region.
2 VLAN-to-instance mapping table
As an attribute of an MST region, the VLAN-to-instance mapping table describes
the mapping relationships between VLANs and MST instances. In Figure 56, for
example, the VLAN-to-instance mapping table of region A0 describes that the
same region name, the same VLAN-to-instance mapping (VLAN 1 is mapped to
MST instance 1, VLAN 2 to MST instance 2, and the rest to CIST). MSTP achieves
load balancing by means of the VLAN-to-instance mapping table.
3 IST
Internal spanning tree (IST) is a spanning tree that runs in an MST region.
ISTs in all MST regions and the common spanning tree (CST) jointly constitute the
common and internal spanning tree (CIST) of the entire network. An IST is a
section of the CIST in the given MST region.
In Figure 56, for example, the CIST has a section in each MST region, and this
section is the IST in the respective MST region.
4 CST
The CST is a single spanning tree that connects all MST regions in a switched
network. If you regard each MST region as a "device", the CST is a spanning tree
calculated by these devices through STP or RSTP. For example, the red lines in
Figure 56 describe the CST.
5 CIST
Jointly constituted by ISTs and the CST, the CIST is a single spanning tree that
connects all devices in a switched network.
In Figure 56, for example, the ISTs in all MST regions plus the inter-region CST
constitute the CIST of the entire network.
6 MSTI
Multiple spanning trees can be generated in an MST region through MSTP, one
spanning tree being independent of another. Each spanning tree is referred to as a
multiple spanning tree instance (MSTI). In Figure 56, for example, multiple
spanning tree can exist in each MST region, each spanning tree corresponding to a
VLAN. These spanning trees are called MSTIs.
7 Regional root bridge
The root bridge of the IST or an MSTI within an MST region is the regional root
bridge of the MST or that MSTI. Based on the topology, different spanning trees in
an MST region may have different regional roots.
For example, in region D0 in Figure 56, the regional root of instance 1 is device B,
while that of instance 2 is device C.
8 Common root bridge