3Com 4500 Configuration Manual page 189

MSTP supports mapping VLANs to Multiple Spanning Tree (MST) instances (MSTIs) by means of
a VLAN-to-instance mapping table. MSTP introduces instances (which integrates multiple VLANs
into a set) and can bind multiple VLANs to an instance, thus saving communication overhead and
improving resource utilization.
MSTP divides a switched network into multiple regions, each containing multiple spanning trees
that are independent of one another.
MSTP prunes a ring network into a network with tree topology, preventing packets from being
duplicated and forwarded in a network endlessly. Furthermore, it offers multiple redundant paths
for forwarding data, and thus achieves load balancing for forwarding VLAN data.
MSTP is compatible with STP and RSTP.
Basic MSTP Terminology
Figure 1-4 illustrates basic MSTP terms (assuming that MSTP is enabled on each switch in this figure).
Figure 1-4 Basic MSTP terminologies
A
B
D
Region D0:
VLAN 1 mapped to MSTI 1, B
as the regional root bridge
VLAN 2 mapped to MSTI 2, C
as the regional root bridge
Other VLANs mapped to CIST
1) MST region
A multiple spanning tree region (MST region) comprises multiple physically-interconnected
MSTP-enabled switches and the corresponding network segments connected to these switches. These
switches have the same region name, the same VLAN-to-instance mapping configuration and the same
MSTP revision level.
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 1-4, all the switches in region A0 are of the same MST region-related configuration,
including:
Region name
VLAN-to-instance mapping (that is, VLAN 1 is mapped to MSTI 1, VLAN 2 is mapped to MSTI 2,
and the other VLANs are mapped to CIST.)
MSTP revision level (not shown in
BPDU BPDU
CST
C
BPDU
Figure 1-4)
1-11
Region A0:
VLAN 1 mapped to MSTI 1
VLAN 2 mapped to MSTI 2
Other VLANs mapped to CIST
Region B0:
VLAN 1 mapped to MSTI 1
VLAN 2 mapped to MSTI 2
Other VLANs mapped to CIST
Region C0:
VLAN 1 mapped to MSTI 1
VLAN 2 and 3 mapped to MSTI 2
Other VLANs mapped to CIST