3Com MSR 50 Series Configuration Manual page 453

MSTP supports mapping VLANs to MST instances by means of a
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VLAN-to-instance mapping table. MSTP can save communication overheads
and resource usage by mapping multiple VLANs to one MST instance.
MSTP divides a switched network into multiple regions, each containing
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multiple spanning trees that are independent of one another.
MSTP prunes loop networks into a loop-free tree, thus avoiding proliferation
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and endless recycling of packets in a loop network. In addition, it provides
multiple redundant paths for data forwarding, thus supporting load sharing of
VLAN data in the data forwarding process.
MSTP is compatible with STP and RSTP.
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Basic concepts in MSTP
Assume that all the four switches in
diagram, the following paragraphs will present some basic concepts of MSTP.
Figure 130 Basic concepts in MSTP
B B
Regi o n D0
VLAN 1 mapped to instance 1,
B as regi o nal root bridge
VLAN 2 mapped to instance 2,
C as regi o nal root bridge
Other VLANs mapped CIST
1 MST region
A multiple spanning tree region (MST region) is composed of multiple devices in a
switched network and network segments among them. These devices have the
following characteristics:
All are MSTP-enabled,
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They have the same region name,
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They have the same VLAN-to-instance mapping configuration,
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They have the same MSTP revision level configuration, and
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They are physically linked with one another.
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In area A0 in Figure
configuration:
The same region name
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Figure 130
BPDU
A A
CST
C C
D D
130, for example, all the device have the same MST region
MSTP Overview
are running MSTP. In light with the
Region A0
VLAN 1 mapped to instance 1
VLAN 2 mapped to instance 2
Other VLANs mapped CIST
BPDU
BPDU
Regi o n B0
VLAN 1 mapped to instance 1
VLAN 2 mapped to instance 2
Other VLANs mapped CIST
Regi o n C0
VLAN 1 mapped to instance 1
VLANs 2 and 3 mapped to instance 2
Other VLANs mapped CIST
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