MSTP Overview 207
■ MSTP prunes loop networks into a loop-free tree, thus avoiding proliferation
and endless recycling of packets in a loop network. In addition, it provides
multiple redundant paths for data forwarding, thus supporting load balancing
of VLAN data in the data forwarding process.
■ MSTP is compatible with STP and RSTP.
Basic concepts in MSTP
Assume that all the four switches in Figure 56 are running MSTP. In light with the
diagram, the following paragraphs will present some basic concepts of MSTP.
Figure 56 Basic concepts in MSTP
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,
■ They have the same region name,
■ They have the same VLAN-to-instance mapping configuration,
■ They have the same MSTP revision level configuration, and
■ They are physically linked with one another.
For example, all the devices in region A0 in Figure 56 have the same MST region
configuration:
CST
Region A0
VLAN 1 mapped to instance 1
VLAN 2 mapped to instance 2
Other VLANs mapped to CIST
Region B0
VLAN 1 mapped to instance 1
VLAN 2 mapped to instance 2
Other VLANs mapped to CIST
Region C0
VLAN 1 mapped to instance 1
VLAN 2,3 mapped to instance 2
Other VLANs mapped to CIST
Region D0
VLAN 1 mapped to instance 1,
B as regional root bridge.
VLAN 2 mapped to instance 2ˈ
C as regional root bridge.
Other VLANs mapped to CIST
BPDU BPDU
C
D
B
A
BPDU
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