3Com Switch 8800 Configuration Guide
Chapter 38 MSTP Region-configuration
38-2
C
A
B
BPDU
BPDU
BPDU
CIST: common and
internal spanning tree
MSTI: multiple spanning
tree instance
CST: common
spanning tree
Area A0
VLAN 1 mapped to Instance 1
VLAN 2 mapped to Instance 2
Other VLANs mapped to CIST
D
Area B0
VLAN 1 mapped to Instance 1
VLAN 2 mapped to Instance 2
Other VLANs mapped to CIST
Area C0
VLAN 1 mapped to Instance 1
VLANs 2 and 3 mapped to Instance 2
Other VLANs mapped to CIST
Area D0
VLAN 1 mapped to Instance
1, domain root as B
VLAN 2 mapped to Instance
2, domain root as C
Other VLANs mapped to
CIST
Figure 38-1
Basic MSTP concepts
I. MST region
Multiple Spanning Tree Regions: A multiple spanning tree region contains several
switches and the network segments between them. These MSTP switches share the
same region name, VLAN-spanning tree mapping configuration, and MSTP revision
level configuration, and are connected directly. There can be several MST regions on a
switching network. You can group several switches into a MST region, using MSTP
configuration commands. For example, in Figure 38-1, the four switches in MST region
A0 are configured with the same region name, the same VLAN mapping table (VLAN1
is mapped to instance 1, VLAN 2 is mapped to instance 2, other VLANs is mapped to
instance CIST), and the same revision level (not indicated in Figure 38-1).
II. VLAN mapping table
The VLAN mapping table is an attribute of MST region. It is used for describing the
mapping relationship of VLANs and spanning tree instances (STIs). For example, in the
VLAN mapping table of MST region A0 in Figure 38-1, VLAN1 is mapped to instance 1,
VLAN 2 is mapped to instance 2, other VLANs is mapped to CIST.
In the same region, the mapping relationship of VLANs and STIs must be consistent on
all the switches in this region. Otherwise, VLAN and STI are not in the same region.
III. IST
Internal Spanning Tree (IST): a spanning tree in a MSTP region. The IST and the
Common Spanning Tree (CST), together make up a Common and Internal Spanning
Tree (CIST) for the entire switching network. The IST in a MST region is a fragment of
the CIST. For example, every MST region in Figure 38-1 has an IST, which is a
fragment of CIST.