3Com Switch 8800 Advanced Software V5 Configuration Guide

104 CHAPTER 11: MSTP CONFIGURATION
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.
3 IST
Internal spanning tree (IST) is a spanning tree that runs in an MST region, with the
instance number of 0. 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 an MST region. In
Figure 28, for
example, the CIST has a section in each MST region, and this section is the IST in
each 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
computed by these devices through MSTP. For example, the red lines in
Figure 28
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 28, 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 28, 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 28, the regional root of instance 1 is device B, while that of instance 2 is
device C.
8 Common root bridge
The root bridge of the CIST is the common root bridge. In Figure 28, for example,
the common root bridge is a device in region A0.
9 Boundary port
A boundary port is a port that connects an MST region to another MST
configuration, or to a single spanning-tree region running STP, or to a single
spanning-tree region running RSTP.
During MSTP computing, a boundary port assumes the same role on the CIST and
on MST instances. Namely, if a boundary port is master port on the CIST, it is also
the master port on all MST instances within this region. In
Figure 28, for example,