Configuring MSTP
3. 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 calculated by these
devices through STP or RSTP. For example, the red lines in Figure 23-1 describe the CST.
4. IST
Internal spanning tree (IST) is a spanning tree that runs in an MST region.
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 the given MST region.
In Figure 23-1, for example, the CIST has a section in each MST region, and this section is
the IST in the respective MST region.
5. 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 23-1, for example, multiple spanning tree can exist in each
MST region, each spanning tree corresponding to a VLAN. These spanning trees are called
MSTIs.
6. CIST root bridge
CIST root，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.
7. CIST External root path cost
External root path cost refers to the cost of the shortest path for a packet to travel to the
common root bridge.
8. CIST Internal root path cost
CIST Internal root path cost refers to the cost of the shortest path for a packet to travel to
the CIST regional root bridge.
9. CIST designated bridge
CIST designated bridge is the STP appointed bridge
10. MSTI regional root
The root bridge of the IST or an MSTI within an MST region is the regional root bridge of
the IST or the MSTI. Based on the topology, different spanning trees in an MST region may
have different regional roots.
11. MSTI internal root path cost
MSTI Internal root path cost refers to the cost of the shortest path for a packet to travel to
the MSTI regional root bridge.
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