295
VLAN-to-instance mapping table
As an attribute of an MST region, the VLAN-to-instance mapping table describes the mapping
relationships between VLANs and MSTIs. In Figure 315, for example, the VLAN-to-in
stan
ce mapping
table of region A0 is: VLAN 1 is mapped to MSTI 1, VLAN 2 to MSTI 2, and the rest to CIST. MSTP
achieves load balancing by means of the VLAN-to-instance mapping table.
IST
An 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 an MST region.
In Figure 315, for example, the CIST has a section in each MST
region, and this section is the IST in
the respective MST region.
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. CSTs are indicated by red lines in Figure 315.
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 315, for example, the ISTs
in all MST regions plus the inter-region CST constitute the CIST
of the entire network.
MSTI
Multiple spanning trees can be generated in an MST region through MSTP, one spanning tree being
independent of another. Each spanning tree is called a multiple spanning tree instance (MSTI).
In Figure 315, for example, multiple MSTIs can exist in each MST region, each MSTI corresponding
to the spe
cified VLANs.
Regional root bridge
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 might have different
regional roots.
For example, in region D0 in Figure 315, the regional ro
ot of MSTI 1 is device B, and that of MSTI 2 is
device C.
Common root bridge
The common root bridge is the root bridge of the CIST.
In Figure 315, for example, the common root bridge is a device in region A0.
Boundary port
A boundary port is a port that connects an MST region to another MST region, or to a single
spanning-tree region running STP, or to a single spanning-tree region running RSTP. It is at the
boundary of an MST region.
During MSTP calculation, the role of a boundary port in an MSTI must be consistent with its role in
the CIST. However, this is not true with master ports. A master port on MSTIs is a root port on the
CIST. For example, in Figure 315, if a
device in region
A0 is interconnected to the first port of a device
in region D0 and the common root bridge of the entire switched network is located in region A0, the
first port of that device in region D0 is the boundary port of region D0.
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