H3C Technologies H3C S3100 Series Switches - Manual (Page 231)

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3) MSTI
A multiple spanning tree instance (MSTI) refers to a spanning tree in an MST region.
Multiple spanning trees can be established in one MST region. These spanning trees are independent
of each other. For example, each region in
Figure 1-4 contains multiple spanning trees known as MSTIs.
Each of these spanning trees corresponds to a VLAN.
4) VLAN-to-instance mapping table
A VLAN-to-instance mapping table is maintained for each MST region. The table is a collection of
mappings between VLANs and MSTIs. For example, in
Figure 1-4 , the VLAN-to-instance mapping table
of region A0 contains these mappings: VLAN 1 to MSTI 1; VLAN 2 to MSTI 2, and other VLANs to CIST.
In an MST region, load balancing is implemented according to the VLAN-to-instance mapping table.
5) IST
An internal spanning tree (IST) is a spanning tree in an MST region.
ISTs together with the common spanning tree (CST) form the common and internal spanning tree (CIST)
of the entire switched network. An IST is a special MSTI; it is a branch of CIST in the MST region.
In Figure 1-4 , each MST region has an IST, which is a branch of the CIST.
6) CST
A common spanning tree (CST) is a single spanning tree in a switched network that connects all MST
regions in the network. If you regard each MST region in the network as a “switch”, then the CST is the
spanning tree generated by STP or RSTP running on the "switches". For example, the red lines in
Figure 1-4 represent the CST.
7) CIST
A common and internal spanning tree (CIST) is the spanning tree in a switched network that connects
all switches in the network. It comprises the ISTs and the CST.
In Figure 1-4 , the ISTs in the MST regions and the CST connecting the MST regions form the CIST.
8) Region root
A region root is the root of the IST or an MSTI in an MST region. Different spanning trees in an MST
region may have different topologies and thus have different region roots.
In region D0 shown in Figure 1-4 , the region root of MSTI 1 is switch B, and the region root of MSTI 2 is
switch C.
9) Common root bridge
The common root bridge is the root of the CIST. The common root bridge of the network shown in Figure
1-4 is a switch in region A0.
10) Port role
MSTP calculation involves the following port roles: root port, designated port, master port, region
boundary port, alternate port, and backup port.
z
A root port is used to forward packets to the root.
z
A designated port is used to forward packets to a downstream network segment or switch.
z
A master port connects an MST region to the common root. The path from the master port to the
common root is the shortest path between the MST region and the common root. In the CST, the
master port is the root port of the region, which is considered as a node. The master port is a
special boundary port. It is a root port in the IST/CIST while a master port in the other MSTIs.