Huawei Quidway S3900 Series Operation Manual page 237

Operation Manual – MSTP
Quidway S3900 Series Ethernet Switches-Release 1510
III. VLAN mapping table
A VLAN mapping table is a property of an MST region. It contains information about
how VLANs are mapped to MSTIs. For example, in Figure 1-1, the information
contained in the VLAN mapping table of region A0 is: VLAN 1 is mapped to MSTI 1;
VLAN 2 is mapped to MSTI 2; and other VLANs are mapped to CIST. In an MST region,
load balancing is achieved by the VLAN mapping table.
IV. 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
belongs to an MST region and is a branch of CIST. In Figure 1-1, each MST region has
an IST, which is a branch of the CIST.
V. CST
A CST is the 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". In Figure 1-1, the
lines in red depict the CST.
VI. CIST
A 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 1-1, the ISTs in the MST regions
and the CST connecting the MST regions form the CIST.
VII. Region root
A region root is the root of the IST or an MSTI in a 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 1-1, the region root of MSTI 1 is switch B, and the region root
of MSTI 2 is switch C.
VIII. Common root bridge
The common root bridge is the root of the CIST. The common root bridge of the network
shown in Figure 1-1 is a switch in region A0.
IX. Port roles
In MSTP, the following port roles exist: root port, designated port, master port, region
edge port, alternate port, and backup port.
A root port is used to forward packets to the root.
A designated port is used to forward packets to a downstream network segment or
switch.
Huawei Technologies Proprietary
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Chapter 1 MSTP Configuration