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22.2.1.1 Operations between MST Regions
If there are multiple regions or legacy 802.1D bridges within the network, MSTP establishes
and maintains the CST, which includes all MST regions and all legacy STP bridges in the
network. The MST instances combine with the IST at the boundary of the region to become the
CST.
The MSTI is only valid within its MST region. An MSTI has nothing to do with MSTIs in other
MST regions. The bridges in a MST region receive the MST BPDU of other regions through
Boundary Ports. They only process CIST related information and abandon MSTI information.
22.2.2 Port Roles
The MSTP bridge assigns a port role to each port which runs MSTP.
CIST port roles: Root Port, Designated Port, Alternate Port and Backup Port
On top of those roles, each MSTI port has one new role: Master Port.
The port roles in the CIST (Root Port, Designated Port, Alternate Port and Backup Port) are
defined in the same ways as those in the RSTP.
22.2.3 MSTP Load Balance
In a MSTP region, VLANs can by mapped to various instances. That can form various
topologies. Each instance is independent from the others and each distance can have its own
attributes such as bridge priority and port cost etc. Consequently, the VLANs in different
instances have their own paths. The traffic of the VLANs are load-balanced.
22.3 MSTP Configuration Task List
MSTP configuration task list:
1. Enable the MSTP and set the running mode
2. Configure instance parameters
3. Configure MSTP region parameters
4. Configure MSTP time parameters
5. Configure the fast migrate feature for MSTP
6. Configure the format of port packet
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