Huawei AR530 series Configuration Manual page 252

Huawei AR530&AR550 Series Industrial Switch Routers
Configuration Guide - Ethernet Switching
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CIST Calculation
After completing the configuration message comparison, the switching device with the highest
priority on the entire network is selected as the CIST root. MSTP calculates an IST for each
MST region, and computes a CST to interconnect MST regions. On the CST, each MST region
is considered a switching device. The CST and ISTs constitute a CIST for the entire network.
Issue 01 (2014-11-30)
Vector Name Description
Regional root ID Identifies the MSTI regional root. The regional root ID consists of
the priority value (16 bits) and MAC address (48 bits).
The priority value is the priority of MSTI 0.
Internal root path Indicates the path cost from the local bridge to the regional root.
cost (IRPC) The IRPC saved on a regional edge port is greater than the IRPC
saved on a non-regional edge port.
Designated Identifies the nearest upstream bridge on the path from the local
switching device bridge to the regional root. If the local bridge is the root or the
ID regional root, this ID is the local bridge ID.
Designated port Identifies the port on the designated switching device connected to
ID the root port on the local bridge. The port ID consists of the priority
value (4 bits) and port number (12 bits). The priority value must be
a multiple of 16.
Receiving port ID Identifies the port receiving the BPDU. The port ID consists of the
priority value (4 bits) and port number (12 bits). The priority value
must be a multiple of 16.
The vector comparison principle is as follows:
For a vector, the smaller the priority value, the higher the priority.
Vectors are compared based on the following rules:
1. Compare the IDs of the roots.
2. If the IDs of the roots are the same, compare ERPCs.
3. If ERPCs are the same, compare the IDs of regional roots.
4. If the IDs of regional roots are the same, compare IRPCs.
5. If IRPCs are the same, compare the IDs of designated switching devices.
6. If the IDs of designated switching devices are the same, compare the IDs of designated
ports.
7. If the IDs of designated ports are the same, compare the IDs of receiving ports.
If the priority of a vector carried in the configuration message of a BPDU received by a
port is higher than the priority of the vector in the configuration message saved on the port,
the port replaces the saved configuration message with the received one. In addition, the
port updates the global configuration message saved on the device. If the priority of a vector
carried in the configuration message of a BPDU received on a port is equal to or lower than
the priority of the vector in the configuration message saved on the port, the port discards
the BPDU.
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7 MSTP Configuration
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