3Com 4210 PWR Configuration Manual page 154

152 C 14: MSTP C
HAPTER ONFIGURATION
Calculate an MSTI
In an MST region, different MSTIs are generated for different VLANs based on the
VLAN-to-MSTI mappings. Each spanning tree is calculated independently, in the
same way as how STP/RSTP is calculated.
Implement STP algorithm
In the beginning, each switch regards itself as the root, and generates a
configuration BPDU for each port on it as a root, with the root path cost being 0,
the ID of the designated bridge being that of the switch, and the designated port
being itself.
1 Each switch sends out its configuration BPDUs and operates in the following way
when receiving a configuration BPDU on one of its ports from another switch:
If the priority of the configuration BPDU is lower than that of the configuration
■
BPDU of the port itself, the switch discards the BPDU and does not change the
configuration BPDU of the port.
If the priority of the configuration BPDU is higher than that of the
■
configuration BPDU of the port itself, the switch replaces the configuration
BPDU of the port with the received one and compares it with those of other
ports on the switch to obtain the one with the highest priority.
2 Configuration BPDUs are compared as follows:
For MSTP, CIST configuration information is generally expressed as follows:
■
(Root bridge ID, External path cost, Master bridge ID, Internal path cost,
Designated bridge ID, ID of sending port, ID of receiving port)
The smaller the Root bridge ID of the configuration BPDU is, the higher the
■
priority of the configuration BPDU is.
For configuration BPDUs with the same Root bridge IDs, the External path
■
costs are compared.
For configuration BPDUs with both the same Root bridge ID and the same
■
External path costs, Master bridge ID, Internal path cost, Designated bridge
ID, ID of sending port, ID of receiving port are compared in turn.
For MSTP, MSTI configuration information is generally expressed as follows:
■
(Instance bridge ID, Internal path costs, Designated bridge ID, ID of sending
port, ID of receiving port)
The smaller the Instance bridge ID of the configuration BPDU is, the higher
■
the priority of the configuration BPDU is.
For configuration BPDUs with the same Instance bridge IDs, Internal path
■
costs are compared.
For configuration BPDUs with both the same Instance bridge ID and the
■
same Internal path costs, Designated bridge ID, ID of sending port, ID of
receiving port are compared in turn.
3 A spanning tree is calculated as follows:
Determining the root bridge
■
Root bridges are selected by configuration BPDU comparing. The switch with the
smallest root ID is chosen as the root bridge.