3Com 5500-SI Configuration Manual page 167

Determining an MSTI
In an MST region, MSTP generates different MSTIs for different VLANs according to
VLAN-to-spanning tree mappings. MSTP calculates each spanning tree independently
in the same way as STP/RSTP does.
Implementation of STP algorithm
In the beginning, each of the ports on each switch generate its own BPDU, taking the
switch as the root, setting the root path cost to 0, the ID of the designated bridge to
that of the switch, and the designated port to itself.
1 Each switch releases its BPDUs and operates as follows when receiving BPDUs of other
switches.
Discards the received BPDUs whose priorities are lower than that of its BPDU. That
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is, the received messages of this type are not processed.
If the priority of a received BPDU is higher than that of the BPDU released by one
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of its ports, the switch replaces the BPDU of the port with the received one and
compares the resulted BPDU with those of its other ports to obtain the one with
the highest priority.
2 BPDUs are compared based on the following principles:
The BPDU with a smaller root ID has a higher priority.
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For BPDUs with the same root ID, the BPDU with a smaller sum of the root path
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cost (in the BPDU) and the path cost of the port has a higher priority.
For BPDUs with the same root ID and root path cost, the designated bridge ID,
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designated port ID, the ID of the port from which the BPDU is received are
compared in turn.
3 A spanning tree is figured out in the following steps:
Determine the root bridge
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The root bridge is determined through BPDU comparing. The switch with the
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smallest root ID is the root bridge.
Determine the root port
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For each switch in a network, the port from which the BPDU with the highest
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priority is received is set to be the root port of the switch.
Determine the designated port
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First, a switch generates a designated port BPDU for each of its port based on the root
port BPDU and the root port path cost, setting the root ID to that of the root port
BPDU, the root path cost to the sum of the path cost of the root port BPDU and the
path cost of the root port, the ID of the designated bridge to that of the switch, and
the ID of the designated port to that of the port.
Then, the switch compares the resulted BPDUs with the original BPDUs of its ports.
For ports whose original BPDUs are better, the switch blocks them and remains their
original BPDUs, allowing them to receive BPDUs and forbidding them from
forwarding data. For a port whose original BPDU is not as good, the switch sets it to
the designated port, replaces the original BPDU with the calculated one, and releases
the new BPDU regularly.
MSTP Overview 167