Huawei Quidway S3900 Series Operation Manual page 240

Operation Manual – MSTP
Quidway S3900 Series Ethernet Switches-Release 1510
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:
The smaller the root ID of the configuration BPDU is, the higher the priority of the
configuration BPDU is.
For configuration BPDUs with the same root IDs, the comparison is based on the
path costs. Suppose S is the sum of the root path cost and the corresponding path
cost of the port. The less the S value is, the higher the priority of the configuration
BPDU is.
For configuration BPDUs with both the same root ID and the same root path cost,
the designated bridge ID, designated port ID, the ID of the receiving port are
compared in turn.
3) A spanning tree is figured out as follows:
Determining the root bridge
The root bridge is selected by configuration BPDU comparing. The switch with the
smallest root ID is chosen as the root bridge.
Determining the root port
For each switch in a network, the port through which the configuration BPDU with the
highest priority is received is chosen as the root port of the switch.
Determining the designated port
First, the switch generates a designated port configuration BPDU for each of its port
using the root port configuration BPDU and the root port path cost, with the root ID
being replaced with that of the root port configuration BPDU, root path cost being
replaced with the sum of the path cost of the root port configuration BPDU and the path
cost of the root port, the ID of the designated bridge being replaced with that of the
switch, and the ID of the designated port being replaced with that of the port.
The switch then compares the resulting configuration BPDU with the original
configuration BPDU received from the corresponding port on another switch. If the
latter takes precedence over the former, the switch blocks the local port and remains
the port's configuration BPDU unchanged, so that the port can only receive
configuration messages and cannot forward packets. Otherwise, the switch sets the
local port to the designated port, replaces the original configuration BPDU of the port
with the resulting one and releases it regularly.
Huawei Technologies Proprietary
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Chapter 1 MSTP Configuration