Huawei Quidway S3100 Series Operation Manual page 162

Operation Manual – MSTP
Quidway S3100 Series Ethernet Switches
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:
Selecting 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.
Selecting the root port
For each switch (except the one chosen as the root bridge) in a network, the port that
receives the configuration BPDU with the highest priority is chosen as the root port of
the switch.
Selecting 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 configuration
BPDU received from the peer 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
1-6
Chapter 1 MSTP Configuration