H3C H3C S3100 8C SI, Operation Manual

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Operation Manual – MSTP
H3C S3100 Series Ethernet Switches
Chapter 1 MSTP Configuration
1-5
Table 1-3 Selection of the root port and designated ports
Step Description
1
A non-root-bridge device takes the port on which the optimum configuration
BPDU was received as the root port.
2
Based on the configuration BPDU and the path cost of the root port, the
device calculates a designated port configuration BPDU for each of the rest
ports.
z
The root bridge ID is replaced with that of the configuration BPDU of the
root port.
z
The root path cost is replaced with that of the configuration BPDU of the
root port plus the path cost corresponding to the root port.
z
The designated bridge ID is replaced with the ID of this device.
z
The designated port ID is replaced with the ID of this port.
3
The device compares the calculated configuration BPDU with the
configuration BPDU on the port whose role is to be determined, and acts as
follows based on the comparison result:
z
If the calculated configuration BPDU is superior, this port will serve as
the designated port, and the configuration BPDU on the port will be
replaced with the calculated configuration BPDU, which will be sent out
periodically.
z
If the configuration BPDU on the port is superior, the device stops
updating the configuration BPDUs of the port and blocks the port, so
that the port only receives configuration BPDUs, but does not forward
data or send configuration BPDUs.
Note:
When the network topology is stable, only the root port and designated ports forward
traffic, while other ports are all in the blocked state – they only receive STP packets but
do not forward user traffic.
Once the root bridge, the root port on each non-root bridge and designated ports have
been successfully elected, the entire tree-shaped topology has been constructed.
The following is an example of how the STP algorithm works. The specific network
diagram is shown in Figure 1-2 . The priority of Device A is 0, the priority of Device B is 1,
the priority of Device C is 2, and the path costs of these links are 5, 10 and 4
respectively.