H3C Technologies H3C S3100 Series Switches - Manual (Page 181)

Operation Manual – MSTP
H3C S3100-52P Ethernet Switch
Chapter 1 MSTP Configuration
1-6
z
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.
z
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:
z
The smaller the root ID of the configuration BPDU is, the higher the priority of the
configuration BPDU is.
z
For configuration BPDUs with the same root IDs, the path costs are compared.
Suppose S is the sum of the root path costs and the corresponding path cost of the
port. The less the S value is, the higher the priority of the configuration BPDU is.
z
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 calculated as follows:
z
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.
z
Determining the root port
For each switch in a network, the port on which the configuration BPDU with the highest
priority is received is chosen as the root port of the switch.
z
Determining the designated port
First, the switch calculates a designated port configuration BPDU for each of its ports
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 root 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 calculated 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 keeps 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
calculated one and advertises it regularly.