TP-Link T3700G-28TQ, Руководство пользователя

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port begins to forward data after twice forward delay, which ensures the new configuration
BPDUs are spread in the whole network.

BPDU Comparing Principle in STP mode
Assuming two BPDUs: BPDU X and BPDU Y
If the root bridge ID of X is smaller than that of Y, X is superior to Y.
If the root bridge ID of X equals that of Y, but the root path cost of X is smaller than that of Y, X
is superior to Y.
If the root bridge ID and the root path cost of X equal those of Y, but the bridge ID of X is
smaller than that of Y, X is superior to Y.
If the root bridge ID, the root path cost and bridge ID of X equal those of Y, but the port ID of X
is smaller than that of Y, X is superior to Y.

STP Generation

In the beginning
In the beginning, each switch regards itself as the root, and generates a configuration BPDU for
each port on it as a root, with the root path cost being 0, the ID of the designated bridge being
that of the switch, and the designated port being itself.

Comparing BPDUs
Each switch sends out configuration BPDUs and receives a configuration BPDU on one of its
ports from another switch. The following table shows the comparing operations.
Step Operation
1 If the priority of the BPDU received on the port is lower than that of the BPDU of
the port itself, the switch discards the BPDU and does not change the BPDU of
the port itself. If the priority of the BPDU is higher than that of the BPDU of the
port itself, the switch replaces the BPDU of the port with the received one.
2 The switch compares the BPDUs of all the ports, and selects the BPDU with the
highest priority as the switch’s BPDU.
Table 8-2 Comparing BPDUs

Selecting the root bridge
The root bridge is selected by BPDU comparing. The switch with the smallest bridge ID is
chosen as the root bridge.

Selecting the root port
The non-root switch will go through the following steps when selecting a root port:
1)
Choose the port that provides the lowest root path cost as the root port.
2) If multiple ports provide the same root path cost, the port which is connected to the
neighboring switch (the switch will go through to reach the root bridge) with the smallest
bridge ID will be selected as the root port.
3) If multiple ports go through the same neighboring switch, the port with the highest priority
will be selected as the root port.
4) If the priorities are the same between the ports, the port with the smallest port number will
be selected as the root port.
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