TP-Link TL-SL5428E manual ¾ STP Generation, Step Operation

¾STP Generation

zIn 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.

zComparing 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

1If the priority of the BPDU received on the port is lower than that of the BPDU if of the port itself, the switch discards the BPDU and does not change the BPDU of the port.

2If 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 and compares it with those of other ports on the switch to obtain the one with the highest priority.

Table 7-1 Comparing BPDUs

zSelecting the root bridge

The root bridge is selected by BPDU comparing. The switch with the smallest root ID is chosen as the root bridge.

zSelecting the root port and designate port The operation is taken in the following way:

Step	Operation
1	For each switch (except the one chosen as the root bridge) in a network, the port
	that receives the BPDU with the highest priority is chosen as the root port of the
	switch.
2	Using the root port BPDU and the root path cost, the switch generates a
	designated port BPDU for each of its ports.
	z	Root ID is replaced with that of the root port;
	z	Root path is replaced with the sum of the root path cost of the root port and
		the path cost between this port and the root port;
	z	The ID of the designated bridge is replaced with that of the switch;
	z	The ID of the designated port is replaced with that of the port.
3	The switch compares the resulting BPDU with the BPDU of the desired port
	whose role you want to determine.
	z	If the resulting BPDU takes the precedence over the BPDU of the port, the
		port is chosen as the designated port and the BPDU of this port is replaced
		with the resulting BPDU. The port regularly sends out the resulting BPDU;
	z	If the BPDU of this port takes the precedence over the resulting BPDU, the
		BPDU of this port is not replaced and the port is blocked. The port only can
		receive BPDUs.
		Table 7-2 Selecting root port and designated port

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