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the 802.1D STP topology change mechanism on S5700 switch - REPRINT

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authorized reprint by author zhushigeng(Vinsoney)


1. Overview of the 802.1D STP topology change mechanism

The switch that first recognizes the topology change sends a Topology Change Notification BPDU (TCN), the specific bridge receives the TCN, and confirms by immediately returning a BPDU which TCA field is set. Before the bridge switch confirm the TCN with sending BPDU, the bridge which send the TCN will continue to send TCN BPDUs.

The switch which receives the TCN will then generate another TCN and send the TCN out from the root port, and the process will be sent all the way to root switch.

Once the root bridge switch is aware of the topology change in the network, it will send the configuration BPDU (Topology Change Configuration BPDU) that the TC field is set. Each switch in the network will deliver these BPDUs, thus facilitating each switch is aware of the topology changes.


2. related messages

In the process of topology changing information to the entire network, a total of three BPDUs are involved:

1) TCN BPDU: Topology Change Notification BPDU. The non-root switch advertises topology change information to the upstream switch on the root port, and sends it every Hello Time (2 seconds) until it receives the topology change confirmation configuration BPDU or topology change configuration BPDU of the uplink switch.

2) TC-ACK BPDU: Topology Change Acknowledgment Configuration BPDU. One type of configuration BPDU. Different from the common configuration BPDU, this configuration BPDU sets a Flag bit. The non-root switch sends the acknowledgement information of the topology change notification to the downlink switch on the designated interface that receives the topology change notification BPDU.

3) TC BPDU: Topology Change Configuration BPDU. This configuration BPDU sets another Flag bit. the root switch uses it sed to flood topology changing information to the entire network. All switches flood the BPDUs on all their designated ports.


3. Examples





1) SwitchA hangs

2) SwitchB first detects the topology change, so the TCN BPDU is generated and sent out from the root port (because the root port is in the direction of the root bridge), B will continuously send the TCN BPDU until the designated switch C sends a TCN ACK to confirm.

3) Switch C receives the TCN BPDU, sends a TCN ACK to confirm, and forwards the TCN BPDU to its own root port.

4) The Root receives this TCN and sends a TCN ACK to C.

5) Root modifies its own configuration BPDU to inform the entire switching network about topology changes. The root sets the topology change (TC flag) for a period of time in the configuration BPDU. This time is equal to the forwarding delay + Max. Age, the default is 35S.

6) When the switch receives the configuration BPDU from the Root that the TC flag is set, they use the Forward Delay Timer (default 15S) to update the entries in its MAC address table. In other words, the lifetime of the entry is changed from the default value of the original 300S to 15S, which ensures that the MAC address entry is refreshed more quickly. The switch will continue this process until it no longer receives a TC BPDU message from the Root.


We will find that when the topology changes, the TCN will be generated. However, in some cases, the transition flooding of the TCN may have an unnecessary impact on the network. The interface connected to the PC terminal device on the access layer switch is set to The edge port can optimize the network to a certain extent to prevent excessive TCN from being generated due to the PC's power on/off.


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