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STP Topology Changes

Latest reply: Aug 2, 2018 10:49:25 847 1 0 0 0
When the status of the interface at point T changes, a downstream device continuously sends TCN BPDUs to the upstream device.
The upstream device processes only the TCN BPDUs received on the designated port and drops TCN BPDUs on other ports.
The upstream device sets the TCA bit of the Flags field in the configuration BPDUs to 1 and returns the configuration BPDUs to instruct the downstream device to stop sending TCN BPDUs.
The upstream device sends a copy of the TCN BPDUs toward the root bridge.
Steps 1, 2, 3 and 4 are repeated until the root bridge receives the TCN BPDUs.
The root bridge sets the TC bit of the Flags field in the configuration BPDUs to 1 to instruct the downstream devices to delete MAC address entries.
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Created Aug 2, 2018 10:49:25

Worth to add, that downstram devices need to populate flushed mac address table once again. While doing it, switch forwards so called unknown-unicast traffic in all ports inside a VLAN, which causes unnecessary traffic spike.
Doesn't sound too bad in case of faulty link between two switches, but the thing is TCN also floods in two following common scenarios:
- access port connected to the end host is not configured with the stp edged-port enable transits to the forwarding state;
- stp edged-port receives BPDU from the switch;

You can check where TCN BPDU  comes from:
<msk01-lan-c1-c2>display stp   
-------[CIST Global Info][Mode MSTP]-------
CIST Bridge         :0    .xxxx
Config Times        :Hello 2s MaxAge 20s FwDly 15s MaxHop 20
Active Times        :Hello 2s MaxAge 20s FwDly 15s MaxHop 20
CIST Root/ERPC      :0    .xxxx-xxxx-xxxx / 0 (This bridge is the root)
CIST RegRoot/IRPC   :0    .xxxx-xxxx-xxxx / 0 (This bridge is the root)
CIST RootPortId     :0.0
BPDU-Protection     :Disabled
CIST Root Type      :Primary root
TC or TCN received  :2557
TC count per hello  :0
STP Converge Mode   :Normal
Share region-configuration :Enabled
Time since last TC  :1 days 2h:1m:33s
Number of TC        :478
Last TC occurred    :Eth-Trunk6 This post was last edited by Beaverperson at 2018-08-02 05:52.
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