Question regarding STP TCNs

[chandra_rc16]

Please let me know if i wrong.

In STP when a switch detects a topology change it will send TCNs to the neighboring switches and then all the switches reduce their mac aging time to 15secs.

Doubt:

1. The switch that detects the toplogy change will reduce mac aging time to 15 secs only for the port that is affecting or for the entire mac table ?

2. What is the exact time the switches will reduce their mac aging time to ? Is it 15 secs, if yes, How?

Regards,

Chandu

[Deben Bhattarai]

1. Entire MAC table.

for Quest 2 , default is 15.

As the root switch hears the TCN BPDU, it will set TCN ACK flag in all its outgoing configuration BPDUs for the duration of Max_Age+Forward_Time. All switches that see this flag, will set their MAC address tables aging time to Forward_Time. Once the switch that originated the TCN BPDU will hear the TCN ACK, it will stop signaling about the topology change.

When a Switch (Bridge) discovers topology change, it generates a TCN BPDU and send on its root port. The upstream Switch (Bridge) responds back the sender with TCA BPDU confirmint the receipt of TCN BPDU. The upstream Switch (Bridge) (bridge which received the TCN BPDU) generates another TCN BPDU and sends out via its Root Port , The process continues until the Root Switch receives the TCN BPDU.

[chandra_rc16]

max age

—The max age timer controls the maximum           length of time that passes before a bridge port saves its configuration BPDU           information. This time is 20 sec by default, but you can tune the time to be           between 6 and 40 sec.

forward delay

—The forward delay is the time that is           spent in the listening and learning state. This time is equal to 15 sec by           default, but you can tune the time to be between 4 and 30 sec.

So max age + forward delay = 35 secs's

Then why and how switches set their aging time to 15 secs

Regards,

Chandu

[Deben Bhattarai · Accepted solution]

The Root Bridge continues to set the Topology Change flag in all Configuration BPDUs that it sends out for a total of Forward Delay + Max Age seconds (default = 35 (20+15) seconds). This flag instructs all bridges to shorten their MAC address table ( Bridge table) aging process from the default value of 300 seconds to the current Forward Delay value of the bridge (default=15 seconds).

The TCA flag is set by the upstream bridge to tell the downstream bridges to stop sending TCN BPDUs. The TC flag is set in configuration BPDU by the Root Bridge to shorten the bridge table age-out period from default 300 seconds to Forward Delay seconds.

[k.darsh931]

Hii Sir,

Please briefly explain about time of 35+15 Seconds. BPDUs are sending towards all the switches to 35 seconds? 

[Dusty87]

If you are asking why switches are forced to shorten the bridge table aging time specifically to 15 seconds ONLY then I guess the answer for this is to keep the Mac table ready for new entries ( same old existing  Mac   Addresses in the Network being learnt from different ports/paths after convergence). Let me try my best to  eloberate -

Mac learning in STP Happens in learning phase which starts after 15 seconds of listening phase. It’s in this listening phase that the Topology building or convergence becomes stable & gets ready for next phase to learn Mac addresses . So TCN gives that 15 seconds time for switch to actively build/converge to a stable topology meanwhile make the old Mac entries invalid by flushing post 15 seconds & make way for new entries  .