# Spanning Tree Topology Change Notification (TCN)

**URL:** <https://forum.networklessons.com/t/spanning-tree-topology-change-notification-tcn/1120>\
**Category:** Lessons Discussion\
**Created:** [December 27, 2016, 4:00pm UTC](https://forum.networklessons.com/t/spanning-tree-topology-change-notification-tcn/1120 "2016-12-27T16:00:15Z")\
**Posts on this page:** 1\
**Showing post:** 32

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**Author:** ![lagapidis](https://cdn-forum.networklessons.com/user_avatar/forum.networklessons.com/lagapidis/32/4949_2.png) [@lagapidis](https://forum.networklessons.com/u/lagapidis)\
**Post date:** [March 26, 2017, 7:31pm UTC](https://forum.networklessons.com/t/spanning-tree-topology-change-notification-tcn/1120/32 "2017-03-26T19:31:41Z")

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Hello Americo

Once the root bridge is aware of a change in the topology of the network, it sets the TC flag on the BPDUs it sends out, which are then relayed to all the bridges in the network. When a bridge receives a BPDU with the TC flag bit set, it reduces its bridging-table aging time from 300 seconds to 15 seconds. This ensures a relatively quick flush of stale information.

I hope this has been helpful!

Laz

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