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4.8.4 Topology Change Notice (TCN)
A bridge/switch creates a forwarding table of MAC addresses and port numbers by learning the
source MAC address that received on a particular port. The table is used to forward frames to a
specific port rather than flooding the frame to all ports. The typical maximum aging time of entries in
the table is 5 minutes. Only when a host has been silent for 5 minutes would its entry be removed
from the table. It is sometimes beneficial to reduce the aging time. For example, when a forwarding
link goes to blocking and a different link goes from blocking to forwarding. This change could take
up to 50 seconds. At the end of the STP re-calculation a new path would be available for
communications between end stations. However, because the forwarding table would still have
entries based on the old topology, communications may not be reestablished until after 5 minutes
when the affected ports entries are removed from the table. Traffic would then be flooded to all ports
and re-learned. In this case it is beneficial to reduce the aging time. This is the purpose of a topology
change notice (TCN) BPDU. The TCN is sent from the affected bridge/switch to the root bridge/
switch. As soon as a bridge/switch detects a topology change (a link going down or a port going to
forwarding) it sends a TCN to the root bridge via its root port. The root bridge then advertises a
BPDU with a Topology Change to the entire network.This causes every bridge to reduce the MAC
table aging time to 15 seconds for a specified amount of time. This allows the switch to re-learn the
MAC addresses as soon as STP re-converges.
Topology Change Notice BPDUs are sent when a port that was forwarding changes to blocking or
transitions to forwarding. A TCN BPDU does not initiate an STP recalculation. It only affects the
aging time of the forwarding table entries in the switch.It will not change the topology of the network
or create loops. End nodes such as servers or clients trigger a topology change when they power off
and then power back on.