Fujitsu BX620, PG-FCS103, PG-FCS102 manual Topology Change Notice TCN, 144

4.8.4Topology 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.

144