What does the OSPF topology change notification (TCN) indicate?

The OSPF topology change notification (TCN) signifies a change in the network topology. Specifically, it indicates that there has been a modification in the state of the OSPF network, such as the addition or removal of a router or interface. When an OSPF router detects a significant change in the network, it sends a TCN to its adjacent routers to notify them of the topology alteration. This proactive communication allows OSPF to quickly reassess and adjust the routing tables accordingly to maintain optimal routing paths.

This TCN mechanism is essential for ensuring that all participating routers remain synchronized with the current network configuration and can respond quickly to changes, thus promoting network stability and efficiency. In summary, the TCN is a crucial part of OSPF's ability to adapt to changes in the network topology, ensuring robust and reliable routing.