15.1.2 How Fault Discovery Works in VRRP

The master router periodically (every second by default) sends advertisement packets from the IP interface on which the virtual router is configured to report its operating status to the backup routers. When the backup routers receive an advertisement packet from the master router, they know the master router is running normally. The following figure shows an example of sending an advertisement packet.

Figure 15-3: Sending ADVERTISEMENT packets

If the master router fails, it cannot send advertisement packets. A failure can occur, for example, if the entire Switch has failed, a failure prevents the IP interface on which the virtual router is configured from sending packets, or cables are disconnected.

If backup routers do not receive an advertisement packet from the master router for a specified period, they determine that the master router has failed, and one of them enters the master state.