Keepalive response in ISL failure scenarios

ISL link is down but the switches are still up and running: In this case, VSX switches use their keepalive connection to determine that they are both up and running. Once that is determined, the user-configured primary VSX switch keeps its multichassis (VSX) LAG links up and the secondary VSX switch forces its VSX LAG links to go down with the appropriate reason. Once the ISL link is up, the MAC and ARP tables of the primary switch are synchronized to the secondary switch. Then, the configured delay timer starts. Once the delay timer expires, the secondary VSX switch brings up its VSX LAG links.

ISL link and one of the VSX switches is down: The running switch sees that the ISL and keepalive connection are both down. Independent of the user configured role (primary or secondary), the switch that is up continues to keep its VSX LAG links up. Subsequently when the peer switch returns, the ISL link comes up first. Then, the returned VSX peer switch synchronizes its MAC and ARP tables from the peer switch that stayed up. After the synchronization completes, the delay time starts. Once the delay timer expires, the VSX peer switch brings up its VSX LAG links.