VSX switch reboot

After a VSX switch reboots, it has no entries for ARP, MAC, and routes. If downstream VSX LAG ports are activated before all this information is relearned, traffic is dropped. To avoid a traffic drop, VSX LAGs on the rebooted switch stay down until the restoration of LACP, MAC, ARP databases, and MSTP states if MSTP is used.

The learning process for the VSX LAGs has two phases:

  • Initial sync phase: The LACP states, MAC address table, ARP table, and potentially MSTP states are downloaded from the forwarding switch to the freshly rebooted switch.

  • Link-up delay phase: The downloaded entries are installed into the ASIC. Router adjacencies with core nodes and learned upstream routes are also established.

    The link-up delay phase is configurable with the linkup-delay-timer <DELAY-TIMER> command. The default value is 180 seconds. Set the link-up delay timer to the maximum value of 600 seconds for a network with many MAC addresses, a large ARP table, or a large routing table.

When both VSX switches reboot, the link-up delay timer is not used because both switches are trying to relearn the LACP states, MAC address table, and ARP table.

To get upstream router adjacencies established during the link-up delay, the upstream LAGs have to be excluded from the scope of the link-up delay. Run the linkup-delay-timer exclude lag-list <LAG-LIST> for identifying the LAGs for exclusion.

For example, assume that you have a topology similar to the one in VSX solution topology overview, the upstream LAGs (LAG 101 and LAG 102), would need to be identified by the linkup-delay-timer exclude lag-list <LAG-LIST> for exclusion before a VSX switch reboot.