When the primary uplink of an MX appliance fails over, a 'Primary uplink status' event type will be logged under Monitor > Event log. The "Details" of the event show the new primary uplink.
- uplink: 0 - indicates Internet1 is being used.
- uplink: 1 - indicates Internet2 is being used.
- uplink: 2 - indicates Cellular is being used.
When the MX appliance experiences a Ethernet port state change, an 'Ethernet port carrier change' event type will be logged under Monitor > Event log. The details of this event define the type of state change and port that was affected. For example, 'device: lan0, carrier: false' means the first LAN port on the MX had a 'down' event and the Ethernet link was lost and 'device: lan1, carrier: false' means the second LAN port on the MX had a 'down' event and the Ethernet link was lost.
The MX uses heuristic detection to determine if an internet link is down. If the Ethernet link state is up but the MX is unable to ping internet hosts, resolve DNS names of Internet hosts or ARP entries for the default gateway, it will trigger a link failover.