Redundant Network Paths

When using an HA configuration, it can be important to use redundant paths to vital resources such as the Internet. The paths through the network from the master device in an HA configuration may fail in which case it may be desirable to have this failure trigger a failover to the slave unit which has a different path to the resource.

Monitoring Paths

Monitoring the availability of specific network paths can be done with the cOS Core Link Monitor. The Link Monitor allows the administrator to specify particular hosts whose reachability is monitored using ICMP "Ping" requests and therefore link status. If these hosts become unreachable then the link is considered failed and a failover to a slave can be initiated. Provided that the slave is using a different network link and also monitoring the reachability of different hosts, traffic can continue to flow.

Using the Shared IP Address

When this property of the Link Monitor object is enabled, it allows the link monitor pings to be sent from the shared IP address instead of sending using the individual IPs of each unit. This is useful if public IPv4 addresses are not available for the sending interface.

It should also be noted that when this option is enabled, pings are sent only from the active cluster peer. When it is disabled pings are sent both from the active and inactive peers.

Further Link Monitor Reading

Using link monitoring is described further in Section 2.4.3, The Link Monitor.