IP monitoring of reth interface LAGs

In addition to the reth interface, IP monitoring through a redundant LAG is supported to take advantage of both throughput and redundancy. IP monitoring checks the end-to-end connectivity of configured IP addresses and allows a redundancy group to automatically fail over when the monitored IP address is not reachable through the reth interface. Both the primary and secondary devices in the chassis cluster monitor specific IP addresses to determine whether an upstream device in the network is reachable.