Managing Serviceguard 12th Edition, March 2006

Understanding Serviceguard Software Components
Responses to Failures
Chapter 3 129
Network Communication Failure
An important element in the cluster is the health of the network itself.
As it continuously monitors the cluster, each node listens for heartbeat
messages from the other nodes confirming that all nodes are able to
communicate with each other. If a node does not hear these messages
within the configured amount of time, a node timeout occurs, resulting in
a cluster re-formation and later, if there are still no heartbeat messages
received, a TOC. In a two-node cluster, the use of an RS-232 line
prevents a TOC from the momentary loss of heartbeat on the LAN due to
network saturation. The RS232 line also assists in quickly detecting
network failures when they occur.