Managing Serviceguard 12th Edition, March 2006

Understanding Serviceguard Hardware Configurations
Redundant Network Components
Chapter 242
NOTE The use of a serial (RS232) heartbeat line is supported only in a two-node
cluster configuration. A serial heartbeat line is required in a two-node
cluster that has only one heartbeat LAN. If you have at least two
heartbeat LANs, or one heartbeat LAN and one standby LAN, a serial
(RS232) heartbeat should not be used.
If the heartbeat network card fails on one node, having a serial line
heartbeat keeps the cluster up just long enough to detect the LAN
controller card status and to fail the node with bad network connections
while the healthy node stays up and runs all the packages.
Even if you have a serial (RS232) line configured for redundant
heartbeat, one LAN is still required to carry a heartbeat signal. The
serial line heartbeat protects against network saturation but not against
network failure, since Serviceguard requires TCP/IP to communicate
between cluster members.
Serial (RS232) lines are inherently unreliable compared to network
cards which run the TCP/IP protocol, such as Ethernet or FDDI. Unlike
TCP/IP, the serial line protocol has no error correction or retry
mechanism. Serial lines can also be complex and difficult to configure
because of a lack of standards.
A serial (RS232) heartbeat line is shown in Figure 2-4.
Figure 2-4 Serial (RS232) Heartbeat Line