Managing HP Serviceguard for Linux, Eighth Edition, March 2008

Understanding Serviceguard Software Components
How the Network Manager Works
Chapter 3 81
Package Switching and Relocatable IP Addresses
A package switch involves moving the package to a new system. In the
most common configuration, in which all nodes are on the same
subnet(s), the package IP (relocatable IP; see “Stationary and
Relocatable IP Addresses and Monitored Subnets” on page 74) moves as
well, and the new system must already have the subnet configured and
working properly, otherwise the packages will not be started.
NOTE It is possible to configure a cluster that spans subnets joined by a router,
with some nodes using one subnet and some another. This is called a
cross-subnet configuration. In this context, you can configure packages
to fail over from a node on one subnet to a node on another, and you will
need to configure a relocatable address for each subnet the package is
configured to start on; see “About Cross-Subnet Failover” on page 136,
and in particular the subsection“Implications for Application
Deployment” on page 137.
When a package switch occurs, TCP connections are lost. TCP
applications must reconnect to regain connectivity; this is not handled
automatically. Note that if the package is dependent on multiple subnets
(specified as monitored_subnets in the package configuration file), all
those subnets must normally be available on the target node before the
package will be started. (In a cross-subnet configuration, all subnets
configured on that node, and identified as monitored subnets in the
package configuration file, must be available.)
The switching of relocatable IP addresses is shown in Figure 3-6 and
Figure 3-7.
Address Resolution Messages after Switching on the
Same Subnet
When a relocatable IP address is moved to a new interface, either locally
or remotely, an ARP message is broadcast to indicate the new mapping
between IP address and link layer address. An ARP message is sent for
each IP address that has been moved. All systems receiving the
broadcast should update the associated ARP cache entry to reflect the
change. Currently, the ARP messages are sent at the time the IP address
is added to the new system. An ARP message is sent in the form of an