Technical Considerations for a Serviceguard Cluster that Spans Multiple IP Subnets, July 2009

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Network domain requirements
Serviceguard recognizes only the hostname element of a fully-qualified domain name. Serviceguard
now supports different subnets on different nodes but does not support different network domains.
 This means that all the nodes in the cluster must belong to the same network domain.
Heartbeat subnet requirements
Heartbeat subnets in a cross-subnet cluster must meet the following requirements:
 There must be less than 200 milliseconds of latency in the heartbeat network. This is an existing
requirement and remains the same for cross-subnet configuration even though the heartbeat network
can now span across multiple subnets.
 The nodes must be fully connected at the IP level for heartbeat subnets.
 A minimum of two heartbeat subnets must be configured for each cluster node.
o A standby interface by itself does not qualify as a heartbeat path. The reason is that
the amount of time a router needs to recalculate the path might exceed the node
timeout. Hence, part of the cluster might go down when the heartbeat path needs to
be recalculated due to a network failure. Therefore, a dual heartbeat configuration is
required for a cross-subnet cluster regardless of whether standby interfaces are
configured or not.
o A heartbeat subnet can be dedicated or can be shared with a package subnet on a
physical network connection.
 Each heartbeat subnet on a node must be routed on its own separate physical network to the
heartbeat subnet on another node; that is, each heartbeat path must be physically separate.
o The heartbeat subnets must be statically routed; static route entries must be
configured on each node to route the heartbeats through different physical paths.
Otherwise, with dynamic routing, the subnets might be routed on the same physical
network.
o Failure of a single router must not affect both heartbeats at the same time.
Figure 3 shows a heartbeat configuration that fulfills all the requirements for a cross-subnet cluster. A
package network is not shown in this example.
Figure 3 – Architectural overview of a cross-subnet cluster with two dedicated heartbeat subnets