Physical and logical networks

A physical network connects two or more physical network interfaces.

As stated in the previous section, configuring multiple TCP/IP-based networks helps to prevent cluster partitioning. PowerHA® SystemMirror® communicates across the storage network when necessary. This additional communication path helps prevent partitioned clusters by providing additional communications paths in cases when the TCP/IP-based network connections become congested or severed between cluster nodes.

Note: If you are considering a cluster where the physical networks use external networking devices to route packets from one network to another, consider the following: When you configure a PowerHA SystemMirror cluster, PowerHA SystemMirror verifies the connectivity and access to all interfaces defined on a particular physical network. However, PowerHA SystemMirror cannot determine the presence of external network devices such as bridges and routers in the network path between cluster nodes. If the networks have external networking devices, ensure that you are using devices that are highly available and redundant so that they do not create a single point of failure in the PowerHA SystemMirror cluster.

A logical network is a portion of a physical network that connects two or more logical network interfaces or devices. A logical network interface or device is the software entity that is known by an operating system. There is a one-to-one mapping between a physical network interface/device and a logical network interface/device. Each logical network interface can exchange packets with each logical network interface on the same logical network.

If a subset of logical network interfaces on the logical network needs to communicate with each other (but with no one else) while sharing the same physical network, subnets are used. A subnet mask defines the part of the IP address that determines whether one logical network interface can send packets to another logical network interface on the same logical network.