Network startup

The network subsystems such as VTAM, TCP/IP, and related components are normally started as part of the IPL sequence under z/OS.

There is normally an automation software product that controls the startup, and this has dependency checks or parent-child relationships built into it. For example, VTAM would have to start prior to TCP/IP, and FTP (an application daemon under TCP/IP) cannot start before TCP/IP. Because FTP requires TCP/IP and TCP/IP requires VTAM, such a starting order makes sense. However, it is not always necessary. For example, TCP/IP waits for VTAM to start up if it detects VTAM is not available.

VTAM and TCP/IP are started tasks, with JCL procedures like many other z/OS components. You should familiarize yourself with the JCL and data sets, and members that are in use for these components.

The network administrator should work with the automation team to address any network component startup sequences and dependency requirements you have. Many of the network component dependencies and relationships could already be in place.

Note: The z/OS network administrator should develop the initial network component runbook for operations. If there are any particular checks or commands that need issuing as part of an network startup or takedown, these should be included.

System startup dependencies for the network

Some of the dependencies that are of concern during system startup are:

The network cannot start before the z/OS operating system and JES2 are up and running.
Network devices need to be varied online by the operating system.
The network must start up before any network applications. There is no point in starting these before the underlying network is available.
Enterprise Extender (EE) cannot be enabled until both VTAM and TCP/IP are up.
OMPROUTE is started after TCP/IP is available.
In a sysplex, ownership of dynamic VIPA addresses is dynamically determined, but there might be a moment during IPL when the dynamic VIPA is associated with a backup host. The same applies to a sysplex distributor environment.