VIPA as the name suggest is a virtual IP address, that is, it is not associated with a physical (OSA) interface and is therefore available for the life of the TCP/IP stack, which in most organisations is the life of the mainframe LPAR.
Users and services (i.e. FTP) connect to the VIPA address, not the physical OSA addresses. When multiple adapters are installed, multiple ‘routes’ to the VIPA exist. Should one ‘route’ (adapter) fail the traffic transparently uses another route.
The implementation of VIPA turns the mainframe TCP/IP stack into a router, which will participate in routing decisions with the network, notifying the network of changes to the state of the physical interfaces, allowing the network to route around failures transparently to the users.
Another advantage of VIPA is that the TCP/IP stack can load balance outgoing connections, based on the ‘best’ route and load of the adapter. This is only done on outgoing traffic, as the TCP/IP stack has no control of the route incoming packets take, this is a network decision.
To make full use of VIPA, multiple connections, OSA ports, are required to the TCP/IP stack. VIPA can handle any number of physical adapters. The TCP/IP stack will exchange routing information with routers on the network to enable routing decisions, therefore the network routers must ‘talk’ the same routing protocol as the VIPA enabled stack.
At present z/OS TCP/IP only supports the RIP and OSPF routing protocols (via OROUTED or OMPROUTE), the network does not use these routing protocols natively. Instead utilises the proprietary Cisco routing protocol EIGRP. Therefore ‘redistribution’ routers are required. These routers (Cisco 6509 multi-layer switches) act as a gateway between the native routing protocol and the selected VIPA routing protocol. OSPF is the preferred routing protocol due to its faster convergence time.
As the connections to the VIPA are routed from the physical adapters, it is necessary that the VIPA be in a different TCP/IP subnet from the physical adapters. Therefore all the mainframe TCP/IP clients will need to be re‑addressed to point to the VIPA. DNS greatly simplifies this. However it is possible that the TN3270 emulator session configuration could be ‘pushed’ out to the clients.
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