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This blog will explain that it is possible to run an SAP system in different subnets and why a reboot of the whole SAP system is necessary if the (A)SCS instance will be moved to a different subnet, for example using a high availability solution.

The (A)SCS instance contains the Message Server (msg_server.exe) and the Enqueue Server (enserver.exe). Both processes are the single point of failure components of an SAP system. To configure them highly available, the (A)SCS will be installed in a Microsoft Failover Cluster or any other HA solution available on the market.

A prerequisite is that the virtual host name used for the (A)SCS instance (which is usually SAPGLOBALHOST name by default), has one IP address configured in DNS, a so-called A-record in DNS.

When you start the SAP System all “clients” will connect to either message server or enqueue server.

Clients mean here:

  • SAP application server instances (ABAP or JAVA)
  • SAP standalone gateway instances
  • SAP Web Dispatcher instances
  • SAP routers
  • any RFC client (from 3rd party vendors)

They all want to connect to the hostname used for the (A)SCS instance. The OS retrieves the IP address for the (A)SCS hostname and all clients are using this IP address for communication.

If a failover will occur to site B, message server and enqueue server are running under a different IP address now. All clients are still trying to reconnect using the IP address of site A.

 

Take a look at this video to understand this basic principle of SAP operations: https://sapvideoa35699dc5.hana.ondemand.com/?entry_id=1_b9e6t03p

 

Conclusion:

If you failover an SAP (A)SCS instance in an HA environment and you don’t want to restart any client, then use the same network subnet.

If you use different subnets, a restart of all clients is always mandatory.

 

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