The system must determine whether a program can be started locally on the same host or if an RFC call must be initiated that transfers program execution to another host. To make this decision, the host machine checks whether the RFC address matches one of its own names. If so, the RFC is performed locally.
On cluster hosts the gateway requires that host names that translate into virtual IP addresses are used. Otherwise it would be necessary to change the instance profile, if the instance is switched over to another host machine with different non-virtual names. Only virtual names stay constant and so comply with the transparent addressing techniques that are required by the SAP system. The host name configured as SAPLOCALHOSTFULL, if not set explicitly, contains the returned value of a gethostname system call as the default. This is read into gw/alternative_hostnames by default and means that you should configure SAPLOCALHOSTFULL to contain a virtual host name as well.
The gateway can only remember a limited number of addresses. The following limit applies:
512 addresses are stored in all SAP R/3 Releases from 4.6D onwards (with patch level 878)
This limit can be a problem for systems that use virtual addressing, because they require many addresses. This limit has implications for the types of network setup that can be run with the SAP system in a switchover environment.
The number of local addresses that must be known to the gateway at any given moment varies with the:
? Number of LANs used
? Number of stationary addresses used on one host
? Number of virtual addresses used on one host
? Switchover status, since the number of virtual addresses is increased if addresses are switched over to the local host machine
In one scenario the CI switches over to the DB instance so that both the CI and DB then run on a single instance. This can occur in a single-LAN or two-LAN setup:
? Single-LAN setup
The virtual IP address of the CI is switched over to the DB host, which already holds 2+1 addresses. As a result, 3+1 addresses need to be held on the gateway of the DB host.
? Two-LAN setup
The 2 virtual IP addresses of the CI are switched over to the DB host, which already holds 4+1 addresses. As a result, 6+1 addresses need to be held on the gateway of the DB host.
‘+1’ above refers to the entry needed for localhost
To look at the first five addresses held by the gateway, execute the following operating system command for the specified host machine as user <sid>adm:
gwmon pf=/usr/sap/<SID>/SYS/profile/<instanceprofile> nr=<NR>
Normally <instanceprofile> is <SID>_<INSTTYPE><NR>_<hostname>. The command provides a menu (letter m), where you select the letter y to display system information. This option returns a list of all gateways in the SAP system and the host names configured on them. Look for the entry of the LOCAL_R3, in the last column called hostname(s), to find out the host names and addresses that are held by the local gateway. Note that the gwmon tool only lists a maximum of five addresses, even if more addresses are actually known to the gateway.