HP OpenVMS Systems Documentation
OpenVMS System Manager's Manual
18.104.22.168 Unsupported Configuration
When configuring a network to use an OpenVMS system running the LAT Version 5.3 software, avoid the configuration shown in Figure 26-5.
Figure 26-5 Unsupported Multiple Address LAT Configuration
Any configuration similar to this diagram will result in unpredictable
results and may not function. In a network environment, LAT Version 5.1
and 5.2 nodes can have only a single logical LAN address. The
configuration in Figure 26-5 violates this rule. The configuration
shown in Figure 26-4 is a valid alternative.
The LAT software regards all paths as equal, point-to-point communications. The LAT software can support a maximum of eight LAN adapters simultaneously (and it is possible to connect all controllers to the same logical LAN). To get the maximum coverage over possible path failures, each logical link should be created prior to setting the LAT node state to ON in SYS$MANAGER:LAT$SYSTARTUP.COM.
For example, if a system has one Ethernet adapter (device ESA0) with two FDDI adapters (FCA0 and FCB0) and the system manager chooses to run LAT over all adapters, the LAT$SYSTARTUP.COM file would contain the following commands:
It is possible to create logical LAT datalinks after the LAT protocol
has been started. Any existing virtual circuit will attempt to find any
new paths through the newly created logical datalink when it is ready
for use. However, Compaq does not recommend that you create links at
this point because during the time it takes existing virtual circuits
to discover new paths through this newly created datalink, the virtual
circuit may fail before the new paths are discovered.
The OpenVMS LAT software uses a combination of the directory service and solicitation to obtain paths for each virtual circuit. To expedite path discovery at virtual circuit startup, Compaq recommends that you configure a system with multiple LAN adapters to maintain a LAT service and node database, by performing the following actions:
An OpenVMS system running with outgoing connections disabled and no
service and node database is still capable of running with multiple
paths for each virtual circuit. These paths must be discovered through
the LAT solicitation process and will take longer (leaving the
possibility for virtual circuit failure to occur before all paths have
In the unlikely event of a path failure, it will take the OpenVMS LAT software time (which will vary depending on the number of adapters to which the remote node has access) to locate another working path. Therefore, Compaq strongly recommends that you modify the following LAT parameters on potential LAT master nodes:
Although it is possible to keep virtual circuits running through multiple adapters to LAT Version 5.1 or LAT Version 5.2 master nodes, there is still a possibility that the connections to these nodes may fail.
LAT Version 5.2 and LAT Version 5.1 master nodes do not have the ability to recognize multiple paths to LAT nodes that provide services. They can only communicate with such nodes through one remote address at a time. Therefore, if a LAN path failure occurs when a LAT master node running LAT Version 5.1 or Version 5.2 attempts to connect to a remote LAT Version 5.3 node providing services, the LAT Version 5.3 node might not discover this failure in time and the LAT master node may time out the connection. You can partially solve this problem by increasing the retransmit limit to as high a setting as possible.
In addition, if a LAT Version 5.3 node providing services views the
virtual circuit as completely idle during the primary path failure, no
attempt will be made to use any of the alternate paths (because of the
previously described LAT Version 5.2 and 5.1 limitation). Therefore,
although multiple LAN adapters will work with older LAT
implementations, you might still need to upgrade to Version 7.0 or
higher of the OpenVMS operating system to obtain the LAT Version 5.3
protocol, which will correct this type of problem. (Note that this type
of problem affects only those connections that are idle. An example of
where this situation could arise is in an office environment if all
users were to leave their systems at the same time, either at lunchtime
or at the end of the workday.)
The OpenVMS LAT software will attempt to use large buffers over any virtual circuit that comes in over an FDDI controller. This feature can cause problems if an alternate virtual circuit path must go through an Ethernet. Figure 26-6 is an example of the configuration that can cause problems.
Figure 26-6 LAT FDDI Ring and Large Buffers
In this diagram, it is possible for the two OpenVMS systems to communicate using large packets through the path described by controllers B and C. Large packets may exceed 1500 bytes of data (the maximum Ethernet message can contain 1500 bytes of data). If the path described by controllers B and C were to fail, it will not be possible for communication to continue through the path described by A and D.
The path described by controllers A and D pass through an Ethernet LAN segment. The messages that are routed through the 10/100 bridges cannot be larger than the maximum Ethernet message. Problems can occur because the OpenVMS LAT software cannot always detect this kind of configuration.
There are two ways to prevent problems with the previously described configuration. The first and easiest option is to create a logical LAT link using an Ethernet adapter (if either system has an Ethernet LAN adapter). This will force the message size negotiation to be no larger than the maximum sized Ethernet message.
If neither system has an Ethernet controller (thus making the first option not possible), the second option is to override the use of large buffer support (which is enabled by default) by using the new LATCP command qualifier, /[NO]LARGE_BUFFER. For example:
Compaq recommends that you use the SET NODE/NOLARGE_BUFFER command after all logical LAT links have been created and before the LAT node has been turned on. For example, note the order of the commands in LAT$SYSTARTUP.COM:
26.4 Understanding the LATCP Utility
The LAT Control Program (LATCP) utility configures and controls the LAT software on OpenVMS systems. LATCP commands let you stop and start the LAT driver (which implements the LAT protocol) and modify or display LAT characteristics of the OpenVMS node.
With LATCP, you can set up your system as a service node, which offers one or more resources (services) for access by users on other systems in the local area network (LAN).
You can also use LATCP to set up the system to allow its users to access services on other systems in the LAN. In this case, the system can act like a terminal server: it can manage multiple user sessions simultaneously for connections to services on other nodes.
You can use LATCP to set up your system to support incoming access only, outgoing access only, or both incoming and outgoing access. You can also set up your system so that it supports neither incoming nor outgoing access.
When you set up your system to support outgoing access, the LAT software manages a database of LAT services and nodes. The software builds the database when you enable outgoing access on your node. The software begins to collect LAT service announcements---multicast messages sent by LAT service nodes---and builds the database based on these service announcements. You can use LATCP to display the services and nodes in this database and to control the size of the database. Allow outgoing access on systems that can tolerate the additional overhead, such as standalone systems.
With the LAT protocol, you can set up LAT application ports on the
local node so that users can access printers and other asynchronous
devices that are connected to LAT terminal servers or service nodes on
the LAN. The remote devices must be configured appropriately.
You can also execute a single LATCP command by using a DCL string assignment statement, as shown in the following example:
LATCP executes the SET NODE command and returns control to DCL.
Table 26-1 summarizes the LATCP commands.
For detailed information about LATCP commands and qualifiers, refer to
the OpenVMS System Management Utilities Reference Manual.
As system manager, you start up the LAT protocol and configure your node as a service node by executing the command procedure SYS$STARTUP:LAT$STARTUP. This procedure executes the following two procedures:
To make sure the LAT protocol is started each time the system boots, add a command to execute this procedure in the general-purpose, site-specific startup command procedure, described as follows. (See Section 5.2.1 for more detailed information about this command procedure, including the file specification used to identify it in your operating system.)
To set up your node as a LAT service node and start the LAT protocol software on your system each time the system boots, edit the general-purpose, site-specific startup command procedure to add the following line:
When the general-purpose, site-specific startup command procedure executes this command, it invokes LAT$STARTUP.COM, which in turn invokes the LAT$CONFIG and LAT$SYSTARTUP command procedures.
You can append any of the following arguments to the command line that invokes LAT$STARTUP to specify unique LAT characteristics for your node. The procedure will pass these arguments to LAT$SYSTARTUP.COM to define the LAT characteristics you specify.
Compaq recommends that you modify LAT$SYSTARTUP.COM directly, rather than passing parameters in P1 through P5. However, if you choose to use P1 through P5, the arguments have the following meanings:
Note that if you want to do any of the following LAT network tasks, you must edit LAT$SYSTARTUP.COM (described in Section 26.6):
For a full description of LATCP commands and qualifiers, refer to the OpenVMS System Management Utilities Reference Manual.
The following command creates the service OFFICE on the service node MOE, which is part of the OFFICE cluster (refer to Figure 26-1):
26.6 Customizing LAT Characteristics
To define special LAT characteristics for your node, edit the site-specific command procedure SYS$MANAGER:LAT$SYSTARTUP.COM. This command procedure contains LATCP commands that define LAT characteristics. LAT$SYSTARTUP.COM is invoked when you execute the LAT$STARTUP command procedure. As explained in Section 26.5, you typically execute LAT$STARTUP.COM from the general-purpose, site-specific startup command procedure.
If you want your node to be a LAT service node that only supports incoming connections from interactive terminals, editing LAT$SYSTARTUP.COM is not necessary. You can assign a service name and other characteristics by specifying parameters when you invoke the command procedure SYS$STARTUP:LAT$STARTUP, as described in Section 26.5.
However, you can edit LAT$SYSTARTUP.COM to add LATCP commands that customize LAT characteristics for your node, for example:
1For example, to assign special service announcements or LAN links (using the SET NODE and SET LINK commands).
If you edit LAT$SYSTARTUP.COM, you should add only LATCP commands. In addition, you should conform to the order of commands in the template file SYS$MANAGER:LAT$SYSTARTUP.TEMPLATE. Section 26.6.5 provides a sample edited LAT$SYSTARTUP procedure. The OpenVMS System Management Utilities Reference Manual contains full descriptions of all the LATCP commands you can include in LAT$SYSTARTUP.COM.