INFO: Troubleshooting SNA Server Communication over an IP Router (148969)

SUMMARY

When you open the SNA Server Admin from a backup SNA Server, you might receive the following error message:

The Primary SNA Server for the domain is not active.

NOTE: The troubleshooting steps described in this article do not apply to SNA Server 3.0.

An SNA Server 3.0 backup or member configuration server locates the primary SNA Server by contacting it's name (or IP address) directly, based on the primary server name entered during SNA Server Setup when the backup or member server was installed. The primary server name is saved under the following registry key:

  HKEY_LOCAL_MACHINE/SYSTEM/CurrentControlSet/Services/SnaBase/PrimaryServer: REG_SZ:

where either the server name or IP address can be entered. If IP address, enter in standard notation (for example: 123.45.67.89). A backup SNA Server tries to open the Com.cfg file on the primary SNA Server. The backup learns about the location of the primary SNA Server when it receives a broadcast mailslot from the primary. The packet looks like the following:

   Src MAC       Dst MAC   Prot    Description                          Src

Other Addr   Dst Other Addr
00AA006B4BE2   *BROADCAST  SMB     C transact, File = \MAILSLOT\SNADMOD
157.57.15.231    157.57.15.255 IP

However, when there is an IP router in between the backup and primary SNA Server, the SNA Server domain must be configured to send directed mailslots instead of broadcasts.

The following outlines how to configure the SNA Server domain to send directed mailslots instead of broadcasts:

In SNA Server Admin, click Server Broadcasts on the Options menu.

NOTE: When changes are made to this dialog box, it effects all SNA Servers in the respective domain.
Select the Microsoft Networking and Route Server Broadcasts Over IP Routers check boxes.
Close the dialog box.
Save the configuration.

NOTE: You must stop and start the SNABASE service for these changes to take effect.

MORE INFORMATION

After you make the above changes, the following is a sample packet sent from the primary to the backup SNA Server.

NOTES:

The destination address is not broadcast, but instead, a specific MAC address is used.

The MAC address will be the router's MAC address if the destination server is on a different segment, but the destination IP address will be the backup SNA server's IP address.

Src MAC          Dst MAC     Prot  Description                          Src
Other Addr  Dst Other Addr
00AA006B4BE2   08002B314A12  SMB   C transact, File = \MAILSLOT\SNADMOD
157.57.15.231   157.57.13.12    IP

When configured this way, the SNA Server relies on the netlogon service of Microsoft Windows NT to retrieve a list of all domain controllers participating in the domain. The SNA Server will send a directed mailslot to each domain controller even if it is not running SNA Server.

There are two common methods that the Windows NT netlogon service uses to retrieve a list of all the servers in the Windows NT domain:

Queries WINS

WINS must have a registration for each computer acting as a domain controller. The WINS server's database should have an entry for the domain_name followed by [1Ch]. If you double-click this entry you should see a list of all the domain controllers registered with this WINS server. If the SNA Server is not in this list, the Windows NT computer has not properly registered with the WINS server as a domain controller.

If your Windows NT domain has 25 or more domain controllers, and you use WINS for name resolution, the SNA server announcements may not be sent to all of the SNA Servers. WINS only keeps a list of 25 domain controllers in the [1Ch] group name. As new backup domain controllers register, they replace a backup domain controller entry in the list. In this case you would need to add LMHOSTS files on the SNA servers or use SNA subdomains.
Uses local name cache via LMHOSTS with the #PRE #DOM switches.

LMHOSTS file must have an entry for every domain controller with the extensions #PRE #DOM:domain. An example of this file can be found in <winntroot>\System32\Drivers\Etc\Lmhosts.sam.

To reload the LMHOSTS file, use the following command:
```
      nbtstat -r
						
```
To verify these entries have been properly cached use the following command:
```
      nbtstat -c
						
```

The following are other things to check:

The SNA Servers must be installed on a domain controller, either a backup or primary, in the same Windows NT domain. It is not sufficient to install on a Windows NT Server Non-domain controller.
From the primary SNA Server, using PING, confirm that the IP address of each SNA Server is reachable.

NOTE: Using PING to resolve the server's name is not relevant, the packet does not use the server's netbios name.
Make sure that the netlogon service on all SNA Servers in the domain is started.
If the SNA Server communication is still failing after you check all of the earlier, network traces of the traffic from the SNA Server should be done to capture the server announcements. These will appear as SMBs, with the following description:
```
      C transact, File = \MAILSLOT\SNADMOD.
						
```
The traces can be used to verify the announcements are being generated and sent to the correct IP addresses. Traces can also be captured on the other side of the router to verify the messages are arriving on that segment.
Try reloading the Netbios name cache on the Windows NT computer where the Primary SNA Server is running:
```
      nbtstat -R
						
```
This will cause the name cache to be purged and reloaded.