TCP/IP Clients Lose Connections To Multihomed SNA Server, Event 706 logged

ID: Q226538

The information in this article applies to:

Microsoft SNA Server, versions 2.0, 2.1, 2.11, 2.11SP1, 2.11SP2, 3.0, 3.0SP1, 3.0SP2, 3.0SP3, 4.0, 4.0SP1, 4.0SP2

SYMPTOMS

Clients using TCP/IP to connect to an SNA Server are able to establish a session, but intermittently, the client session is lost. When this happens, the following event is logged at the server:

Event ID: 706
Source: SNA Server
Description:
Connection to client xxx.xxx.xxx.xxx:xxxx has been aborted due to too many (100) pending writes.

In a 3270 terminal session, users may lose their connections to the mainframe application and be sent back to the VTAM Logon screen.

CAUSE

This problem can occur if the computer running Windows NT Server has the TCP/IP transport bound to multiple network interface cards (NICs), which are connected to different (disparate) networks. When trying to send a message, if TCP/IP detects a problem with the route between the server and the client, the transport is designed to use a secondary NIC to send the message. This works fine as long as the secondary NIC is connected to a network with a working route to the client. But if the secondary NIC is on a disparate network, the server message will not reach the client, causing the TCP/IP connection to timeout and drop.

Please note that this is only one situation where an Event 706 may be generated by Microsoft SNA Server.

RESOLUTION

Disable the TCP/IP protocol binding from the adapters where it is not needed. For example, many SNA Servers have one or more Ethernet adapters to support TCP/IP client connections, and one or more Token Ring adapters to support DLC connections between the server and a mainframe or AS/400. In this scenario, because TCP/IP is not needed on the Token Ring adapter, it should be disabled from this adapter.

MORE INFORMATION

When Microsoft Network Monitor is installed on the SNA Server, and network traffic is captured on both NICs, you can observe the server receiving client traffic over one card and responding back over the other card. The TCP/IP stack switches to the alternate card when it believes the first card is busy.

For additional information about running Microsoft Windows NT Server on multihomed computers, please see the following articles in the Microsoft Knowledge Base:

Q139960 Limiting LAN Traffic to One Network Adapter

Q157025 Default Gateway Configuration for Multihomed Computers

Additional query words:

Keywords :
Version : WINDOWS:2.0,2.1,2.11,2.11SP1,2.11SP2,3.0,3.0SP1,3.0SP2,3.0SP3,4.0,4.0SP1,4.0SP2
Platform : WINDOWS
Issue type : kbprb