Troubleshooting Translational Bridging

Allowing nodes from Ethernet segments to communicate with nodes on Token rings presents a number of challenges. Following are the most common problems associated with translational bridging.

The primary factor responsible for problems in this situation are differing MTUs between segments. Token Ring MTUs range from 4,464 to 17,914 bytes, while the Ethernet MTU is 1,500. A FDDI segment has an MTU of 4,532 bytes. When a bridge or Layer 2 switch connects two of these differing networking technologies, packets can be dropped because the Layer 2 switch cannot fragment the data and cannot alert the sending node of the reduced MTU.

In the example shown in Figure 3.6, an Ethernet backbone connects two 16-MB token rings. Instead of a router, a translational bridge in the form of a Layer 2 switch connects the segments. In this case, local traffic on the Token Rings uses an MTU of 17,914 and is not affected by the bridge. However, when Computer A must communicate with Computer B, the bridge drops large packets without notifying Computer A of the need to fragment. In this situation, Computer A has no way to discover the MTU on the other side of the bridge.

Figure 3.6 Connecting Two Token Ring Networks with an Ethernet Bridge

Other symptoms of translational bridging problems might include the ability to ping a computer on the far side of the bridge, being able to establish a connection, but not being able to send bulk data. This occurs because Echo Request messages and TCP connection establishment segments are small. When sending bulk data, however, large segments at the size of the MTU of the locally attached network are sent and dropped by the Layer 2 switch. Another example is when a computer is able to use FTP to establish a session, but is unable to use a get < filename > command, which requires sending a larger packet over the switch.

In Windows 2000, the MTU registry entry can be adjusted to meet the MTU requirement of the Ethernet segment connecting the two Token Ring segments, reducing all MTUs to the lowest common denominator. Each node's MTU is reduced to 1,500 to meet the requirements of the Ethernet backbone. However, this solution requires that all traffic (even traffic that is local on a Token Ring) is sent within the reduced MTU.

Using Ping to Determine Maximum Transmission Units

You can use the Windows 2000 ping -l command to send packets with a defined ICMP Echo Request data size. By sending packets of varying sizes, you can determine the MTU for any given bridge by noting which packet sizes cross the bridge successfully. For example, in Figure 3.6, a ping packet can be sent from Computer A to Computer C with a size of 1,472 bytes, which generates an Echo Reply packet from Computer C. However, if a size of 1,473 bytes or greater is used, the intermediate switch drops the packet. Computer C does not receive the Echo Request and no Echo Reply is generated.

The default ICMP Echo Request contains 32 bytes of data; you can use the ping < IP address or Host Name > -l < data size > command to specify a different data size. For example, you can ping with the maximum Ethernet data size by entering this command:

ping 134.56.78.1. -l 1472

The data size specified by the -l switch is 1,472 rather than the Ethernet IP MTU of 1,500 because 20 bytes are reserved to make room for the IP header and 8 bytes must be allocated for the ICMP Echo Request header.

When you have determined the MTU, you can set the packet size on either side of the bridge by changing the value in the registry entry. The MTU registry entry can be found at:

HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\Tcpip\Parameters\Interfaces\ Adapter _GUID

For more information about MTU, see "Windows 2000 TCP/IP" in this book.