TCP/IP problems with a Token-Ring/Token-Ring bridge
When operating through a bridge, change the default value of 1500 for the maximum transmission unit (MTU) to a value that is eight less than the maximum information field (maximum I-frame) advertised by the bridge in the routing control field.
To find the routing control field value, use the iptrace daemon to look at incoming packets. Bits 1, 2, and 3 of Byte 1 are the Largest Frame Bits, which specify the maximum information field that can be transmitted between two communicating stations on a specific route. See the following for the format of the routing control field:
Values for the Largest Frame Bits are as follows:
|000||Specifies a maximum of 516 bytes in the information field.|
|001||Specifies a maximum of 1500 bytes in the information field.|
|010||Specifies a maximum of 2052 bytes in the information field.|
|011||Specifies a maximum of 4472 bytes in the information field.|
|100||Specifies a maximum of 8144 bytes in the information field.|
|111||Used in all-routes broadcast frames.|
For example, if the maximum I-frame value is 2052 in the routing control field, the MTU size should be set to 2044. This is for token-ring network interfaces only.