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:

Figure 1. Routing control field
Routing control field
This illustration shows byte 0 and byte 1 of a routing control field. The eight bits of byte one are B, B, B, B, L, L, L, L. The eight bits of byte 1 are D, F, F, F, r, r, r, r.

Values for the Largest Frame Bits are as follows:

Item Description
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.
101 Reserved.
110 Reserved.
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.

Note: When using iptrace, the output file must not be on a Network File System (NFS).