Token Ring IEEE 802.5

When a token-ring frame passes through a bridge, the bridge adds information to the routing information field (RIF) of the frame (assuming that the bridge supports source route bridging). The RIF contains information concerning the route taken by the frame and, more importantly, the maximum amount of data that the frame can contain within its data field. This is called the maximum information field (I-field). The value specified for the maximum I-field is sometimes referred to as the largest frame size, but this means the largest frame size excluding headers. See Figure 1 for details on the relationship of the I-field to the header fields.

Note: It is important to be aware that IBM®'s implementation limits the number of bridges through which a frame can be passed to 7. An attempt to pass a frame through an eighth bridge will fail.

The maximum I-field is always decreased by a bridge when it cannot handle the value specified. So, for a given path through several token-ring bridges, the maximum I-field is the largest value that all of the bridges will support. This value is specified in the Routing Control (RC) field within the RIF as shown in Figure 1.

Figure 1. Format of an IEEE 802.5 Token-Ring Frame

The size of the MTU is the maximum amount of data that is allowed within a frame. The token-ring architecture specifies the maximum value of the I-field in the data frame, which corresponds to the maximum size of the L-PDU. The maximum I-field is determined by the bit configuration in the RC field, and is present in all routed frames.

Table 1 shows the relationship between the RC field and the maximum I-field values.

Table 1. Relationship between RC Field and Maximum I-Field Value
Routing Control Field	Maximum I-Field in Bytes
x000 xxxx xxxx xxxx	516
x001 xxxx xxxx xxxx	1500
x010 xxxx xxxx xxxx	2052
x011 xxxx xxxx xxxx	4472
x100 xxxx xxxx xxxx	8144
x101 xxxx xxxx xxxx	11 407
x110 xxxx xxxx xxxx	17 800

In Figure 1, we can see that, within the L-PDU, the Logical Link Control (LLC) header uses 8 bytes, and so the MTU value is 8 bytes less that the maximum I-field. (Note that the L-PDU contains a SNAP header, as described in Sub-Network Access Protocol (SNAP)) This is how to calculate the MTU for a token ring. The token-ring bridges always adjust the value of the maximum I-field to that of the smallest one in the path. You should always ensure that the MTU value is less than the value specified by the bridge.

Typically, within a 4Mbps token-ring network, the value of maximum I-field will be 2052 bytes, and so the MTU would be set to 2044 bytes (2052 minus 8 bytes for the LLC header).