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.

Guideline: It is important to be aware that the IBM® implementation limits the number of bridges through which a frame can be passed to seven. An attempt to pass a frame through an eighth bridge fails.

The maximum I-field is always decreased by a bridge when it cannot handle the value specified. So, for a given path through a number of token-ring bridges, the maximum I-field is the largest value that all of the bridges 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

Diagram of the 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 value 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	11407
x110 xxxx xxxx xxxx	17800

Figure 1 shows that, within the L-PDU, the Logical Link Control (LLC) header uses 8 bytes. Thus the MTU value is 8 bytes less than the maximum I-field. The L-PDU contains a SNAP header, as described in Subnetwork Access Protocol (SNAP). Follow this example 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. Ensure that the MTU value is less than the value specified by the bridge.

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