Data Communications Networking Devices - 4th Ed.pdf

3.5 TOKEN-RING _________________________________________________________________ 341Routing informationThe routing information ®eld is optional and is included in a frame when the RIbit of the source address ®eld is set. Figure 3.38 illustrates the format of theoptional routing information ®eld. If this ®eld is omitted, the frame cannot leavethe ring it was originated on under IBM's source routing bridging method. Undertransparent bridging, the frame can be transmitted onto another ring. Both sourcerouting bridging and transparent bridging are covered in detail in Chapter 5. Therouting information ®eld is of variable length and contains a control sub®eld andone or more two-byte route designator ®elds when included in a frame, as the latterare required to control the ¯ow of frames across one or more bridges.Figure 3.38Routing information ®eld