Data Communications Networking Devices - 4th Ed.pdf

5.1 BRIDGES _____________________________________________________________________ 557Figure 5.11Token-Ring route information ®eld. The ®eld is variable in lengthdifferent memory in bridges which may limit frame sizes, the LF bits enable differentdevices to negotiate the size of the frame. Normally a default setting indicatesa frame size of 512 bytes. Each bridge can select a number and, if supported byother bridges, that number is then used to represent the negotiated frame size.Otherwise, a smaller number used to represent a smaller frame size is selected andthe negotiation process is repeated.Up to eight route number sub®elds, each consisting of a 12-bit ring number anda 4-bit bridge number, can be contained in the routing information ®eld. Both ringnumbers and bridge numbers are expressed as hexadecimal characters, with threehex characters used to denote the ring number and one hex character used toidentify the bridge number.OperationTo illustrate the concept behind source routing, consider the network illustrated inFigure 5.12. In this example let us assume two Token-Ring networks are located inAtlanta and one network is located in New York.Each Token-Ring and every bridge are assigned ring and bridge numbers. Forsimplicity ring numbers R1, R2 and R3 were used, although, as previously explained,those numbers are actually represented in hexadecimal. Similarly, forsimplicity bridge numbers are shown as B1, B2, B3, B4 and B5 instead of a hexadecimalcharacter.When a workstation wants to originate communications it is responsible for®nding the destination by transmitting a discovery packet to network bridges andother network workstations whenever it has a message to transmit to a new destinationaddress. Assuming workstation A wishes to transmit to station C, it sends aroute discovery packet which contains an empty route information ®eld and itssource address as indicated in the upper left portion of Figure 5.12. This packet is