Data Communications Networking Devices - 4th Ed.pdf

5.1 BRIDGES _____________________________________________________________________ 559information to provide statistical information concerning network activity. Thoseproducts may assist you in determining wide area network link utilization, the needto modify the capacity of those links, or whether one or more workstations arehogging communications between networks.DisadvantagesAlthough the preceding advantages are considerable, they are not without a price.That price includes a requirement to speci®cally identify bridges and links, higherbursts of network activity and an incompatibility between Token-Ring and Ethernetnetworks. In addition, due to the structure of the route information ®eld whichsupports a maximum of seven entries, routing of frames is restricted to crossing amaximum of seven bridges.When using source routing bridges to connect Token-Ring networks you mustcon®gure each bridge with a unique bridge/ring number. In addition, unless youwish to accept the default method by which workstations select a packet during theroute discovery process, you will have to recon®gure your LAN software. Thus,source routing creates an administrative burden not present when using transparentbridges.Due to the route discovery process the ¯ooding of packets occurs in bursts whenstations are powered on or after a power outage. Depending upon the complexity ofan interconnected series of networks, the discovery process can degrade networkperformance. Perhaps the biggest problem is for organizations that require theinterconnection of Ethernet and Token-Ring networks.Asource routing bridge can only be used to interconnect Token-Ring networkssince it operates on route information ®eld data which is not included in anEthernet frame. Although transparent bridges can operate in Ethernet, Token-Ring and mixed environments, their use precludes the ability to construct loop ormesh topologies and inhibits the ability to establish operational redundant paths forload sharing.Different LAN operating systems use the RIF data in different ways. Thus, theuse of a transparent bridge to interconnect Ethernet and Token-Ring networks mayrequire the same LAN operating system on each network. To alleviate theseproblems several vendors introduced source routing transparent SRT) bridges.Source routing transparent bridgesAsource routing transparent bridge supports both IBM's source routing and IEEE802.1D transparent spanning tree protocol operations. This type of bridge can beconsidered as two bridges in one and has been standardized by the IEEE 802.1committee.OperationUnder source routing the MAC packets contain a status bit in the source ®eldwhich identi®es whether or not source routing is to be used for a message. If source