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A problem that has not been uniformly addressed by vendors is the processing of the control fields. Token Ring control fields are used to determine frame priority, frame destination receipt, frames forwarded, frame errors, and maximum transfer units (MTU). These parameters have no parallel functions on the Ethernet side, which requires these values to be set to a default value for frames coming from the Ethernet segment. The default values vary between different vendors and compromise the function of the Token Ring LAN because the default values do not indicate dynamic settings.

Another issue that must be addressed with translational bridging is that Token Ring and Ethernet have different MTU values. Ethernet supports an MTU of approximately 1,500 bytes, whereas Token Ring supports an MTU of up to 8,000 bytes. Token Ring can be configured to support a 1,500 MTU, but large file transfers on the Token Ringside become less efficient.

The last complexity that arises with translational bridging between Token Ring and Ethernet is frame conversion. Both LAN types support 802.2 and SNAP fields, but much of the existing Ethernet base uses the original Ethernet encapsulation with the DEC-Intel-Xerox (DIX) type codes, which have no correlation in Token Ring LANs. A special SNAP code (00-0-F8) is used to handle the DIX code conversions between Token Ring and Ethernet segments. Translational bridging is needed so long as older source route code exists in end stations and source route bridges.

Source Route Transparent Bridging

This chapter began by stating that internetworking is not as simple as implementing 10Base-T. This is a good preliminary warning before the concepts of source route transparent bridging (SRT) are explored. SRT is a draft standard for bridging between different Token Ring and Ethernet environments and is intended to replace the current translational bridging approaches. The motivation for replacing translational bridging is intuitively obvious in light of the discussion in the translational bridging section.

The basic concept of SRT is to extend transparent bridging to Token Ring LANs by having all internetwork traffic forwarded by transparent bridging and the spanning tree algorithm. The SRT operational premise is concise; frames received by an SRT are only forwarded to Ethernet segments if no RIF exists, otherwise forwarding is accomplished by source route bridging (SRB). Specifically the SRT uses the most significant bit of the source address (the ring-in (RII) bit) to determine forwarding status. If the RII bit is 0, then no RIF exists and forwarding is accomplished through transparent bridging.

SRB is installed in SRT bridges to handle legacy Token Ring segments. The SRT has SRB functions but no translating can occur;in other words a Token Ring host cannot communicate with an Ethernet host if the Token Ring host uses SRB. The reason for segmenting the two bridging functions is that the loop and broadcast problems that plagued translational bridges have been eliminated.

It now is evident that the greatest obstacles to migrating to SRT is cost. If Token Ring hosts need to communicate with Ethernet hosts, then all ordinary SRB bridges and host software must be upgraded to support SRT. Once the software and hardware conversion has taken place then the entire bridged internet has the benefit of loop free operation by using spanning tree. Another benefit that SRT offers is that the end node is effectively taken out of the bridging process, thereby reducing bridging complexity and points for failure. SRT should not be considered a bridging panacea. The address bit order embedded MAC layer addressing in ARP and IPX, and MTU problems encountered in translational bridging still remain. But even with the remaining problems SRT offers a viable mechanism to facilitate heterogeneous LAN communications.

SUMMARY

Repeaters and bridges are the means of network interconnection that function at the lowest levels of the OSI hierarchy, which enables them to attain high performance levels but limits their use in multiple access method and multiprotocol internets. Routers are much more flexible, albeit more expensive, internetworking mechanisms.


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