Handbook of Local Area Networks, 1998 Edition:LAN Interconnectivity Basics Click Here! Search the site:   ITLibrary ITKnowledge EXPERT SEARCH Programming Languages Databases Security Web Services Network Services Middleware Components Operating Systems User Interfaces Groupware & Collaboration Content Management Productivity Applications Hardware Fun & Games EarthWeb sites Crossnodes Datamation Developer.com DICE EarthWeb.com EarthWeb Direct ERP Hub Gamelan GoCertify.com HTMLGoodies Intranet Journal IT Knowledge IT Library JavaGoodies JARS JavaScripts.com open source IT RoadCoders Y2K Info Previous Table of Contents Next Remote Source Route Bridging Remote source route bridging is the process of bridging Token Ring LANs across non-Token Ring network segments. This is usually done by high-end router/bridges and is accomplished by encapsulating the source route data inside another transport service. MAC layer encapsulation can be used to transport Token Ring traffic over point-to-point serial links. In this case, the Token Ring frame is placed inside a synchronous data link control (SDLC) frame in order to traverse the serial link. The remote router/bridge strips off the SDLC envelope and delivers the Token Ring frame to the remote LAN segment. Although MAC layer encapsulation is an efficient transport mechanism, the scope of the application is limited to simple serial links. TCP/IP encapsulation of Token Ring frames offers greater implementation flexibility for remote bridging functions. TCP/IP encapsulation can be implemented using IP or TCP encapsulation. IP is a connectionless network layer protocol and TCP is a connection oriented transport layer protocol. Either encapsulation mechanism allows the Token Ring frames to be routed over diverse media transparently to the original frame structure. The encapsulation mechanism is analogous to a mail delivery service,wherein the mail is delivered to a destination but the contents of the mail are not known by the carrier. IP encapsulation is used in cases where guaranteed delivery is not as important as fast delivery. TCP encapsulation is used in the cases where the reliability of a connection oriented delivery is paramount. As with any encapsulation technique the encapsulation envelope is added by the bridge/router that receives the original frame, is removed by the bridge/router that delivers the frame to the local LAN segment, and the originating and destination LANs must support the same link layer protocol (i.e., Token Ring or Ethernet). Translational Bridging Translational bridging was devised to address the problem of bridging between different LAN protocols. The most common translating bridge combinations are Token Ring to Ethernet and Ethernet to FDDI. Designing fault free translational bridges is far more complicated than the previously discussed encapsulation bridges. Bridging between Token Ring and Ethernet LANs seems fairly innocuous, until the difference in the frame types and operating functions is considered. Although Token Ring and Ethernet use 48-bit addresses, bit order interpretation of the addresses in hardware is inverted. Token Ring interface cards consider the first bit encountered to be the high-order bit, Ethernet interface cards consider the first bit encountered as the low-order bit. As an example, the address 84 (hexadecimal) would appear as 0010 0001(binary) on Ethernet (with the lease significant bit first) whereas the same address would appear as 1000 0100 (binary) on Token Ring(with the most significant bit first). This address inversion problem is fairly straightforward to solve for the discrete MAC level addresses. However, a number of higher level protocols user part or all of the MAC layer address as data or to form the higher level address. ARP is part of TCP/IP and performs a mapping function between The MAC and IP addresses. ARP is treated as data by bridges because it is a network layer protocol. Therefore the MAC address embedded in the ARP packet will not be translated, unless nonlink layer processing is performed by the translational bridge. Novell’s IPX addresses derive the node portion of the network layer address from the MAC layer address. Because network layer addresses are seen as data by bridges, the node portion of the Novell address is translated, unless special processing outside normal bridge functions is performed. Token Ring frames contain a number of fields that have no correlation on the Ethernet side. Specifically Token Ring has access control, route control, route data, and delimiter fields. The delimiter fields do not pose a significant problem, however,the RIFs require special handling. By definition the RIFs are supposed to contain the complete explicit route from source to destination. Effectively the translational bridge must provide route spoofing, because the Ethernet side of the route is not discernible. Spoofing is done by stripping and caching the RIF information before reformatting to an Ethernet frame. The appropriate cached RIF value is attached to a frame destined for the attached Token Ring segment. Previous Table of Contents Next Use of this site is subject certain Terms & Conditions. Copyright (c) 1996-1999 EarthWeb, Inc.. All rights reserved. Reproduction in whole or in part in any form or medium without express written permission of EarthWeb is prohibited. Please read our privacy policy for details.