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 The connecting management (CMT) protocol is responsible for establishing connections and defining connect parameters. In fault-recovery mode, the CMT causes the primary and secondary rings to be wrapped into a single ring, thereby bypassing the fault. The FDDI connections controlled by the CMT process must be defined as A, B,S, or M type connections, as follows: •  A: Dual connection, primary ring in and secondary ring out. •  B: Dual connection, primary ring out and secondary ring in. •  M: Single connection, master concentrator. •  S: Single connection, slave end device. The station management (SMT) protocol keeps track of upstream and downstream neighbors and most FDDI management statistics. In some routers SMT information is integrated into the private extension of the SNMP agent. This facilitates management of an FDDI network with an SNMP management station. On a LAN all SMT versions must be the same to identify the presence of upstream and downstream neighbors. FDDI is not an IEEE protocol, but the network-dependent frame format is compatible with IEEE interface standards and supports encapsulation of the IEEE 802.2 and SNAP data link layer protocols necessary to support higher-level address resolution. This standards compliant structure ensures full interoperability with 802.3 and 802.5 LANs. The FDDI standard specifies a fault-tolerant dual ring; the primary ring carries user data at 100M bps, while the secondary ring carries control signals at 100M bps. In the event of component failure, the dual ring self-heals and operates in a single ring configuration until the problem is resolved. Exhibit 3-3-8 shows the normal FDDI operation and recovery sequences on an FDDI ring with dual-attached routers. In this configuration, the primary ring has one inbound and one outbound MAC interface per device through which all data traffic and control information flows. The secondary ring functions as a backup to the primary ring. Exhibit 3-3-8.  Normal Dual-Attached FDDI Operation When a single failure in the FDDI network occurs (Exhibit 3-3-9), nodes that sense an open ring loop the primary path onto the secondary path, thereby automatically restoring data communications. In this case, the control and data traffic share the single restored ring, and all devices with operational FDI connections can communicate. Exhibit 3-3-9.  Dual-Attached FDDI Ring Single Failure Recovery The failure of two nodes or fiber links (Exhibit 3-3-10) results in partition of the original FDDI network into two counter-rotating rings that support communication on each ring but not between rings. When an FDDI network has multiple simultaneous failures,only local communication is maintained. Exhibit 3-3-10.  Dual-Attached FDDI Ring Dual Failure Recovery The probability of a failure of two nodes or fibers can be substantially reduced with a dual-homed FDDI architecture. A dual-homed architecture, for both the routers and LAN based file servers, is the best possible traffic segmentation. It eliminates single points of failure and is flexible with respect to network expansion. The master hubs are the only dual-attached stations on the FDDI ring. Because the probability of an outage increases with the number of dual-attached devices, the dual-homed LAN maximizes network availability. But the two fiber cables have to be routed separately to ensure that a single cable cut does not segment the network. The A and B interfaces on dual-attached router cards are split between the two master hubs and function as a single attached FDDI device with a single attached host backup connection to the second fiber hub. The dual master hub configuration also allows FDDI ports to be turned on or shut down by a management station without causing the ring to wrap. Exhibit 3-3-11 shows the connection strategy that supports the dual-homed FDDI architecture. Exhibit 3-3-11.  Dual-Attached, Dual-Homed FDDI Configuration BRIDGING The different link layer protocols make necessary the use of different bridging techniques in homogenous and heterogeneous link layer protocol environments. Transparent bridging and the associated spanning tree algorithm were originally designed to support local and remote bridging between Ethernet segments. Source route bridging was developed by IBM to support local and remote bridging between Token Ring LAN segments. All other bridging types discussed in this chapter have been developed to handle bridging between different link layer LAN implementations. It should be noted that source route bridging has been decertified as a standard and has bee targeted for replacement by source route transparent bridging. However, because of the large installed base of pure source route bridge networks, this technique is described in this chapter. Bridges, like repeaters, can operate in local and remote modes. Local bridging uses a single bridge, and remote bridging uses bridge pairs. The remainder of this chapter covers bridging concepts as a whole and does not distinguish, in general, between local and remote bridging. Specific points unique to remote or local bridging are highlighted, however, the reader is referred to the repeater section of this chapter for a more detailed discussion of local and remote internetworking functions. 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.