Bridge and Extended LAN Reference

This document, "Bridge and Extended LAN Reference" (Order Number: EK-DEBAM-HR-003), published in September 1991 by Digital Equipment Corporation, serves as a comprehensive guide for network managers and personnel on the use, operation, and management of bridges in extended Local Area Network (LAN) configurations.

The guide's primary objective is to explain how bridges overcome the limitations of single LANs (such as physical extent, number of stations, and performance) by interconnecting multiple LANs to form a larger, logical extended LAN.

Key areas covered include:

• Bridge Concepts: An overview of bridges as protocol-independent devices operating at the Data Link layer, enabling logical data link services across connected LANs. It discusses their advantages, including supporting more stations, creating larger networks, reducing traffic, and interconnecting different LAN types.
• Bridge Operation: A detailed description of the bridge model, its internal interfaces, various port states (e.g., INIT, DISABLED, FORWARDING, BACKUP), frame aging, and the forwarding and translating process, which includes features like frame translation and IP fragmentation.
• The Spanning Tree Algorithm: A core component for maintaining a loop-free network topology in extended LANs. The document explains its properties (loop detection, automatic backup, determinism, low overhead, self-maintenance, and management), how bridges communicate to elect a root bridge and designated bridges, and how the network adjusts to topology changes. It also addresses the compatibility between Digital's LAN Bridge 100 and IEEE 802.1d spanning tree implementations.
• Extended LAN and Bridge Management: Instructions on configuring an extended LAN, specifying root and designated bridges, controlling bridges and ports (e.g., restricting access, disabling/enabling ports, setting passwords, and firmware loading), and monitoring bridge and port counters.
• Performance: Discussion of factors affecting extended LAN and bridge performance, such as end-to-end delay, frame lifetime, and frame loss caused by data errors or congestion.
• Configuration Strategies: Guidelines for designing efficient network topologies, including physical and logical configurations, and practical examples of using bridge features like packet filtering (destination, source, protocol) to solve common network problems (e.g., heavy broadcast traffic, isolating VAXclusters, blocking nodes, controlling load server traffic). It also touches on FDDI bridges and the use of repeaters versus bridges.
• Network Troubleshooting Methodology: A step-by-step approach to identifying and resolving network issues, emphasizing understanding network topology, performance, and usage, analyzing different types of network errors, and utilizing various network management and troubleshooting tools.

Appendices provide details on Digital's bridge family features and related documentation.