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Cisco IOS Cookbook, 2nd Edition by Ian Brown, Kevin Dooley

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Introduction

Frame Relay is a popular WAN protocol because it makes it easy to construct reliable and inexpensive networks. Its main advantage over simple point-to-point serial links is the ability to connect one site to many remote sites through a single physical circuit. Frame Relay uses virtual circuits to connect any physical circuit in a cloud to any other physical circuit. Many virtual circuits can coexist on a single physical interface.

This section will offer only a quick refresher of how Frame Relay works. If you are unfamiliar with Frame Relay, we recommend reading the more detailed description of the protocol and its features that are found in T1: A Survival Guide by Matthew Gast (O’Reilly).

The Frame Relay standard allows for both Switched (SVC) and Permanent (PVC) Virtual Circuits, although support for SVCs in Frame Relay switching equipment continues to be relatively rare. Most fixed Frame Relay WANs use PVCs rather than SVCs. This allows you to configure the routers to look like a set of point-to-point physical connections. SVCs, on the other hand, provide a mechanism for the network to dynamically make connections between any two physical circuits as they are needed. In general, SVCs are more complicated to configure and manage. Most network engineers prefer to use PVCs unless the carrier offers significant cost benefits for using SVCs. SVCs tend to be most practical when the site-to-site traffic is relatively light and intermittent.

Each virtual circuit is identified ...

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