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 (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 by a Data Link Connection Identifier (DLCI), which is simply a number between 0 and 1023. In fact, Cisco routers can only use DLCI numbers in the range 16 through 1007 to carry user data.
If the router at Site A wants to send a packet to Site B, it simply specifies the appropriate DLCI number for the virtual circuit that connects to Site B in the Frame Relay header. Although a physical circuit can have many virtual circuits, each connecting to a different remote circuit, there is no ambiguity about where the network should send each individual packet.
It's important to remember, though, that the DLCI number only has local significance. That is, the DLCI number doesn't uniquely identify the whole virtual circuit, just the connection from the local physical circuit to the Frame Relay switch at the Telco central office. The DLCI number associated with this virtual circuit can change several times before it reaches the remote physical circuit.
We like to use this fact to our advantage when constructing a Frame Relay network. Instead of thinking of the DLCI number as a virtual circuit identifier, we use it to uniquely label each physical circuit. Suppose, for example, that Site A has virtual circuits to both Sites B and C. Then we would use the same DLCI number at both Sites B and C to label the virtual circuits that terminate at Site A. This is just one of many possible DLCI numbering schemes, but we prefer it because it makes troubleshooting easier. Unfortunately, while this scheme works well in hub-and-spoke network topologies, it tends to become unworkable in meshed or partially meshed networks.
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