Partial Redundancy

Another approach to using redundant routed protocols is to install the redundant protocol only where it is needed. In such cases, some end systems would have two routed protocols installed and configured. Others would rely on just one. Figure 14-2 demonstrates such partial routed protocol redundancy in a small network.

Figure 14-2: Partially redundant routed protocols.

Figure 14-2: Partially redundant routed protocols.

In this example, the laser printer has been given an address in both routed protocols. Therefore, users on both networks can use it. However, this is the only resource that can be accessed universally. All the other end systems have just one routed protocol and, consequently, can only communicate with other end systems on their LAN.

The benefit of this approach is that end systems that don't need the second protocol are spared the overhead of running it. Additionally, the network's administrator is spared the bother of installing and supporting extra protocols on devices that do not require them. The one drawback is that devices that only use one protocol can only communicate with other devices that run that same protocol. In other words, an AppleTalk-only end system cannot communicate with any IP-only end systems. This drawback, however, can be eliminated through proper planning and identification of users' needs.

One of the most important benefits of the separation of a network's layered functions is that two or more network protocols can simultaneously operate over the same LAN. Therefore, there is no need to rewire or add another network to support dual protocols.

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