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In order to provide users with the most connectivity options, designers often incorporate dial-on-demand routing (DDR) services on the router. This configuration makes use of another design concept—floating static routes.

Recall the presentation on IP routing and the administrative distance (AD) parameter. Each route could be provided by one or more routing protocols, and the router maintained an administrative distance that it used to select routing information. Floating static builds upon this concept of administrative distance. Normally, a static route has an administrative distance of one, making it one of the best routes from the protocol’s perspective. This would tend to override dynamic routing information, which is undesirable in many instances.

However, if the administrator informed the router that the static route had an AD of 240 (the highest number is 254), then the dynamic protocols would have lower ADs and would be used instead. As shown in Figure 8.4, the IGRP route through the Frame Relay cloud is used under normal circumstances. However, the floating static route between the two modems on the dial-on-demand connection is used when the Frame Relay link fails.

FIGURE 8.4  Floating static routes

Note that floating static routes may be used on any link and are not dependent on DDR connections.

Backup Interfaces

An alternative to floating static routes is the backup interface. Under this configuration, the router is instructed to bring up a link if the interface goes down. The backup interface is associated with the primary interface. While this configuration has merits, the use of floating static routes typically works better in Frame Relay configurations. This addresses the concern of failed PVCs—the link may remain up/up (interface is up/line protocol is up); however, a switch failure in the cloud will collapse the PVC.

The Local Management Interface was designed to prevent this type of failure, yet there are specific scenarios that LMI cannot detect.

Since the router has no method for detecting this failure (unlike ATM OAM cells, discussed later in this chapter), it continues to believe that the interface is valid. The routing protocol may eventually record the fact that the link is unavailable, but this requires the use of a routing protocol, which adds overhead.

Network Design with ATM

Asynchronous Transfer Mode technology was developed to combine video, voice, and data in the network. The ATM Forum, a working group of vendors, developed a cell-based system for transporting these types of information. Cells are fixed in length, and therefore latency and delay can be determined and controlled accurately.

ATM provides many services for the network designer and should be considered in any wide area network design. This is especially true when considering the integration of voice and data. An emerging trend in networking is to focus on the services that are provided by the network and not the methodology employed. This technique simplifies the business-to-technology modeling process. Business-to-technology modeling is a process that incorporates the concepts presented in Chapter 1, where the business demands and needs are integrated into the technology and its abilities.

When selecting ATM as a WAN technology, there are two interesting issues that warrant careful consideration. First, every conversion from cell to frame requires processing and adds a small amount of latency. This is an important factor to consider when choosing a partial-mesh topology. The second consideration is vendor availability, especially with new features. For example, vendors deployed only IMA technology in mid-1999. Inverse multiplexing for ATM (IMA) is a major feature for network designers to consider since it provides a middle ground between T1 and DS-3 circuits. In many locations, IMA is the only way to provide greater than T1 bandwidth in remote locations—IMA bonds multiple T1s into a single data conduit.

Network Design in the Real-World: The Benefits of ATM

As networks have advanced, the lines between voice and data have blurred significantly. From a historical perspective, voice services and data have operated over separate circuits. When the two were integrated, it was via time division multiplexing (TDM), which maintained distinct channels for each service. Asynchronous Transfer Mode (ATM) allows for the true integration of these services, in addition to video, so Cisco recommends that designers use ATM whenever possible. Thus far the marketplace has continued to use Frame Relay and other technologies, yet providers are developing better tariffs and offerings to make ATM more attractive. Vendors are also providing ATM in more regions and with more equipment options.

Virtual Path and Virtual Circuit Identifiers

Every ATM cell contains a virtual path identifier (VPI) and a virtual circuit identifier (VCI). These values are combined, depending on the switch configuration, to create unique conduit information for the cell. This is very similar to the DLCI in Frame Relay, although the difference between path and circuit does not apply in ATM. Frame Relay understands only the equivalent concept of circuit.

The virtual path identifier encompasses a large number of virtual circuit identifiers. A four-line roadway tunnel is one way to visualize this. Each lane is analogous to the VCI, and the tunnel itself is the VPI. The lanes can diverge at either end of the tunnel, but within the tunnel they are fixed to the single path.

The terms “virtual path” and “virtual circuit” do not relate to permanence or switched characteristics. Both PVCs and SVCs require a VPI/VCI pair.

Figure 8.5 illustrates the flow of data through the ATM switches. As with the DLCI in Frame Relay, the VPI/VCI pair is used by the ATM switch to forward cells.

FIGURE 8.5  ATM data flow

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