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Interrelationships with IGRP

EIGRP is built in part on the foundation laid by IGRP. Many designers migrate to EIGRP to add features to their networks while retaining some of the benefits of IGRP. Most conversions are promoted by the need for VLSM, although faster convergence and other benefits may also lead to the recommendation for conversion.

There are two methods for redistributing IGRP and EIGRP routes. The first is to assign the same autonomous system (AS) number to both the IGRP and EIGRP processes. The second method is similar to the technique used for other routing protocols—the administrator manually places a redistribution command into the routing process.

Of the two redistribution methods, most experienced designers lean toward the second, or manual redistribution. This solution affords a greater degree of control over the process, which frequently becomes desirable. For example, EIGRP, unlike IGRP, provides a method for identifying routes as internal or external. An external route is one that was learned from another routing process. IGRP contains no such mechanism, which may impact the administrative distance and other factors the router will use when selecting a route. Manual redistribution also affords the opportunity to use distribution lists, route maps, and other techniques to control the routing process.

Designers should use some care when converting from IGRP to EIGRP. Perhaps the most significant design criterion is to select only a few routers to handle the redistribution—ideally, routers in the core or distribution layers.

EIGRP designs tend to be most successful when using the three-tier, hierarchical model.

This section has noted that designers typically select EIGRP as a replacement for IGRP without describing some of the reasons a designer would do so. Here is a list of advantages provided by EIGRP:

Low bandwidth consumption (stable network) When the network is stable, the protocol relies only on hello packets. This greatly reduces the amount of bandwidth needed for updates.
Efficient use of bandwidth during convergence When a change is made to the routing topology, EIGRP will enter a period of active convergence. During this time, the routers will attempt to rebuild their routing tables to account for the change—typically the failure of an interface. To conserve bandwidth, EIGRP will communicate only changes in the topological database to other routers in that AS, as opposed to communication of the entire routing table, which consumes a great deal of bandwidth, especially in larger networks.
Support for VLSM As noted previously, EIGRP supports variable-length subnet masks. This support, along with support for classless Inter-net domain routing (CIDR), can greatly assist the network designer by offering greater flexibility in IP addressing.

Designers should use some caution in deploying VLSM in the network. Ideally, there should be only two or three masks for the entire enterprise. These typically include /30 and /24. The reason for this is not specifically a routing protocol limitation, but rather a consideration for troubleshooting and other support issues. The concepts of VLSM and CIDR have been around for many years, but an understanding of both features, especially in the server and workstation arenas, is still wanting—network designers may find that their workstation support staffs are unfamiliar with these concepts and may find resistance to a readdressing effort. Remember that IP addressing affects not only the network, but also all other devices in the network, including Dynamic Host Configuration Protocol (DHCP), workstations, and servers. In well-administered networks, the use of VLSM is transparent to end users. However, the lack of familiarity by administrators and users can cause problems—consider the impact on the network if end users changed their subnet mask to the default value because they found it to be wrong. The problem is not technical but educational. Fortunately, these concerns and issues are being quickly eliminated from the landscape as VLSM gains in popularity and designers become more familiar with it. Recall from Chapter 3 that VLSM helps designers construct efficient IP addressing schemes.

EIGRP and IGRP share the same composite routing metrics and mathematical weights; however, EIGRP supports metrics up to 32 bits. This differs from IGRP, which supports only 24 bits for the metric. EIGRP will automatically handle this issue, and after conversion metrics from either protocol are interchangeable.

Pay special attention to memory and CPU capacity on routers that will run EIGRP. The protocol can be very memory intensive, especially as the number of neighbors increases.

Network Design in the Real World: EIGRP

On the surface, it would appear that most Cisco-only networks should automatically use EIGRP. The protocol provides extremely fast convergence, relatively easy configuration, and variable-length subnetting.

Unfortunately, as with most things, it is not that simple to deploy EIGRP. The most significant problem frequently relates to memory and CPU; however, other factors can hinder deployment.

The simplest recommendations for designers thinking of deploying EIGRP fall into four basic areas, as follows:

  Maximize the amount of memory available on each router and increase the capacity of each router as the number of neighbors increases. There are formulas that predict the amount of memory that an EIGRP installation will require based on the number of neighbors and the number of routes, but these solutions are far from accurate. One installation I consulted on, after the deployment failed, had over 40MB of free router memory—the formula predicted that just over 1MB was sufficient. The deployment was ultimately removed, but it is important to note that the most critical issue involved the number of neighbors.
  Limit the number of neighbors. This is easier said than done, especially when the network has evolved over time. One technique is to use passive interfaces, although doing so significantly diminishes the overall benefits of EIGRP. Cisco recommends the use of ODR in hub-and-spoke designs, which can also reduce the number of neighbor relationships, but again, this reduces the overall benefits of EIGRP. The generic guidelines recommend that EIGRP neighbors be kept to fewer than 30; however, this is dependent on the amount of memory and the number of routes. Networks have failed with fewer neighbors, and a small number of networks have deployed over 70 neighbors successfully.
  Don’t use the automatic redistribution feature unless the network is very simple. Automatic redistribution is a feature Cisco provides in order to make IGRP-to-EIGRP migration easier. You configure this feature by setting the AS number to the same value in the two protocols. The automatic feature works well, but many administrators find that it does not afford enough control over the redistribution process, which may be necessary for the migration.
  Administrators and designs should disable automatic route summarization and manually summarize routes whenever possible. Route summarization is an automatic process within the major network address, and it may require readdressing. However, summarization reduces the size of the routing table and can further enhance stability and convergence.

External EIGRP Routes

One of the most unique features in EIGRP is the concept of an external route, which is how IGRP routes are tagged in EIGRP upon redistribution. External routes are learned from one of the following:

  A static route injected into the protocol
  A route learned from redistribution from another EIGRP AS
  Routes learned from other protocols, including IGRP

All routes tagged as external are given a higher administrative distance than internal EIGRP routes. This effectively weights the internal routes for preference, which typically benefits the overall network. However, designers will wish to monitor this characteristic to ascertain the appropriateness of the routing table and to avoid asymmetric routing, if desired. Asymmetric routing is a situation wherein the outbound packets traverse a different path than the inbound packets. Such a design can make troubleshooting more difficult.

When EIGRP tags a route as external, it includes additional information about the route in the topology table. This information includes the following:

  The router ID of the router that redistributed the route (EIGRP redistribution) and the AS number of that router
  The protocol used in the external network
  The metric or cost received with the route
  An external route tag that the administrator can use for filtering

IGRP does not provide an external route mechanism. Therefore, the protocol cannot differentiate between internally and externally learned routes.

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