Chapter 8. Enhanced Interior Gateway Routing Protocol (EIGRP)

• Operation of EIGRP

  Protocol-Dependent Modules

  Reliable Transport Protocol

  Neighbor Discovery and Recovery

  The Diffusing Update Algorithm

  EIGRP Packet Formats

  Address Aggregation

• Configuring EIGRP

  Case Study: A Basic EIGRP Configuration

  Case Study: Redistribution with IGRP

  Case Study: Disabling Automatic Summarization

  Case Study: Address Aggregation

  Authentication

• Troubleshooting EIGRP

  Case Study: A Missing Neighbor

  Stuck-in-Active Neighbors

EIGRP is occasionally described as a distance vector protocol that acts like a link state protocol. To recap the extensive discussion in Chapter 4, a distance vector protocol shares everything it knows but only with directly connected neighbors. Link state protocols announce information only about their directly connected links, but they share the information with all routers in their routing domain or area.

All the distance vector protocols discussed so far run some variant of the Bellman- Ford (or Ford-Fulkerson) algorithm. These protocols are prone to routing loops and counting to infinity. As a result, they must implement loop-avoidance measures such as split horizon, route poisoning, and hold-down timers. Because each router must run the routing algorithm on received routes before passing those routes along to its neighbors, larger internetworks may be slow to converge. More important, distance vector protocols advertise routes; the change of a critical link may mean the advertisement of many changed routes.

Compared to distance vector protocols, link state protocols are far less susceptible to routing loops and bad routing information. The forwarding of link state packets is not dependent on performing the route calculations first, so large internetworks may converge faster. And only links and their states are advertised, not routes, which means the change of a link will not cause the advertisement of all routes using that link. However, compared to distance vector algorithms, the complex Dijkstra algorithms and the associated databases place a higher demand on CPU and memory.

Regardless of whether other routing protocols perform route calculations before sending distance vector updates to neighbors or after building a topological database, their common denominator is that they perform the calculations individually. In contrast, EIGRP uses a system of diffusing computations—route calculations that are performed in a coordinated fashion among multiple routers—to attain fast convergence while remaining loop free at every instant.

Although EIGRP updates are still vectors of distances transmitted to directly connected neighbors, they are nonperiodic, partial, and bounded. Nonperiodic means that updates are not sent at regular intervals; rather, updates are sent only when a metric or topology change occurs. Partial means that the updates will include only routes that have changed, not every entry in the route table. Bounded means that the updates are sent only to affected routers. These characteristics mean that EIGRP uses much less bandwidth than typical distance vector protocols use. This feature can be especially important on low-bandwidth, high-cost WAN links.

Another concern when routing over low-bandwidth WAN links is the maximum amount of bandwidth used during periods of convergence, when routing traffic is high. By default, EIGRP uses no more than 50% of the bandwidth of a link. Later IOS releases allow this percentage to be changed with the command ip bandwidth-percent eigrp.

EIGRP is a classless protocol (that is, each route entry in an update includes a subnet mask). Variable-length subnet masks may be used with EIGRP not only for sub-subnetting as described in Chapter 7,"Routing Information Protocol Version 2," but also for address aggregation—the summarization of a group of major network addresses.

Beginning with IOS 11.3, EIGRP packets can be authenticated using an MD5 cryptographic checksum. The basics of authentication and MD5 are covered in Chapter 7; an example of configuring EIGRP authentication is included in this chapter.

Finally, a major feature of EIGRP is that it can route not only IP but also IPX and AppleTalk.