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EIGRP - Posted at 14:59 on 20/1/2007 by @lfons


Cisco Systems's EIGRP is one of the most feature-rich and robust routing protocols to ever be developed. Its unique combination of features blends the best attributes of distance vector protocols with the best attributes of link-state protocols. The result is a hybrid routing protocol that defies easy categorization with conventional protocols.

EIGRP is also remarkably easy to configure and use, as well as remarkably efficient and secure in operation. It can be used in conjunction with IPv4, AppleTalk, and IPX. More importantly, its modular architecture will readily enable Cisco to add support for other routed protocols that may be developed in the future.

Review Questions

Q—Name the four key technologies that are used by EIGRP.

A—EIGRP employs four key technologies, including neighbor discover/recovery, Reliable Transport Protocol (RTP), Diffusing Update ALgorithm (DUAL) finite-state machine, and a modular architecture that enables support for new protocols to be easily added to an existing network.

Q—Explain why EIGRP is more efficient in operation than IGRP.

A—Unlike most other distance vector routing protocols, EIGRP does not mandate a periodic update of routing tables between neighboring routers. Instead, it employs a neighbor discovery/recovery mechanism to ensure that neighbors remain aware of each other's accessibility. As long as a router receives periodic hello packets from its neighbors, it can assume that those neighbors remain functional. More importantly, it can assume that all of its routes that rely upon passage through those neighbors remain usable. Thus, EIGRP is much more efficient than conventional distance vector routing protocols because it imposes much less overhead on routers and transmission facilities during normal operation.

Q—How does RTP enable improved convergence times?

A—RTP is responsible for providing guaranteed delivery of EIGRP packets between neighboring routers. However, not all of the EIGRP packets that neighbors exchange must be sent reliably. Some packets, such as hello packets, can be sent unreliably. More importantly, they can be multicast rather than having separate datagrams with essentially the same payload being discretely addressed and sent to individual routers. This helps an EIGRP network converge quickly, even when its links are of varying speeds.

Q—Why does EIGRP tag certain routes?

A—EIGRP supports both internal and external routes. Routes that are internal to an AS are completely contained within that AS. External routes are those that are learned from neighbors that lie outside the AS. External routes are tagged with information that identifies their origin. This enables a network administrator to develop customized interdomain routing policies.

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