Enhanced Interior Gateway Routing Protocol(3)

Route Tagging
Enhanced IGRP supports internal and external routes. Internal routes originate within
an Enhanced IGRP AS. Therefore, a directly attached network that is configured to run Enhanced IGRP is considered an internal route and is propagated with this information throughout the Enhanced IGRP AS. External routes are learned by another routing protocol or reside in the routing table as static routes. These routes are tagged individually with the identity of their origin.

External routes are tagged with the following information:

Router ID of the Enhanced IGRP router that redistributed the route

AS number of the destination

Configurable administrator tag

ID of the external protocol

Metric from the external protocol

Bit flags for default routing

Route tagging allows the network administrator to customize routing and maintain flexible policy controls. Route tagging is particularly useful in transit ASs, where Enhanced IGRP typically interacts with an interdomain routing protocol that implements more global policies, resulting in a very scalable, policy-based routing.

Enhanced IGRP Packet Types
Enhanced IGRP uses the following packet types: hello and acknowledgment, update, and query and reply.

Hello packets are multicast for neighbor discovery/recovery and do not require acknowledgment. An acknowledgment packet is a hello packet that has no data. Acknowledgment packets contain a nonzero acknowledgment number and always are
sent by using a unicast address.

Update packets are used to convey reachability of destinations. When a new neighbor is discovered, unicast update packets are sent so that the neighbor can build up its topology table. In other cases, such as a link-cost change, updates are multicast. Updates always are transmitted reliably.

Query and reply packets are sent when a destination has no feasible successors. Query packets are always multicast. Reply packets are sent in response to query packets to instruct the originator not to recompute the route because feasible successors exist. Reply packets are unicast to the originator of the query. Both query and reply packets are transmitted reliably.

Summary
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.

For More Information

Pepelnjak, Ivan. EIGRP Network Design Solutions. Indianapolis: Cisco Press, 2000.
Sportack, Mark A. IP Routing Fundamentals. Indianapolis: Cisco Press, 1999.