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EIGRP Data Structures


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Enhanced Interior Gateway Routing Protocol



EIGRP Background



New Features Found in EIGRP



EIGRP Data Structures



The Routing Table



The Topology Table

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IP Routing Fundamentals

From: IP Routing Fundamentals
Author: Mark Sportack
Publisher: Cisco Press (53)
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EIGRP Data Structures

EIGRP is a fairly information-intensive routing protocol; it must keep track of the current state (or nearly current state) of many different facets of the network. This information is organized into collections of related information, which are stored in tables. EIGRP maintains the currency of these tables via a series of specialized packet types. Each packet type is used for a specific function. This section examines the basic functionality and use of each of EIGRP's tables and packet types.

EIGRP Tables

EIGRP uses many different tables, each dedicated to organizing and storing data pertinent to a specific facet of the network. The proprietary nature of EIGRP precludes an exhaustive survey of these tables as well as their structures. It is possible, however, to examine the role of some of the more important EIGRP tables, including the following:

  • The neighbor table

  • The routing table

  • The topology table

There are other tables, but these three should adequately convey the bulk of EIGRP's internal mechanics.

The Neighbor Table

The single most important table in EIGRP is the neighbor table. The neighbor relationships tracked in this table are the basis for all of EIGRP's routing update and convergence activity.

The neighbor table tracks state information about adjacent neighboring EIGRP nodes. Whenever a new neighbor is discovered, the address and interface of that neighbor are recorded in a new entry in the neighbor table. Actually, an EIGRP router may contain several neighbor tables because one is required for each protocol-dependent module. Therefore, a network that runs both AppleTalk and IP would have two different neighbor tables. A separate EIGRP process would have to be configured for each routed protocol used in the network.

Additionally, a neighbor table is used to support reliable, sequenced delivery of packets. One field in each row of the table is populated with the last sequence number of the packet received from that neighbor. EIGRP uses this field for two purposes:

  • Sequence numbers are used to acknowledge specific packets that were delivered reliably. Acknowledgments are transmitted bearing the sequence number of the packet of which they are acknowledging receipt. This enables the router that receives the acknowledgment to know which packet is being acknowledged.

  • This field can also alert EIGRP to packets being received out of sequence.

For both of these reasons, EIGRP records the sequence number of the last message received from each specific neighbor.


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