EIGRP: An Enhanced Distance Vector Routing Protocol

Although EIGRP is described as an enhanced distance vector routing protocol, it is still a distance vector routing protocol. This can sometimes be a source of confusion. In order to appreciate enhancements of EIGRP and eliminate any confusion, we must first look at its predecessor, IGRP.
Roots of EIGRP: IGRP
Cisco developed the proprietary IGRP in 1985, in response to some of the limitations of RIPv1, including the use of the hop count metric and the maximum network size of 15 hops.
Instead of hop count, both IGRP and EIGRP use metrics composed of bandwidth, delay, reliability, and load. By default, both routing protocols use only bandwidth and delay. However, because IGRP is a classful routing protocol that uses the Bellman-Ford algorithm and periodic updates, its usefulness is limited in many of today's networks.
Therefore, Cisco enhanced IGRP with a new algorithm, DUAL and other features. The commands for both IGRP and EIGRP are similar, and in many cases identical. This allows for easy migration from IGRP to EIGRP. Cisco discontinued IGRP starting with IOS 12.2(13)T and 12.2(R1s4)S.
Although discussed in more detail throughout this chapter, let us examine some of the differences between a traditional distance vector routing protocol such as RIP and IGRP, and the enhanced distance vector routing protocol, EIGRP.
The figure summarizes the main differences between a traditional distance vector routing protocol, such as RIP, and the enhanced distance vector routing protocol EIGRP.
The Algorithm
Traditional distance vector routing protocols all use some variant of the Bellman-Ford or Ford-Fulkerson algorithm. These protocols, such as RIP and IGRP, age out individual routing entries, and therefore need to periodically send routing table updates.
EIGRP uses the Diffusing Update Algorithm (DUAL). Although still a distance vector routing protocol, EIGRP with DUAL implements features not found in traditional distance vector routing protocols. EIGRP does not send periodic updates and route entries do not age out. Instead, EIGRP uses a lightweight Hello protocol to monitor connection status with its neighbors. Only changes in the routing information, such as a new link or a link becoming unavailable cause a routing update to occur. EIGRP routing updates are still vectors of distances transmitted to directly connected neighbors.
Path Determination
Traditional distance vector routing protocols such as RIP and IGRP keep track of only the preferred routes; the best path to a destination network. If the route becomes unavailable, the router waits for another routing update with a path to this remote network.
EIGRP's DUAL maintains a topology table separate from the routing table, which includes both the best path to a destination network and any backup paths that DUAL has determined to be loop-free. Loop-free means that the neighbor does not have a route to the destination network that passes through this router.
Later in this chapter, you will see that for a route to be a considered as a valid loop-free backup path by DUAL, it must meet a requirement known as the feasibility condition. Any backup path that meets this condition is guaranteed to be loop-free. Because EIGRP is a distance vector routing protocol, it is possible that there might be loop-free backup paths to a destination network that do not meet the feasibility condition. These paths are therefore not included in the topology table as a valid loop-free backup path by DUAL.
If a route becomes unavailable, DUAL will search its topology table for a valid backup path. If one exists, that route is immediately entered into the routing table. If one does not exist, DUAL performs a network discovery process to see if there happens to be a backup path that did not meet the requirement of the feasibility condition. This process is discussed more thoroughly later in this chapter.
Traditional distance vector routing protocols such as RIP and IGRP use periodic updates. Due to the unreliable nature of periodic updates, traditional distance vector routing protocols are prone to routing loops and the count-to-infinity problem. RIP and IGRP use several mechanisms to help avoid these problems including holddown timers, which cause long convergence times.
EIGRP does not use holddown timers. Instead, loop-free paths are achieved through a system of route calculations (diffusing computations) that are performed in a coordinated fashion among the routers. The detail of how this is done is beyond the scope of this course, but the result is faster convergence than traditional distance vector routing protocols.
EIGRP Message Format:
The data portion of an EIGRP message is encapsulated in a packet. This data field is called Type/Length/Value or TLV. As shown in the figure, the types of TLVs relevant to this course are EIGRP Parameters, IP Internal Routes, and IP External Routes. The components of the TLV data field are discussed further on the next page.
The EIGRP packet header is included with every EIGRP packet, regardless of its type. The EIGRP packet header and TLV are then encapsulated in an IP packet. In the IP packet header, the protocol field is set to 88 to indicate EIGRP, and the destination address is set to the multicast If the EIGRP packet is encapsulated in an Ethernet frame, the destination MAC address is also a multicast address: 01-00-5E-00-00-0A.
Note: In the following discussion of EIGRP messages, many fields are beyond the scope of this course. All fields are shown to provide an accurate picture of the EIGRP message format. However, only the fields relevant to the CCNA candidate are discussed.
Every EIGRP message includes the header. Important fields for our discussion include the Opcode field and the Autonomous System Number field. Opcode specifies the EIGRP packet type:
The Autonomous System (AS) Number specifies the EIGRP routing process. Unlike RIP, Cisco routers can run multiple instances of EIGRP. The AS number is used to track multiple instances of EIGRP.


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