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Cisco ICND2 Foundation Learning Guide: Implementing an EIGRP Solution

To assist in your studies for the ICND2 exam, this chapter reviews the basics of dynamic routing and examines the operation, configuration, and troubleshooting of EIGRP for IPv4 and IPv6.
This chapter is from the book

This chapter contains the following sections:

  • Dynamic Routing Review
  • EIGRP Features and Function
  • Troubleshooting EIGRP
  • Implementing EIGRP for IPv6
  • Chapter Summary
  • Review Questions

EIGRP, Enhanced Interior Gateway Protocol, is an advanced distance vector routing protocol that was developed by Cisco over 20 years ago. It is suited for many different topologies and media. EIGRP scales well and provides extremely quick convergence times with minimal overhead. EIGRP performs in both well-designed networks and poorly designed networks. It is a popular choice for a routing protocol on Cisco devices. EIGRP did have a predecessor, Interior Gateway Protocol (IGRP), which is now obsolete and is not included in Cisco IOS 15.

EIGRP was historically a Cisco proprietary and closed protocol. However, as of this writing, Cisco is in the process of releasing the basic functions to the IETF as an RFC (Request For Comments, a standards document; see http://tools.ietf.org/html/draft-savage-eigrp-00).

This chapter begins with a review of dynamic routing. It then examines the operation, configuration, and troubleshooting of EIGRP for IPv4 and IPv6.

Chapter Objectives:

  • Review key concepts for Dynamic Routing Protocols
  • Understand how a Cisco Router populates its routing table
  • Understand the features, operation, theory, and functions of EIGRP
  • Configure and troubleshoot EIGRP for IPv6 and IPv4

Dynamic Routing Review

A dynamic routing protocol is a set of processes, algorithms, and messages that is used to exchange routing and reachability information within the internetwork. Without a dynamic routing protocol, all networks, except those connected directly with the router, must be statically defined. Dynamic routing protocols can react to changes in conditions in the network, such as failed links.


All routing protocols have the same purpose: to learn about remote networks and to quickly adapt whenever there is a change in the topology. The method that a routing protocol uses to accomplish this purpose depends upon the algorithm that it uses and the operational characteristics of the protocol. The performance of a dynamic routing protocol varies depending on the type of routing protocol.

Although routing protocols provide routers with up-to-date routing tables, there are costs that put additional demands on the memory and processing power of the router. First, the exchange of route information adds overhead that consumes network bandwidth. This overhead can be a problem, particularly for low-bandwidth links between routers. Second, after the router receives the route information, the routing protocol needs to process the information received. Therefore, routers that employ these protocols must have sufficient resources to implement the algorithms of the protocol and to perform timely packet routing and forwarding.

Routing Domains

An autonomous system (AS), otherwise known as a routing domain, is a collection of routers under a common administration. A typical example is an internal network of a company and its interconnection to the network of an ISP. The ISP and a company’s internal network are under different control. Therefore, they need a way to interconnect. Static routes are often used in this type of a scenario. However, what if there are multiple links between the company and the ISP? What if the company uses more than one ISP? Static routing protocols would not be suitable. To connect the entities, it is necessary to establish communication with the bodies under different administration. Another example would be a merger, acquisition, or development of a subsidiary that maintains its own IT resources. The networks may need to be connected, but they also may need to be maintained as separate entities. There must be a way to communicate between the two. The third example, which is intimated by the first, is the public Internet. Many different entities are interconnected here as well. Figure 3-1 is a representation of three autonomous systems, one for a private company and two ISPs.

Figure 3-1

Figure 3-1 Connection of Three Distinct Autonomous Systems (AS)

To accommodate these types of scenarios, two categories of routing protocols exist:

  • Interior Gateway Protocols (IGP): These routing protocols are used to exchange routing information within an autonomous system. EIGRP, IS-IS (Intermediate System-to-Intermediate System) Protocol, RIP (Routing Information Protocol), and OSPF (Open Shortest Path First) Protocol are examples of IGPs.
  • Exterior Gateway Protocols (EGP): These routing protocols are used to route between autonomous systems. BGP (Border Gateway Protocol) is the EGP of choice in networks today. The Exterior Gateway Protocol, designed in 1982, was the first EGP. It has since been deprecated in favor of BGP and is considered obsolete. BGP is the routing protocol used on the public Internet.

Classification of Routing Protocols

EGPs and IGPs are further classified depending on how they are designed and operate. There are two categories of routing protocols:

  • Distance vector protocols: The distance vector routing approach determines the direction (vector) and distance (hops) to any point in the internetwork. Some distance vector protocols periodically send complete routing tables to all of the connected neighbors. In large networks, these routing updates can become enormous, causing significant traffic on the links. This can also cause slow convergence, as the whole routing table could be inconsistent due to network changes, such as a link down, between updates. RIP is an example of a protocol that sends out periodic updates.
  • Distance vector protocols use routers as signposts along the path to the final destination. The only information that a router knows about a remote network is the distance or metric to reach that network and which path or interface to use to get there. Distance vector routing protocols do not have an actual map of the network topology. EIGRP is another example of the distance vector routing protocol. However, unlike RIP, EIGRP does not send out full copies of the routing table once the initial setup occurs between two neighboring routers. EIGRP only sends updates when there is a change.
  • Link-state protocols: The link-state approach, which uses the shortest path first (SPF) algorithm, creates an abstract of the exact topology of the entire internetwork, or at least of the partition in which the router is situated. Using a link-state routing protocol is like having a complete map of the network topology. Signposts along the way from the source to the destination are not necessary because all link-state routers are using an identical “map” of the network. A link-state router uses the link-state information to create a topology map and select the best path to all destination networks in the topology. Link-state protocols only send updates when there is a change in the network. BGP, OSPF, and IS-IS are examples of link-state routing protocols.

Classful Routing Versus Classless Routing

IP addresses are categorized in classes: A, B, and C. Classful routing protocols only recognize networks as directly connected by class. So, if a network is subnetted, there cannot be a classful boundary in between. In Figure 3-2, Network A cannot reach Network B using a classful routing protocol because they are separated by a different class network. The term for this scenario is discontiguous subnets.

Figure 3-2

Figure 3-2 Sample Classful Routing Domain

Classful routing is a consequence when subnet masks are not disclosed in the routing advertisements that most distance vector routing protocols generate. When a classful routing protocol is used, all subnetworks of the same major network (Class A, B, or C) must use the same subnet mask, which is not necessarily a default major-class subnet mask. Routers that are running a classful routing protocol perform automatic route summarization across network boundaries. Classful routing has become somewhat obsolete because the classful model is rarely used on the Internet. Because IP address depletion problems occur on the Internet, most Internet blocks are subdivided using classless routing and variable-length subnet masks. You will most likely see classful address allocation inside private organizations that use private IP addresses as defined in RFC 1918 in conjunction with Network Address Translation (NAT) at AS borders.

Classless routing protocols can be considered second-generation protocols because they are designed to address some of the limitations of the earlier classful routing protocols. A serious limitation in a classful network environment is that the subnet mask is not exchanged during the routing update process, thus requiring the same subnet mask to be used on all subnetworks within the same major network. Another limitation of the classful approach is the need to automatically summarize to the classful network number at all major network boundaries. In the classless environment, the summarization process is controlled manually and can usually be invoked at any bit position within the address. Because subnet routes are propagated throughout the routing domain, manual summarization may be required to keep the size of the routing tables manageable. Classless routing protocols include BGP, RIPv2, EIGRP, OSPF, and IS-IS. Classful routing protocols include Cisco IGRP and RIPv1.

Administrative Distance

Multiple routing protocols and static routes may be used at the same time. If there are several sources for routing information, including specific routing protocols, static routes, and even directly connected networks, an administrative distance value is used to rate the trustworthiness of each routing information source. Cisco IOS Software uses the administrative distance feature to select the best path when it learns about the exact same destination network from two or more routing sources.

An administrative distance is an integer from 0 to 255. A routing protocol with a lower administrative distance is more trustworthy than one with a higher administrative distance. Table 3-1 displays the default administrative distances.

Table 3-1 Default Administrative Distances

Route Source

Default Administrative Distance

Directly connected interface


Static route


eBGP (external BGP; between two different AS)






RIP (both v1 and v2)


EIGRP External


iBGP (internal BGP, inside AS)


Unknown/untrusted source


As shown in the example in Figure 3-3, the router must deliver a packet from Network A to Network B. The router must choose between two routes. One is routed by EIGRP, and the other is routed by OSPF. Although the OSPF route appears to be the logical choice, given that it includes fewer hops to the destination network, the EIGRP route is identified as more trustworthy and is added to the routing table of the router.

Figure 3-3

Figure 3-3 Administrative Distance

A good way to detect which routing protocols are configured on the router is to execute show ip protocols. Example 3-1 gives output from a sample router running OSPF, EIGRP, and BGP. The command provides details regarding each routing protocol, including the administrative distance (Distance), values the routing protocol is using, and other features such as route filtering.

Example 3-1 show ip protocols Command Output

Branch# show ip protocols
Routing Protocol is "eigrp 1"
  Outgoing update filter list for all interfaces is not set
  Incoming update filter list for all interfaces is not set
  Default networks flagged in outgoing updates
  Default networks accepted from incoming updates
  EIGRP metric weight K1=1, K2=0, K3=1, K4=0, K5=0
  EIGRP maximum hopcount 100
  EIGRP maximum metric variance 1
  Redistributing: eigrp 1
  EIGRP NSF-aware route hold timer is 240s
  Automatic network summarization is in effect
  Automatic address summarization: for Loopback0, Loopback100 for Loopback0, Vlan1 for Loopback100, Vlan1
      Summarizing with metric 128256
  Maximum path: 4
  Routing for Networks:
  Routing Information Sources:
    Gateway         Distance      Last Update
    (this router)         90      00:00:18
  Distance: internal 90 external 170

Routing Protocol is "eigrp 100"
  Outgoing update filter list for all interfaces is not set
  Incoming update filter list for all interfaces is not set
  Default networks flagged in outgoing updates
  Default networks accepted from incoming updates
  EIGRP metric weight K1=1, K2=0, K3=1, K4=0, K5=0
  EIGRP maximum hopcount 100
  EIGRP maximum metric variance 1
  Redistributing: eigrp 100
  EIGRP NSF-aware route hold timer is 240s
  Automatic network summarization is in effect
  Automatic address summarization: for Loopback0 for Loopback100
      Summarizing with metric 128256
  Maximum path: 4
  Routing for Networks:
  Routing Information Sources:
    Gateway         Distance      Last Update
    (this router)         90      00:00:19
  Distance: internal 90 external 170

Routing Protocol is "ospf 100"
  Outgoing update filter list for all interfaces is not set
  Incoming update filter list for all interfaces is not set
  Router ID
  Number of areas in this router is 1. 1 normal 0 stub 0 nssa
  Maximum path: 4
  Routing for Networks: area 0
 Reference bandwidth unit is 100 mbps
  Routing Information Sources:
    Gateway         Distance      Last Update
  Distance: (default is 110)

Routing Protocol is "bgp 100"
  Outgoing update filter list for all interfaces is not set
  Incoming update filter list for all interfaces is not set
  IGP synchronization is disabled
  Automatic route summarization is disabled
  Maximum path: 1
  Routing Information Sources:
    Gateway         Distance      Last Update
  Distance: external 20 internal 200 local 200

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