Configuring EIGRP
This chapter describes how to configure Enhanced Interior Gateway Routing Protocol (EIGRP). For a complete description of the EIGRP commands listed in this chapter, refer to the "EIGRP Commands" chapter of the Cisco IOS IP Command Reference, Volume 2 of 3: Routing Protocols publication. To locate documentation of other commands that appear in this chapter, use the command reference master index, or search online.
Refer to the Cisco IOS AppleTalk and Novell IPX Configuration Guide for information on AppleTalk EIGRP or Internetwork Packet Exchange (IPX) EIGRP.
For protocol-independent features that work with EIGRP, see the chapter "Configuring IP Routing Protocol-Independent Features" in this document.
EIGRP is an enhanced version of the IGRP developed by Cisco. EIGRP uses the same distance vector algorithm and distance information as IGRP. However, the convergence properties and the operating efficiency of EIGRP have improved substantially over IGRP.
The convergence technology is based on research conducted at SRI International and employs an algorithm referred to as the Diffusing Update Algorithm (DUAL). This algorithm guarantees loop-free operation at every instant throughout a route computation and allows all devices involved in a topology change to synchronize at the same time. Routers that are not affected by topology changes are not involved in recomputations. The convergence time with DUAL rivals that of any other existing routing protocol.
To identify the hardware platform or software image information associated with a feature, use the Feature Navigator on Cisco.com to search for information about the feature or refer to the software release notes for a specific release. For more information, see the "Identifying Supported Platforms" section in the "Using Cisco IOS Software" chapter in this book.
The Cisco EIGRP Implementation
EIGRP provides the following features:
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Automatic redistribution—IGRP routes can be automatically redistributed into EIGRP, and EIGRP routes can be automatically redistributed into IGRP. If desired, you can turn off redistribution. You can also completely turn off EIGRP and IGRP on the router or on individual interfaces.
Automatic redistribution—IGRP routes can be automatically redistributed into EIGRP, and EIGRP routes can be automatically redistributed into IGRP. If desired, you can turn off redistribution. You can also completely turn off EIGRP and IGRP on the router or on individual interfaces. •Increased network width—With IP Routing Information Protocol (RIP), the largest possible width of your network is 15 hops. When EIGRP is enabled, the largest possible width is increased to 100 hops, and the metric is large enough to support thousands of hops.
Increased network width—With IP Routing Information Protocol (RIP), the largest possible width of your network is 15 hops. When EIGRP is enabled, the largest possible width is increased to 100 hops, and the metric is large enough to support thousands of hops. 
Note Redistribution between EIGRP and IGRP differs from normal redistribution in that the metrics of IGRP routes are compared with the metrics of external EIGRP routes. The rules of normal administrative distances are not followed, and routes with the lowest metric are selected.
Redistribution between EIGRP and IGRP differs from normal redistribution in that the metrics of IGRP routes are compared with the metrics of external EIGRP routes. The rules of normal administrative distances are not followed, and routes with the lowest metric are selected. EIGRP offers the following features:
•Fast convergence—The DUAL algorithm allows routing information to converge as quickly as any currently available routing protocol.
Fast convergence—The DUAL algorithm allows routing information to converge as quickly as any currently available routing protocol. •Partial updates—EIGRP sends incremental updates when the state of a destination changes, instead of sending the entire contents of the routing table. This feature minimizes the bandwidth required for EIGRP packets.
Partial updates—EIGRP sends incremental updates when the state of a destination changes, instead of sending the entire contents of the routing table. This feature minimizes the bandwidth required for EIGRP packets. •Less CPU usage than IGRP—This occurs because full update packets need not be processed each time they are received.
Less CPU usage than IGRP—This occurs because full update packets need not be processed each time they are received. •Neighbor discovery mechanism—This is a simple hello mechanism used to learn about neighboring routers. It is protocol-independent.
Neighbor discovery mechanism—This is a simple hello mechanism used to learn about neighboring routers. It is protocol-independent. •Variable-length subnet masks (VLSMs).
Variable-length subnet masks (VLSMs). •Arbitrary route summarization.
Arbitrary route summarization. •Scaling—EIGRP scales to large networks.
Scaling—EIGRP scales to large networks. EIGRP has the following four basic components:
•Neighbor discovery of neighbor recovery
Neighbor discovery of neighbor recovery •Reliable transport protocol
Reliable transport protocol •DUAL finite state machine
DUAL finite state machine •Protocol-dependent modules
Protocol-dependent modules Neighbor discovery of neighbor recovery is the process that routers use to dynamically learn of other routers on their directly attached networks. Routers must also discover when their neighbors become unreachable or inoperative. Neighbor discovery of neighbor recovery is achieved with low overhead by periodically sending small hello packets. As long as hello packets are received, the Cisco IOS software can determine that a neighbor is alive and functioning. Once this status is determined, the neighboring routers can exchange routing information.
The reliable transport protocol is responsible for guaranteed, ordered delivery of EIGRP packets to all neighbors. It supports intermixed transmission of multicast and unicast packets. Some EIGRP packets must be sent reliably and others need not be. For efficiency, reliability is provided only when necessary. For example, on a multiaccess network that has multicast capabilities (such as Ethernet) it is not necessary to send hello packets reliably to all neighbors individually. Therefore, EIGRP sends a single multicast hello with an indication in the packet informing the receivers that the packet need not be acknowledged. Other types of packets (such as updates) require acknowledgment, which is indicated in the packet. The reliable transport has a provision to send multicast packets quickly when unacknowledged packets are pending. This provision helps to ensure that convergence time remains low in the presence of varying speed links.
The DUAL finite state machine embodies the decision process for all route computations. It tracks all routes advertised by all neighbors. DUAL uses the distance information (known as a metric) to select efficient, loop-free paths. DUAL selects routes to be inserted into a routing table based on feasible successors. A successor is a neighboring router used for packet forwarding that has a least-cost path to a destination that is guaranteed not to be part of a routing loop. When there are no feasible successors but there are neighbors advertising the destination, a recomputation must occur. This is the process whereby a new successor is determined. The amount of time required to recompute the route affects the convergence time. Recomputation is processor-intensive; it is advantageous to avoid unneeded recomputation. When a topology change occurs, DUAL will test for feasible successors. If there are feasible successors, it will use any it finds in order to avoid unnecessary recomputation.
The protocol-dependent modules are responsible for network layer protocol-specific tasks. An example is the EIGRP module, which is responsible for sending and receiving EIGRP packets that are encapsulated in IP. It is also responsible for parsing EIGRP packets and informing DUAL of the new information received. EIGRP asks DUAL to make routing decisions, but the results are stored in the IP routing table. Also, EIGRP is responsible for redistributing routes learned by other IP routing protocols.
EIGRP Configuration Task List
To configure EIGRP, perform the tasks described in the following sections. The tasks in the first section are required; the tasks in the remaining sections are optional:
•Enabling EIGRP (Required)
Enabling EIGRP (Required) •Making the Transition from IGRP to EIGRP (Optional)
Making the Transition from IGRP to EIGRP (Optional) •Logging EIGRP Neighbor Adjacency Changes (Optional)
Logging EIGRP Neighbor Adjacency Changes (Optional) •Adjusting the EIGRP Metric Weights (Optional)
Adjusting the EIGRP Metric Weights (Optional) •Applying Offsets to Routing Metrics (Optional)
Applying Offsets to Routing Metrics (Optional) •Disabling Route Summarization (Optional)
Disabling Route Summarization (Optional) •Configuring Summary Aggregate Addresses (Optional)
Configuring Summary Aggregate Addresses (Optional) •Configuring Floating Summary Routes (Optional)
Configuring Floating Summary Routes (Optional) •Configuring EIGRP Route Authentication (Optional)
Configuring EIGRP Route Authentication (Optional) •Configuring EIGRP Stub Routing (Optional)
Configuring EIGRP Stub Routing (Optional) •Monitoring and Maintaining EIGRP(Optional)
Monitoring and Maintaining EIGRP(Optional) See the section "EIGRP Configuration Examples" at the end of this chapter for configuration examples.
Enabling EIGRP
To create an EIGRP routing process, use the following commands beginning in global configuration mode:
EIGRP sends updates to the interfaces in the specified networks. If you do not specify the network of an interface, the interface will not be advertised in any EIGRP update.
Making the Transition from IGRP to EIGRP
If you have routers on your network that are configured for IGRP, and you want to make a transition to routing EIGRP, you must designate transition routers that have both IGRP and EIGRP configured. In these cases, perform the tasks as noted in the previous section, "Enabling EIGRP," and also see the chapter "Configuring IGRP" in this document. You must use the same autonomous system number in order for routes to be redistributed automatically.
