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The Enhanced Interior Gateway Routing Protocol (EIGRP) is the successor of IGRP, it is more scalable and offers faster convergence. Unlike IGRP, EIGRP is a classless routing protocol, hence it supports VLSM. It is developed by Cisco and is supported on Cisco equipment only. In addition to IP, EIGRP can also be used to route IPX and AppleTalk. In contrary to IGRP, EIGRP is consider to be a hybrid routing protocol, because it has distance vector as well as link-state characteristics. EIGRP is a distance vector protocol with link-state characteristics, routing updates can be partial, they do not need to contain the complete routing table such as with RIP and IGRP. Also, updates are not send periodically, but only when necessary, and only to those neighboring routers that need to know. This results in low bandwidth and CPU usage, and makes EIGRP a fast routing protocol suitable for large networks. The maximum hopcount in EIGRP is 224. EIGRP allows for secure routing updates using authentication, to prevent unauthorized or false routing messages, although this is disabled by default. EIGRP updates use the multicast address

Besides maintaining a routing table, EIGRP maintains a topology table based on the information it receives in hello packets, and a neighbor table listing the directly connected neighbors. The neighbors are discovered using hello packets, which are send out periodically to check if the connection to the neighbor is still available. EIGRP uses five packet types: Hello/Acks, Updates, Queries, Replies, and Requests. When an EIGRP router stops receiving hello packets from a neighbor for a configurable amount of time, it will consider the router as unreachable. The topology database will be searched for backup route known as a feasible successor, if there isn't one, a multicast will be send out to find a new route. If another router responds with an alternative route, a change will be made to the topology table and a new route will be added to the routing table.

EIGRP uses the Diffusing Update Algorithm (DUAL) for route calculation and to prevent routing loops. The best route is determined based on 2 metrics by default, bandwidth and delay, but others can be used as well:

bandwidth Minimum bandwidth of the route in kbps * 256
delay Sum of route delay (in tens of microseconds) * 256.
reliability The value 255 means 100 percent reliability; 0 means no reliability.
load Effective bandwidth of the route expressed as a number from 0 to 255 (255 is 100 percent loading).
MTU Maximum transmission unit (MTU) size of the route in bytes. It can be 0 or any positive integer.

The formula used to calculate the composite metric is: metric = [K1 * bandwidth + (K2 * bandwidth) / (256 - load) + K3 * delay] * [K5 / (reliability + K4)]
By default K1 = 1, K2 = 0, K3 = 1, K4 = 0, K5 = 0. You can change these values, and hence the outcome of the formula by using the metric weights command in router configuration mode: Router(config-router)#metric weights tos k1 k2 k3 k4 k5
Tos is short for Type of Service and must be 0 (zero). Note that the default bandwidth for an interface is T1 speed, you can change this by using the bandwidth command in Interface Configuration mode.

To configure EIGRP on a router, use the following command:
Router(config)#router eigrp as-number
The as-number value is the Autonomous System (AS), also known as domain and process. This must be a positive decimal number. Routes from routers in one AS are not injected into another AS by default. Through a process called route tagging, a router is able to be part of more than one AS, which for example, can be used to route IPX and IP over the same network simultaneously. If a route from one AS is injected into another AS using route redistribution, the route will be tagged as external, which influences the administrative distance. The administrative distance for Internal EIGRP is 90 and for External EIGRP 170. These default values can be changed by using the following command:
Router(config-router)#distance eigrp internal external
The internal and external value can be an integer from 0 to 255, remember that routes with an administrative distance of 255 are marked unknown, and will not be used.

The network command is used to specify which networks are directly connected to the router, and to allow the interface of this network to be advertised in EIGRP routing updates. The following is an example of a simple EIGRP configuration:
Router(config)#router eigrp 22

Optionally, since IOS 12.0, the network command supports a network mask.

As mentioned earlier EIGRP sends routing updates to its neighbors only. A system using hello packets is used to discover, identify and built relationships with neighboring routers. The hello packets are sent periodically to determine if a neighbor (and its interfaces) is still available. The default hello packets interval is 60 seconds for low-speed (bandwidth T1 or slower) nonbroadcast multiaccess (NBMA) networks such as ATM and such as multipoint Frame Relay, and 5 seconds for all other networks. You can change the hello interval by using the following command in interface configuration mode:
Router(config-if)#ip hello-interval eigrp as-number seconds

After a hello packet is send, a router will wait until the hold timer expires for a response before it considers a router to be unreachable. The hold time default to 3 times the hello interval, you can change this by using the following command in interface configuration mode:
Router(config-if)ip hold-time eigrp as-number seconds

EIGRP supports load balancing over unequal paths, this means adding multiple primary routes for a single destination to the routing table even if the metrics are not the equal. For example, if you want to load balance between connection A and B, you can use the variance command to allow connection B to be included in the routing table as a feasible route to the same destination, even if it has a greater metric than connection A. Use the following command in router configuration mode:
Router(config-router)#variance multiplier
The multiplier value can be a integer from 1-128, the default is 1, which means equal-cost load balancing. If the value is set to 3, routes with a metric with 3 times greater than the local best metric are considered equal.

Another useful feature of EIGRP is automatic route summarization, this summarizes subnets to the classful network boundary. This is enabled by default, you can turn this off per AS by using the following command in router configuration mode:
Router(config-router)#no auto-summary
(and turn it on again with: Router(config-router)#auto-summary )
EIGRP summary routes have an administrative distance value of 5.
You can also configure a summary aggregate address for a specific interface by using the following command in Interface configuration mode:
Router(config-if)#ip summary-address eigrp as-number network-address subnet-mask [admin-distance]

Static routers and routes from other routing protocols such as RIP, IGRP, and OSPF can be redistributed into the EIGRP Autonomous System by using the redistribute command. For example if you want to redistribute OSPF process 10 into EIGRP AS 20:
Router(config)#router eigrp 20
Router(config-router)#redistribute ospf 10
Router(config-router)#default-metric 10000 100 255 1 1500

The default-metric command is used to configure a default metric for external routes being redistributed into the AS. The syntax for EIGRP is: Router(config-router)#default-metric bandwidth delay reliability loading mtu
IGRP routes can be automatically redistributed into EIGRP and vice versa, as long as the autonomous system is the same.


First a very useful command which is often used to troubleshoot routing, show ip protocols. Per routing protocol and AS it displays the parameters such as the value of the K0-K5 metrics, the networks involved, timers, hop count, outgoing filters, redistributed networks and more. Use the command in EXEC mode:
Router#show ip protocols

To show all routes in the routing table, learned by EIGRP:
Router#show ip route eigrp
Show all ip routes in the routing table by omitting the eigrp option.

To display information about neighboring routers discovered using hello packets, including the interface type and number, the smooth round-trip timer (SRTT), and the hold time (the latter can be used to determine the hello interval if it is not manually configured), use the following command:
Router#show ip eigrp neighbors

The following command displays entries in the EIGRP topology table:
Router#show ip eigrp topology
If the command is used without any options, only routes that are feasible successors are displayed. The following command would display only the active entries in the topology table and less detailed:
Router#show ip eigrp topology active summary
You can also specify an IP address and subnet mask to display a detailed description of the entry, for example:
Router#show ip eigrp topology

Shows the packet count for the five different types of EIGRP packets sent and received.
Router#show ip eigrp traffic

Use the following command in EXEC mode to display information about the interfaces configured with EIGRP. You can use this to determine on which interfaces EIGRP is active, if you do not specify an interface and/or AS, all interfaces running EIGRP and/or from all ASs will be displayed.
Router#show ip eigrp interfaces [interface-type interface-number] [as-number]

EIGRP References

- Introduction to EIGRP
- Enhanced Interior Gateway Routing Protocol white paper
- EIGRP Commands
- Redistributing Routing Protocols

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Current related exam topics for the 643-801 BSCI exam:

Exam Topics
- Describe the features and operation of EIGRP

Implementation and Configuration

- Given a set of network requirements, identify the steps to configure an Enhanced IGRP environment and verify proper operation (within described guidelines) of your routers

- Identify the steps to verify Enhanced IGRP operation

Click here for the complete list of exam objectives.

Discuss this TechNote here Author: Johan Hiemstra


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