Cisco® CCNA Exam Cram Notes : EIGRP for IPv4

X. Appendix

5. EIGRP for IPv4

Important features

1. EIGRP uses Bandwidth, MTU, Reliability, Delay, and Load for computing routing metrics.

2. EIGRP, by default, uses bandwidth and delay as metrics for taking routing decisions.

3. EIGRP uses auto summarization of routes at major network boundaries

4. EIGRP uses multicasts to send queries to neighbor routers.

5. EIGRP, and OSPF use Autonomous System (AS) numbers. An EIGRP AS is a complex network that has diverse bandwidth and delay characteristics.

6. EIGRP uses multicasts to send queries to neighbor routers. EIGRP Hello packets are multicast to

7. EIGRP has certain features that belong to link-state algorithms (like OSPF) than distance-vector algorithms. Ex: EIGRP sends a partial routing table update, which includes just routes that have been changed, not the full routing table like distance-vector algorithms.

8. The feasible successor route will become the primary route when its advertised distance is higher than the feasible distance of the successor route. The feasible successor is kept in the topology table as a backup route and can be used in the event that the successor route goes down.

9. Support VLSM, route summarization, and routing update authentication

10. EIGRP uses a distributed algorithm called DUAL when a route fails and has no feasible successor to discover a replacement for a failed route. When a new route is found, DUAL adds it to the routing table.

11. When both EIGRP and OSPF are configured, EIGRP route takes precedence over OSPF because EIGRP has an administrative distance of 90, whereas OSPF has an administrative distance of 110. Therefore, the route discovered by EIGRP is entered into the routing table.

12. Unlike RIP and IGRP, EIGRP updates are not periodic. EIGRP updates are sent only when there is a topological change in the network.

13. In EIGRP, the router doing the summarization will build a route to null0 for the summarized address. This ensures that the packets that are not destined for any network are routed to null and thus dropped.

14. EIGRP provides the option of disabling route summarization. The command no auto-summary can be used for this purpose. This option is not available in RIP and IGRP.

15. You can summarize routes in EIGRP at any arbitrary bit boundary

EIGRP uses the following components as metrics:

1. Delay: This is affected by the band width and queuing delay. Calculated by adding up the delay along the path to the next router.

2. Reliability: This is a measurement of reliability of a network link. It is assigned by the administrator or can be calculated by using protocol statistics. This is representative of how many errors are occurring on the interface. The best reliability value is 255. A value of 128 represents only 50% reliability.

3. Load: Load is based on many things, CPU usage, packets processed per sec. Load metric also has a range from 1 to 255. If a serial link is being operated at 50% capacity, the load value is 255X0.5 or 12.5. Lower load value is better.

4. MTU(Maximum Transmission Unit): This is the maximum message length that is acceptable to all links on the path. The larger MTU means faster transmission of packets. Ethernet and serial interface has a default MTU of 1500. Larger MTU size means that the link is more efficient.

5. Bandwidth: This represents the maximum throughput of a link. The bandwidth is specified in Kbps. Larger the bandwidth, better the link.

The following are main features of route summarization in EIGRP:

By default, EIGRP summarizes routes at the major network boundaries (classful boundaries).

To enable summarization at any level other than major network boundary, you need to disable auto summarization using the command: "No auto-summary"

The following command enables summarization at an arbitrary network boundary: "Ip summary-address <as-number> <address-mask>"

4. Note that you need to specify the IP address and routing mask of the summary route. No need to specify the metrics.

The following protocols support automatic route summarization at major network boundaries.

1. RIPv1
2. RIPv2

DUAL (Diffusing Update Algorithm) used by EIGRP tracks all the routes advertised by neighbors and selects routes based on feasible successors. It inserts lowest cost paths into the routing table (these routes are known as primary routes or successor routes).

Please note that EIGRP does not summarize received routes. That is, if a network was not summarized at the major network boundary (this may happen, if you use "no auto-summary" command), then all the subnet routes will be carried into the routing tables of subsequent routers in the rest of the world

The correct command to disable auto-summary in EIGRP environment is : no auto-summary

The above command will turn off route summarization in EIGRP network.


For EIGRP routing, you need to provide the AS (Autonomous System) number in the command. Routers need AS number to exchange routing information. Routers belonging to same AS exchange routing information.

Following are possible solutions for preventing routing loops.

1. Split Horizon - Based on the principle that it is not useful to send the information about a route back in the direction from which the information originally came.

2. Poison Reverse - A router that discovers an inaccessible route sets a table entry in a consistent state (infinite metric) while the network converges.

3. Hold-down Timers - Holddown timers prevent regular update messages from reinstating a route that has gone bad. Here, if a route fails, the router waits a certain amount of time before accepting any other routing information about that route.

4. Triggered Updates - Normally, new routing tables are sent to neighboring routers at regular intervals (IP RIP every 30 sec / and IPX RIP every 60 sec). A triggered update is an update sent immediately in response to some change in the routing table. Triggered updates along with Hold-down timers can be used effectively to counter routing loops.

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