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EIGRP Tutorial

December 3rd, 2010 Go to comments

In this article we will mention about the EIGRP protocol.

Enhanced Interior Gateway Routing Protocol (EIGRP) is a Cisco-proprietary routing protocol. EIGRP is a

classless routing protocol, meaning that it sends the subnet mask of its interfaces in routing updates, which use

a complex metric based on bandwidth and delay.

EIGRP is referred to as a hybrid routing protocol because it has the characteristics of both distance-vector

and link-state protocols but now Cisco refers it as an advanced distance vector protocol.

Notice: the term hybrid is misleading because EIGRP is not a hybrid between distance vector and link-state

routing protocols. It is a distance vector routing protocol with enhanced features.

EIGRP is a powerful routing protocol and it is really standout from its ancestor IGRP. The main features are

listed below:

+ Support VLSM and discontiguous networks

+ Use Reliable Transport Protocol (RTP) to delivery and reception of EIGRP packets

+ Use the best path selection Diffusing Update Algorithm (DUAL), guaranteeing loop-free paths and

backup paths throughout the routing domain+ Discover neighboring devices using periodic Hello messages to discover and monitor connection

status with its neighbors

+ Exchange the full routing table at startup and send partial* triggered updates thereafter (not full updates

like distance-vector protocols) and the triggered updates are only sent to routers that need the information.

This behavior is different from the link-state protocol in which an update will be sent to all the link-state

routers within that area. For example, EIGRP will send updates when a new link comes up or a link

becoming unavailable

+ Supports multiple protocols: EIGRP can exchange routes for IPv4, IPv6, AppleTalk and IPX/SPX

networks

+ Load balancing: EIGRP supports unequal metric load balancing, which allows administrators to better

distribute traffic flow in their networks.

* Notice: The term partial means that the update only includes information about the route changes.

EIGRP use metrics composed of bandwidth, delay, reliability, and load. By default, EIGRP uses only

bandwidth and delay.

EIGRP use five types of packets to communicate:

+ Hello: used to identify neighbors. They are sent as periodic multicasts

+ Update: used to advertise routes, only sent as multicasts when something is changed

+ Ack: acknowledges receipt of an update. In fact, Ack is Hello packet without data. It is always unicast and

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uses UDP.

+ Query: used to find alternate paths when all paths to a destination have failed

+ Reply: is sent in response to query packets to instruct the originator not to recompute the route because

feasible successors exist. Reply packets are always unicast to the originator of the query

EIGRP sends every Query and Reply message using RTP, so every message is acknowledged using an

EIGRP ACK message.

EIGRP Route Discovery

Suppose that our network has 2 routers and they are configured to use EIGRP. Lets see what will happen

when they are turned on.

Firstly, the router will try to establish a neighboring relationships by sending Hello packets to others running

EIGRP. The destination IP address is 224.0.0.10 which is the multicast address of EIGRP. By this way,

other routers running EIGRP will receive and proceed these multicast packets. These packets are sent over

TCP.

After hearing Hello from R1, R2 will respond with another Hello packet.

R2 will also send its routing table to R1 by Update packets. Remember that R2 will send its completerouting table for the first time.




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R1 confirms it has received the Update packet by an ACK message.

R1 will also send to R2 all of its routing table for the first time

R2 sends a message saying it has received R1s routing table.




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Now both R1 & R2 learn all the paths of the neighbor and the network is converged. But there are some

notices you should know:

+ After the network converged, Hello messages will still be sent to indicate that the it is still alive.

+ When something in the network changes, routers will only send partial updates to routers which need that

information.+ Hellos are sent as periodic multicasts and are not acknowledged directly.

+ The first hellos are used to build a list of neighbors; thereafter, hellos indicate that the neighbor is still alive

To become a neighbor, the following conditions must be met:

+ The router must hear a Hello packet from a neighbor.

+ The EIGRP autonomous system must be the same.

+ K-values must be the same.

EIGRP builds and maintains three tables:

+ Neighbor table: lists directly connected routers running EIGRP with which this router has an adjacency

+ Topology table: lists all routes learned from each EIGRP neighbor

+ Routing table: lists all best routes from the EIGRP topology table and other routing processes

Configuring EIGRP

Router(config)#router eigrp 1Syntax: router eigrp

Turn on the EIGRP process

1 is the Autonomous System (AS) number. It can be from 1 to 65535.

All routers in the same network must use the same AS number.

Router(config-router)#network

192.168.1.0

Router will turn on EIGRP 1 process on all the interfaces belonging to

192.168.1.0/24 network.

In the next part we will learn about the Feasible Distance & Administrative Distance of EIGRP

Pages: 1 23




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Comments (138) Comments

Comment pages

1 23Next 380

1. vibhu rastogi

December 7th, 2010

this is very gud

and all the information in very dip

thanku so much

2. pavan

December 8th, 2010

god bless u

3. c.t.meiyappan

December 15th, 2010

really its good for undersatnding the eigrp principle

it helps to configure and depth knowledge and also outlook for basic

4. c.t.meiyappan

December 15th, 2010

really its good for undersatnding the eigrp principle

it helps to configure and depth knowledge and also outlook for basic concepts

5. Anonymous

January 10th, 2011

Hello,

How to configure Diffreent AS number in EIGRP with Redistribuition

Thanks

6. Sunnie

January 20th, 2011

Guys thanx i was so confused on EIGRP can you pliz help with Frame relay now

7. Jael

February 7th, 2011

Thanks for the in-depth explanation!

8. Denver

February 7th, 2011




6/13

Hie

thanx a million

9. Kaustubh

February 12th, 2011

Was confused in todd lammlety 9tut :)

10. asif

February 15th, 2011

detailed, easy to understand, yah keep it up, thanx.thanx

11. Yahya Misbah,Dubai

April 19th, 2011

Thanks 9tut.com.I now have more clear picture of EIGRP.

12. San Diego

May 3rd, 2011

Much appreciated, thank you guys for all ur time and effort

13. hmeister

May 6th, 2011

9tut Thanks for the overview concise and to the point I hope I can retain all of this as pieces of

this will be on exam questions

14. Mohamed nour,Egypt

May 6th, 2011

Very good summary,Many thanks

15. The King of Birds

May 7th, 2011

Very well explained, nice for beginners and good to improve and revise your concepts for existing

students. Very good summery of very lengthy topic.

16. daothanh_vietnam

May 11th, 2011

Much appreciated, thank you guys for all ur time and effort

17. Stephen

May 17th, 2011

Perhaps add in the default Admin Distance of 90 ? I didnt see that mentioned.

18. AS

May 18th, 2011




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Now a days Cisco is asking some new questions which are not in pass4sure. Is any one has the source

for these new questions?

19. kiran the king

May 22nd, 2011

god

20. naim

May 24th, 2011

good to read this!!!!!!!!!!!!!

21. Rohith

May 25th, 2011

Thank you very much,,

22. FRAUD

May 27th, 2011

G00D WORK INDEED.!!!!!! THNX

23. Turo!

June 2nd, 2011

Thank You! This is very useful!

24. esta

June 21st, 2011

this is great

25. tohil

June 23rd, 2011

in which condition , we can use no auto-summary and for what purpose we use them.?

26. Dipraj

June 23rd, 2011

Good One

27. Ram Moktan

July 7th, 2011

Greetings,

Very good. Thank you for your effort .

28. rupul

July 8th, 2011




8/13

Thanks again the tutorial was of immense help. Only I think that the figure for outbound interface in

the Routing table should be e0 and not 50 which is the AD for Router A to reach D. If I am wrong

then please do correct me. Thank you.

29. vinicius

July 8th, 2011

Amazing! :)

30. hany khalaf

July 12th, 2011

thanks for this tutorial

31. khaled

July 21st, 2011

Realllllly Thank You! This is very useful! I appreciate u for ur work

32. Eden

July 24th, 2011

Thank You for sharing these clear explanation with us.

33. Anuj

July 28th, 2011

Great workeasy to understand..Thanks a lot.

34. pyiphyo

July 31st, 2011

nice 9tut tutorial.

35. sumit menaria

August 4th, 2011

Nice explanation !!!

36. Satpreet

August 26th, 2011

Thank You 9tut.com for sharing these clear explanation with us.

Great Work.

Thank you very much

37. DarwinSeptember 1st, 2011

Thanks a million 9tut.I have to take hours to read this in Sybex.




9/13

Now this post take me just a few minutes.I really appreciate 9tut.

38. Rami

September 1st, 2011

Hello 9tut,

Can u plz double check the metric formula?

I guess it should be:

metric = [K1 * bandwidth + (K2 * bandwidth)/(256 - load) + K3 * delay] + [K5/(reliability + K4)]

otherwise, the metric will equal to zero if k4 & k5 are zeros!!!

Thx 4 the gr8 infokeep it up

39. xallax

September 1st, 2011

if K4 is 0 then

[K5/(reliability + K4)] = K5/reliability

if K5 is 0 then

[K1 * bandwidth + (K2 * bandwidth)/(256 - load) + K3 * delay] + [K5/(reliability + K4)] =

[K1 * bandwidth + (K2 * bandwidth)/(256 - load) + K3 * delay] + 0 =

[K1 * bandwidth + (K2 * bandwidth)/(256 - load) + K3 * delay]

40. morning999

September 2nd, 2011

Really Good !

41. Jeff2

September 3rd, 2011

if I used a class C with /24 instead of a class A (or B) /24, I wont need no auto-summary right?

42. xallax

September 3rd, 2011

@jeffyeap. youre wasting the entire address space on one interface with that /24 mask so no need for

no auto-summary in that case.

auto-summary is used to have less advertisement packets sent during updates and to keep the routing

table less crowded

43. Jeff2

September 3rd, 2011

Ive been creating these labs for ICND2 and I always use class C /24 so I never used no auto-

summary.

44. Jeff2

September 5th, 2011




10/13

On page 2 routing table, shouldnt the metric be Administrative Distance instead of Feasible Distance?

By default, EIGRP has an AD of 90 as the table shows on page 3. For the routing table, it is

[Administrative Distance (default of 90) / Advertised Distance (from neighboring router to

destination)].

45. 9tut

September 5th, 2011

@Jeff2: In the routing table, the metric is the Feasible Distance. Notice that the FD from routerA to

IOWA is 90 because 90 = 40 (routerA -> IDAHO) + 50 (IDAHO -> IOWA). It is not the AD of

EIGRP.

46. Jeff2

September 5th, 2011

But on the show ip route that you did on page 3, it says [90/30720] on the routing table for every

route learned through EIGRP. What is 90 there? 30720 is Advertised Distance.

47. Rami

September 5th, 2011

@xallax

I meant,

In the tutorial the equation is written wrong, there is (*) instead of (+), so in that case the metric will be

zeromaybe typo

48. xallax

September 5th, 2011

could be

49. 9tut

September 6th, 2011

@Jeff2: In [90/30720], 90 is the Adminstrative Distance of EIGRP.

50. Jeff2

September 6th, 2011

But in your Routing table on page 2, the metric is Feasible Distance not Administrative Distance. When

we talk about routing tables, shouldnt it always be 90 for EIGRP? And it should be [AD of EIGRP /

Advertised Distance] for EIGRP. Again, I am asking about the routing table. Like for RIP [AD of RIP

(120) / Hop count].

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