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Home > EIGRP Tutorial
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
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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
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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.
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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
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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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