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EIGRP Tutorial

December 3rd, 2010 Go to comments

Calculate EIGRP metric

In this part we will continue to learn about the EIGRP Routing Protocol

I built the topology with Packet Tracer to illustrate what will be mentioned. You can download the lab file

here: http://www.9tut.com/download/EIGRP_CCNA_self_study.zip (please unzip & use at least Packet

Tracer v5.3 to open it)

Check the neighbor table of Router0 with the show ip eigrp neighbors command

Lets analyze these columns:

+ H: lists the neighbors in the order this router was learned

+ Address: the IP address of the neighbors

+ Interface: the interface of the local router on which this Hello packet was received

+ Hold (sec): the amount of time left before neighbor is considered in down status

+ Uptime: amount of time since the adjacency was established+ SRTT (Smooth Round Trip Timer): the average time in milliseconds between the transmission of a packet

to a neighbor and the receipt of an acknowledgement.

+ RTO (Retransmission Timeout): if a multicast has failed, then a unicast is sent to that particular router, the

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RTO is the time in milliseconds that the router waits for an acknowledgement of that unicast.

+ Queue count (Q Cnt): shows the number of queued EIGRP packets. It is usually 0.

+ Sequence Number (Seq Num): the sequence number of the last update EIGRP packet received. Each

update message is given a sequence number, and the received ACK should have the same sequence number.

The next update message to that neighbor will use Seq Num + 1.

As CCNA level, we only care about 4 columns: Address, Interface, Hold & Uptime. Other columns will be

discussed in CCNP so you dont need to remember them now!

Notice that you can see a line IP-EIGRP neighbors for process 100. Process 100 here means AS

100.

Next we will analyze the EIGRP topology with the show ip eigrp topology command. The output of Router0

is shown below

The letter P as the left margin of each route entry stands for Passive. Passive state indicates that the route

is in quiescent mode, implying that the route is known to be good and that no activities are taking place withrespect to the route.

Each route shows the number of the successor it has. For example, the network 192.168.2.0,

192.168.1.0,192.168.3.0 & 192.168.4.0 have only 1 successor (and no feasible successor). Only network

192.168.5.0 has 2 successors.

We notice that there are 2 numbers inside the brackets (30720/28160). The first one is the metric from

Router0 to the destination, the second is the AD of this route, advertised by the neighbor. For example, the

third route entry has:


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Lets see how to calculate them!

First you should learn the formula to calculate the metric. Its a bit complex, I think :)

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

Note: you can check these K values with the show ip protocols command. Below is an example of this

command on Router0.

To change these values, use the metric weights tos k1 k2 k3 k4 k5 in the EIGRP router mode.

By default, K1 = 1, K2 = 0, K3 = 1, K4 = 0, K5 = 0 which means that the default values use only

bandwidth & delay parameters while others are ignored. The metric formula is now reduced:

metric = bandwidth + delay

But the bandwidth here is defined as the slowest bandwidth in the route to the destination & delay is the sum

of the delays of each link. Here is how to calculate the EIGRP metric in detail:

EIGRP uses the slowest bandwidth of the outgoing interfaces of the route to calculate the metric. In this

case we need to find out the bandwidth of Fa0/0 of Router0 & Fa0/1 of Router1 as the destination network

is 192.168.3.0/24.


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Find the bandwidth

We can find the bandwidth of each interface by the show interfaces . Below is an output of the show

interfaces fa0/0 on Router0.

All the interfaces in this topology have the bandwidth of 100,000 Kbps so we will get the same result on

interface Fa0/1 of Router1 -> The slowest bandwidth here is 100,000 Kbps. Now we can calculate the first

portion of the formula:

Notice that if the result is not an integer then the result will be rounded down. For example, 10,000,000

divided by 1024 (the speed of T1) equals 9765.625. The result will be rounded down to 9765.

Find the delay

EIGRP also used the delay of the outgoing interfaces and it can also be found with the show interfaces , the

delay lies next to the bandwidth value (for example, DLY 100usec). In this case, the delay value of both

Fa0/0 of Router0 & Fa0/1 of Router1 is 100 usec (microsecond) so the sum of delay is 100 + 100 = 200

usec. The second portion of the formula is:

Get the metric


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Now just sum up two portions of the formula and multiplied by 256 to get the result:

The result is 30720 and it matches the value shown in the topology table of the route to 192.168.3.0/24

Using the formula above, we can easily calculate the AD of that route (with slowest bandwidth =

100,000Kpbs; sum of delay = 10)

metric = (100 + 10) * 256 = 28160

This metric matches with the second parameter of the above route.

Note: The output of show ip eigrp topology command shows only feasible successors while the output of

show ip eigrp topology all-links shows all neighbors, whether feasible successors or not. To learn more

about the show ip eigrp topology all-links please read http://www.digitaltut.com/route-eigrp-simlet.

Although it belongs to CCNP exam but CCNA level can read it too.

EIGRP Routing table

The last table we will discuss is the routing table. This is the most used table to check the operation of

EIGRP. Here is the output of the show ip route command on Router0:


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The routing table has two parameters [90/30720] but the first one is the administrative distance of EIGRP.

EIGRP has a default administrative distance of 90 for internal routes and it is often the most preferred routing

protocol because it has the lowest administrative distance.

Administrative distance is the measure used by Cisco routers to select the best path when there are two or

more different routes to the same destination from two different routing protocols.

Below is the administrative distances of the most popular routing protocols used nowadays. Notice that the

smaller is the better.

So, if a network running two routing protocols at the same time, for example EIGRP and OSPF, which

routing protocol will the router choose? Well, the answer is EIGRP as it has lower Administrative Distance

than OSPF ( 90 < 110).

The second parameter, as you can guess, is the metric of that route as we discussed above.

no auto-summary with EIGRP

One of the features of EIGRP is support VLSM and discontiguous networks. Discontiguous networks are

networks that have subnets of a major network separated by a different major network. Below is an example

of discontiguous networks where subnets 10.10.1.0/24 and 10.10.2.0/24 are separated by a 2.0.0.0/8

network.


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Now lets see what will happen when we turn on EIGRP on both of the routers. To turn on EIGRP you will

use these commands:

R1(config)#router eigrp 1

R1(config-router)#network 2.0.0.0

R1(config-router)#network 10.10.1.0 (or network 10.0.0.0)


R2(config-router)#network 2.0.0.0

R2(config-router)#network 10.10.2.0 (or network 10.0.0.0)

You can try to use the more specific network 10.10.1.0 instead of network 10.0.0.0, hoping that EIGRP

will understand it is a sub-network. But if we check the configuration with the show running-config

command we will notice that EIGRP has auto-summarized our network.

R1#show running-config

-> Network 10.10.1.0 has been summarized to network 10.0.0.0 because it knows 10.x.x.x network

belongs to class A.

The same thing happens for R2. Now we should check the routing table of R1 with the show ip route

command

R1#show ip route

From the output above we learn that R1 only knows about the directly connected 10.10.1.0/24 network but

it doesnt have any information about the far-away 10.10.2.0/24 network and a ping to 10.10.2.1 cannot be

successful (but notice that we can ping to that directly connected network, 10.10.1.2, for example).

So we can conclude that if a router receives the same route with what it is advertising then it will not learn that

route. In the above example, the collision occurs because both of the routers summarize into network

10.0.0.0/8 and advertise it to other router. The neighboring router realizes that it is also advertising this

network so it drops this network information.

Now if we use the no auto-summary command on both routers then the problem will surely be solved but


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first lets try to use that command only on R1 router.


R1(config-router)#no auto-summary

R1#show ip route

-> Nothing changes!

R2#show ip route

-> R2 has just learned about the new 10.10.1.0/24 network which is advertised from R1 so R2 can ping

this network

In conclusion when we enable no auto-summary on R1 then R1 will advertise its network with their subnet

mask so R2 can learn them correctly.

To grasp how to configure EIGRP, please read my article Configuring EIGRP with GNS3

Pages: 12 3

Comments (138) Comments

Comment pages

Previous12 3 380

1. 9tut

May 29th, 2012

@Rose: The AD is one hop less than the FD so it has only 1 outgoing interface (Fa0/1 of Router1)

and the delay of this interface is 10.

2. Anonymous

June 14th, 2012

Very amusing and educational. Good job.Thanks,

Hansen Kannie CCNA, CCDA, CCNP, CCDP, CCIE R&S written.


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3. Ank

June 19th, 2012

In Routing table Is it true, for outbound interface the value would be 50 or the interface name E0

(intead of 50) ?

4. nawel algeria

July 3rd, 2012

thank you for the tuto its cool

5. Anonymous

July 9th, 2012

What a great Summary of EIGRP.

6. Jason

July 11th, 2012

This page should be updated to note that if K5 = 0, the formula reduces to Metric = [k1 * bandwidth

+ (k2 * bandwidth)/(256 - load) + k3 * delay].

Otherwise, the metric formula solves to zero when K5 is zero.

7. Shemie

July 16th, 2012

(CD-ROM) This product is ok. I got a while back for my CCNA with the CCNA kit. It has a dcenet

supporting lineup of router and switches. It seriously lacks commands especially in routers. just a few I

noticed when creating my virtual network: any logging command and any exec command (which means

you always have a logout timer) these are just a few that I can think of off the top of my head. There

are many more but i can only remember the conf t ones right now and I have since uninstalled. I

expected more from Sybex. Their CCNA study guide (Lammle) is amazing. All-in-all not good. To get

my CCNA I used CCNA study guide (Lammle),CCNA IOS Commands Survival Guide (Lammle),

CCNA Fast Pass (Lammle) for the book/lab exercises. I used Cisco Packet Tracer 5.3 for my virtual

lab.Also, the Study guide would have been enough to pass the exam, but I wanted the extra command

info and labs in the Fast Pass and Survival Guide.Packet Tracer is available through Cisco Academy

and a few other places. I dont really know all the ways to get it unless you are DOD. If your DODyou can get it for free.

8. Geet

July 18th, 2012

Thanks for the comment. To know that 1 pesron has benefited from the information that I post is good

enough for me. I completely agree with your statement that keeping a blog on going is a challenge. The

biggest challenge is having content to post that is beneficial.When it comes to linux finding something

that your looking for is sometimes not very easy, and for me, its never easy. It usually takes me a fair

amount of time to locate what I need for whatever im working on. I always find myself saying why cant

someone just post the things in a easy step by step format in one place instead of having to search all

over for the answer. So thats what I attempt to do.Thanks again for the comment. Its awesome. Please

keep coming back and feel free to post your things you have found or run into


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9. Please Explain

July 19th, 2012

EIGRP Load Balancing

@9tut + Load balancing: EIGRP supports UNEQUAL metric load balancing, which allows

administrators to better distribute traffic flow in their networks.

@Todd Lammle Book Load Balance up to four EQUAL cost link

Please confirm. thanks

10. learner

July 22nd, 2012

@please Explain, EIGRP can load balance upto 4 equal cost path links by default, you can use

variance command, set it to one, it will load balance equal cost paths, but it can provide load balancing

upto 16 paths(variable cost path links)

11. learner

July 22nd, 2012

@9tut and @Xallax Hii, i found these tutorials extremely good, i would like to know what you guys

are doing these days and in terms of certifications by Cisco , really i would love to help u guys in these

tutorials someday .. I appreciate your efforts !!

12. learner

July 22nd, 2012

@9tut.. Superb explanation of AD , FD n successor route , even Todd Lammle didnt explained it in

such a nice way..!!

13. thuti_2000

July 25th, 2012

This was the best description of AD FD that I ever read.well done and thank you.

14. Lage

August 3rd, 2012

Hi guys,

Thank you for the wonderful work you people are doing for everyone who is trying to learn all of this

stuff. Thanks @9tut youre great.

By the way can you please tell me where i can get the Packetracer you are using please let me know.

You may contact me at [email protected].

Thank you and bless yall.

15. Lage


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August 3rd, 2012

Sorry actually my email is [email protected]

16. Lage

August 3rd, 2012

@9tut my email is [email protected] please pass on the Packetracer tool.

17. Anonymous

August 4th, 2012

This is good tutorial.

For those want to understand further the strengths of EIGRP for your network, you may want to read

these articles:

http://www.netxplane.com/2012/07/the-strength-of-eigrp-part-i.html (EIGRP Feasible Successor)

http://www.netxplane.com/2012/07/the-strength-of-eigrp-part-ii.html (EIGRP Unequal Load Sharing)

18. amila

August 9th, 2012

loop-free path = valid path ??????????? help meeee

19. nithalia

August 10th, 2012

good job. . ..

20. Lucas

August 22nd, 2012

I love this, its so great ! and easy to understand

21. Good job

August 31st, 2012

This tutorial was written very good I didnt expect it to be on this site, better than some books.

22. RJ

September 4th, 2012

When EIGRP comes up for the first time.(the router will try to establish a neighboring relationships by

sending Hello packets to others running EIGRP)-its fine butit exchanges its TOPOLOGY table

with its neighbors and not ROUTING tablein this above diagram R2 will also send its TOPOLOGY

table to R1 by Update packets. Remember that R2 will send its complete TOPOLOGY table for the

first time and not ROUTING table.R1 confirms it has received the Update packet by an ACKmessage.R1 will also send to R2 all of its TOPOLOGY table for the first time.R2 sends a message

saying it has received R1s TOPOLOGY table.


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23. PLEASE CORRECT THIS POINT 9TUT

September 4th, 2012

When EIGRP comes up for the first time.(the router will try to establish a neighboring relationships by

sending Hello packets to others running EIGRP)-its fine butit exchanges its TOPOLOGY table

with its neighbors and not ROUTING tablein this above diagram R2 will also send its TOPOLOGY

table to R1 by Update packets. Remember that R2 will send its complete TOPOLOGY table for the

first time and not ROUTING table.R1 confirms it has received the Update packet by an ACKmessage.R1 will also send to R2 all of its TOPOLOGY table for the first time.R2 sends a message

saying it has received R1s TOPOLOGY table.

24. Magneta

September 9th, 2012

This is very easy tutorial.

25. Asok Mohan

September 14th, 2012

hats off 9tut worth 9$

26. Ramesh


this tutorial is very good, i understand so much things. So please post this type of notes & improve our

knowledge.

Regards & ThanksRamesh

27. Raj


Shemie,

can yo uplseaes let meknow when can i download Packet Tracer 5.3.. as i am just able to find only the

broken links and that disappoints me :(

28. sotir1984


One question? Do i have a brain freeze or if K5 is 0 then all is zero?

29. Anonymous


The literature is amazing , some times we find things to be difficult because we do not have right people

to expalin it !!!, wel done

Gaid

30. AnkushK


9tut.com//3 12/16


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@anyone

how to change the AS number of eigrp without using no command?

plz help me my xm is on 29th..tnx in advance

31. kushal aroraSeptember 29th, 2012

widout no command we cant chnge it ..we hav to put

no router eigrp AS

thn agn new as no as above

32. C3C

October 11th, 2012

Since I cant afford to enroll into any online or onsite classes I have had to use whatever help I can find

on the Internet. The Sybex test engine is very limited so I turned to GNS3. Since it is a Cisco cert I

require i have found that using the Cisco learning forum a big help. I just now found 9tut and it also

helps. I feel that it is possible to pass any Cisco test without buying into any online or onsite training.

Again thanks to 9tut for your help.

33. S Shrestha

October 13th, 2012

Much gratitude to 9tut.com. Your work is like a Gem for those who wants to learn about Ciscosystem and face the CCNA exam. Although, I have once gone through Todd Lamelles book, the

concise presentation of your materials helps to quickly review the concepts and retain in the brain. I

have already taken the membership to support your effort. Keep going 9tut.com, you are the hope for

all of us!!!Cheers!!

34. tinasoft

November 2nd, 2012

thank so much.

35. avish malrotra

November 9th, 2012

plz i want a full summary of eigrp

36. TelcoGuy

December 2nd, 2012

When calculating the metric you say 1024 is the speed of T1, are you talking about a T1 carrier? T1s

are 1.544Mbps

37. Shri

December 4th, 2012


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Didnt get the use of no auto summary command plz explain

38. Pijush

January 2nd, 2013

@9tut

You guys are Rockstar! Even Todd lamle have not explained this way, about AD and FD. I have used

Reachard Deals(easy and vast) book to understand AD and FD. Its is truly great

@ Shir

auto summary is the term used to refer summarized boundary of a class full network. so 10.0.5.0

would be treated as 10.0.0.0, where subnet bit of an address is situated to that class(known as default

mask(means no subnet)). whether it is under class A,B or C.

192.168.1.1 = 192.168.1.0 (for class c)

172.16.25.0 = 172.16.0.0 (for class b)

21.25.35.74 = 21.0.0.0 (for class a)

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