# Introduction to EIGRP

(Heng S) #97

Hi Rene
In this topology

I don’t understand where all of this value come from it see so complicate

Do u have better way help me to understand this ?

(Rene Molenaar) #98

Hi Heng,

These values are metric values that I made up. On real routers running EIGRP, you will see very large numbers for the metrics which makes it harder to explain.

The advertised distance is the metric that a router has received from its neighbor. The feasible distance is the total distance to get to the destination. In other words, the advertised distance + the metric on your own interface.

To understand the values of the table in your screenshot, it’s best to fill in the topology tables of R1, R2, and R3 yourself first:

R3’s topology table:

R1
R2
R4

R1’s topology table:

R2
R3

R2’s topology table:

R1
R3

For example, here’s the first entry for R3:

R1
R3
R4 5 9

In the topology table of R3, you see an entry for the advertised distance of 5. That’s what R4 advertises to R3. R3 adds its own metric on the interface that connects to R4 (4) to the advertised distance. 4+5 = 9 so that becomes the feasible distance.

R3 advertises this entry to R1 and R2.

R1’s topology table looks like this:

R2
R3 9 12

R3 advertises a metric of 9 so that’s what we add in the advertised distance. It adds its own interface metric (3) to this value, so the feasible distance becomes 12.

Now see if you can enter the remaining values yourself. Keep in mind that that the route is also advertised from R3 > R1 > R2 and back to R3. It’s also advertised from R3 > R2 > R1 > R3.

hi rene i have question regarding the below example in the picture:

when R3 received 25AD from R1 and 19AD from R2 will R3 send those values back to R4?

(Lazaros Agapides) #100

R3 will share its own advertised distance to the destiation with R4 and not the advertised distance reported by R1 and R2. So R3 will share an AD of 9 to the destination with R4.

In general, in EIGRP a router will share its own AD with other routers and the the AD reported by other routers.

I hope this has been helpful!

Laz

thank you for your reply, from my understanding, router will share the reported AD from other router which means when R3 receives (AD=25 from R1) and (AD=19 from R2) so as a result --> R3 should send both AD’s to R4 and this does not make sense!

(Rene Molenaar) #102

In reality, when R3 receives an update from R1 or R2 about the destination behind R4, it won’t install them since these don’t pass the feasibility condition (AD of the feasible successor has to be lower than FD of successor).

R3 will only advertise its successor route to other neighbors. In this topology, R4 is the successor route so normally R3 would advertise this route to R4. However, because of split horizon (don’t advertise a route to a neighbor if you learned that route from the neighbor)…this route is not advertised.

It might be helpful to see all of this in action. Connect four routers like in the topology picture, then load these four configs:

R1-show-run-2018-01-17-15-58-39-clean.txt (270 Bytes)
R2-show-run-2018-01-17-15-58-43-clean.txt (270 Bytes)
R3-show-run-2018-01-17-15-58-46-clean.txt (366 Bytes)
R4-show-run-2018-01-17-15-58-50-clean.txt (261 Bytes)

If you want to see the EIGRP updates, you can start Wireshark and use this filter:

`eigrp.opcode == 1`

It will show you the route that is advertised and the metric.

Seeing this in action probably makes it easier to understand.

Rene

Thank you for the clarification , i will try to demonstrate this test in lab , when i see the eigrp exchange packets in live i will get the full picture.

(Khan Ali P) #104

Hi Rene, The topic is very interesting, Thank you.

(Prashant W) #105

Hello Rene

can you tell me link where i find below topic

LISP encapsulation principles supporting EIGRP OTP

(Rene Molenaar) #106

Hi Prashant,

It’s here:

Enjoy

Rene

(Kapil M) #107

In the introduction of EIGRP it is mention that it is CISCO propitiatory. But now it is open standard. I think you have not updated it. I am confused now about the other content also.

(rudhra moorthy d) #108

hi rene would you create video for ad and fd for second network diagram

(Rene Molenaar) #109

Hello Kapil,

You are correct, I originally wrote this lesson before Cisco made EIGRP an open standard. I just changed the text in the lesson. What other content are you confused about?

Rene

(Rene Molenaar) #110

Hi Rudhra,

Rene

(Swapnil K) #111

Hello Rene,
What is Eigrp hello and dead interval?
I am bit confused about it.
Is it different for LAN and WAN?

Thanks,
Swapnil k

(Lazaros Agapides) #112

Hello Swapnil

The hello timer or interval is the rate at which hello packets are sent from routers that are participating in EIGRP. On an Ethernet interface, the default hello timer is 5 seconds while on a serial interface, T1, ISDN, Frame relay and other such technologies, this value is 60.

Now EIGRP doesn’t have a dead interval. What I believe you mean is the hold timer. This is the amount of time that a router will consider a neighbor alive without receiving a hello packet. This is usually three times the hello interval. By default for Ethernet connections this value is 15 seconds while for other technologies as mentioned above, it is 180 seconds. If the hold time elapses without a hello packet being received, the neighbor is considered dead.

I hope this has been helpful!

Laz

(Dennis P) #113

With regards to the picture below. In the explanation for why R1 and R2 cannot be feasible successors because there would be a loop, I am confused as to why there would be a loop if either R1 or R2 were feasible successors. I understand that EIGRP has the condition that AD must be less than FD. What if R4 becomes unreachable and by default the destination network becomes unreachable. How would there be a loop if there is no network to go? In any case, if R4 is gone then the route to the destination would be removed from the routing table and no loop would take place. Is the explanation that there would be no feasible successor on R3 because there is no way to get to the destination if the destination is gone because R4 is down?

(Ronald C) #114

The topology is being viewed from the standpoint of R3. We have the benefit here of seeing the full network topology, and you can trace if R3 used R1 or R2 as a feasible successor, to reach the destination, you’ll be going in a loop back to R3 (where you started).

(Jesus C) #115

I have a question, doing the eigrp lab for CCNA in gns3vault, in the next point Configure EIGRP AS 1 on both fastethernet links connecting router Bates and oHara , I find that for all the networks learned through the bates router I have two successors except for the loopback 1.1.1.0 belonging to the bates router. which shows me that he announces two identical ADs for both interfaces but the FD of the OHARA interfaces is different only for that network (they have the same BW and delay) (these are the only ones that are enabled).
Also at the time of changing the bandwidth in f1 / 0 so that it is only the successor route through F0 / 0, I change the successor for all routes except for the bats network 1.1.1.0/24, which is still the route through the f1 / 0 interface.
I attached the output of the show ip eigrp topoloy in ohara before changing the bw and after the change of the bw:
before

``````_**R2#sh ip eigrp topology**_
_**IP-EIGRP Topology Table for AS(1)/ID(192.168.21.2)**_

_**Codes: P - Passive, A - Active, U - Update, Q - Query, R - Reply,**_
_**       r - reply Status, s - sia Status**_

_**P 1.1.1.0/24, 1 successors, FD is 156160**_
_**        via 192.168.21.1 (156160/128256), FastEthernet1/0**_
_**        via 192.168.12.1 (386560/128256), FastEthernet0/0**_
_**P 3.3.3.0/24, 2 successors, FD is 2300416**_
_**        via 192.168.12.1 (2300416/2297856), FastEthernet0/0**_
_**        via 192.168.21.1 (2300416/2297856), FastEthernet1/0**_
_**P 4.4.4.0/24, 2 successors, FD is 2300416**_
_**        via 192.168.12.1 (2300416/2297856), FastEthernet0/0**_
_**        via 192.168.21.1 (2300416/2297856), FastEthernet1/0**_
_**P 192.168.12.0/24, 1 successors, FD is 258560**_
_**        via Connected, FastEthernet0/0**_
_**P 192.168.21.0/24, 1 successors, FD is 28160**_
_**        via Connected, FastEthernet1/0**_
_**P 192.168.134.0/24, 2 successors, FD is 2172416**_
_**        via 192.168.12.1 (2172416/2169856), FastEthernet0/0**_
_**        via 192.168.21.1 (2172416/2169856), FastEthernet1/0**_
``````

After change the BW

``````_**R2#sh ip eigrp topology**_
_**IP-EIGRP Topology Table for AS(1)/ID(192.168.21.2)**_

_**Codes: P - Passive, A - Active, U - Update, Q - Query, R - Reply,**_
_**       r - reply Status, s - sia Status**_

_**P 1.1.1.0/24, 1 successors, FD is 156160**_
_**        via 192.168.12.1 (386560/128256), FastEthernet0/0**_
_**        via 192.168.21.1 (25730560/128256), FastEthernet1/0**_
_**P 3.3.3.0/24, 1 successors, FD is 2300416**_
_**        via 192.168.12.1 (2300416/2297856), FastEthernet0/0**_
_**        via 192.168.21.1 (26242560/2297856), FastEthernet1/0**_
_**P 4.4.4.0/24, 1 successors, FD is 2300416**_
_**        via 192.168.12.1 (2300416/2297856), FastEthernet0/0**_
_**        via 192.168.21.1 (26242560/2297856), FastEthernet1/0**_
_**P 192.168.12.0/24, 1 successors, FD is 258560**_
_**        via Connected, FastEthernet0/0**_
_**P 192.168.21.0/24, 1 successors, FD is 25602560**_
_**        via Connected, FastEthernet1/0**_
_**        via 192.168.12.1 (261120/28160), FastEthernet0/0**_
_**P 192.168.134.0/24, 1 successors, FD is 2172416**_
_**        via 192.168.12.1 (2172416/2169856), FastEthernet0/0**_
_**        via 192.168.21.1 (26114560/2169856), FastEthernet1/0**_
_**R2#**_
``````