Introduction to EIGRP

andrew · March 14, 2016, 1:53am

Andrew,
Hearts does advertise the 5+4 route to Diamond. In fact, Diamond would hear about the route from two different directions: via Hearts with an advertised distance of 9 and from Clubs with an advertised distance of 12. Diamond would then advertise what it learned about from Clubs on to Hearts (AD = 19), and also what it learned about from Hearts to Clubs (AD = 18).

Hearts and Clubs would throw away these two learned routes from Diamond because they have routes with a better metric, and the routes fail the Feasibility Condition.

andrew2 · March 14, 2016, 8:38am

Aaaah now I see the light !!! Thanks Andrew you have been a great help.

asmikharat · March 21, 2016, 2:22pm

Nicely explained

mj8mj4 · April 11, 2016, 8:07am

Rene I still have problem with this

((((Router Spade will advertise a distance of 5 towards router Hearts. Router Hearts will advertise 5+4 = 9 towards router Diamond.
Router Diamond will advertise 5+4+9 = 18 towards router Clubs. Finally router Clubs will advertise 5+4+9+7 = 25 towards router Hearts. Split Horizon doesn’t apply here since router Clubs learned about the destination on another interface (router Diamond).)))))

my question is ,why it didnt go with 9 or 18 …I still disagree with 25 ???

Thank you

ReneMolenaar · April 11, 2016, 1:35pm

Hi Amjad,

Router hearts will learn the destination from spade, so it’s shortest path is:

5 + 4 = 9

However it also loops around:

Spade > Hearts > Diamond > Clubs > Hearts

Which equals:

5 + 4 + 9 + 7 = 25

Hope this helps.

Rene

shannon.sariman · May 30, 2016, 8:22am

Hi Rene,

Many thanks for the lesson!

Do you have any more practice examples, just like the last example? I need more practice to solidify my understanding on the building of a topology table for examples like the last one.

Regards,

Shannon

ReneMolenaar · May 30, 2016, 10:25am

Hi Shannon,

I would recommend to boot up some routers and to look at some live topology tables, that will probably be more helpful. In my examples I used simple numbers but on real EIGRP routers, you’ll see very large metrics. That’s something you want to get used to

Rene

shannon.sariman · May 31, 2016, 1:15am

Hi Rene,

Thank you and after thinking about your response, yes, it makes sense to try out in a lab. Much appreciated for the good advice.

Regards,

Shannon

sengvvue · June 10, 2016, 3:35am

Hi Rene,

I need to know what happens when the AD is equal to FD.

Best Regards,

Seng

andrew · June 10, 2016, 2:22pm

Seng,
There are two possibilities for what you are asking: 1) you are asking about a primary (successor) route, or 2) a secondary (feasible successor) route. Here’s the short answer:

Impossible
Not used because it is considered a loop.

Here is the more detailed answer:
The scenario you describe would only be possible for a potential feasible successor route (meaning not the primary route), otherwise the cost from me to my neighbor would be zero, which is impossible.

So, I assume, you are asking whether a route that has its FD = AD would be used as a backup route (a feasible successor). The answer is NO. In order for a route to be considered a feasible successor, its AD must be less than the FD. If the AD is greater than or equal to the FD, it is considered a loop and is ignored.

sims · June 19, 2016, 12:36am

Hi,
" So why didn’t I place “9” in the advertised distance field in the table? Good question! Remember split-horizon? Don’t advertise to your neighbor whatever you learned from them .router Clubs is not sending information about this network back to router Hearts. "

as per 2. Router Hearts will advertise the network to router Clubs and Diamond.

If the hearts advertise same AD 9 to router clubs and diamond what is the issue here .

“Router Spade will advertise a distance of 5 towards router Hearts. Router Hearts will advertise 5+4 = 9 towards router Diamond.”

If the router hearts can advertise 9 to router Diamond why the Hearts cannot advertise 9 to club also at the same time .

Why club should get the route only through Diamond

Thanks

sims · June 22, 2016, 2:55am

Hi,
I did not read completely before posting the above question . I did understand .
Sorry for the inconvenience

Thanks

damienmallon · August 4, 2016, 1:02pm

Brilliant explanations. The simplified way Rene explains these topics along with the illustrations make the concepts much easier to grasp and maintain.

stuart.w.gall · October 19, 2016, 9:09pm

Hi Rene,
I was wondering what happens with the scenario you describe where the third best route is a FS and the successor route fails.
Does the router use the FS route or the new best route ?
I tried to simulate a similar setup (cant get the weights that low in reality).
Looking at R1, when R3 goes down. The routes instantly converge on the new best route and not the FS.
Maybe the FS is used for an instant, but basically EIGRP recalculates and uses the route that was not a FS.

P 192.168.100.0/24, 1 successors, FD is 768
        via 192.168.12.3 (768/512), FastEthernet0/1
        via 192.168.13.4 (26112/512), FastEthernet1/0
P 192.168.11.0/24, 1 successors, FD is 512
        via Connected, FastEthernet0/0
P 192.168.12.0/24, 1 successors, FD is 256
        via Connected, FastEthernet0/1
P 192.168.13.0/24, 1 successors, FD is 25600
        via Connected, FastEthernet1/0
        via 192.168.11.2 (8192/7680), FastEthernet0/0
        via 192.168.12.3 (5632/5376), FastEthernet0/1
P 192.168.1.0/24, 2 successors, FD is 3072
        via 192.168.11.2 (3072/2560), FastEthernet0/0
        via 192.168.12.3 (3072/2816), FastEthernet0/1
        via 192.168.13.4 (28416/2816), FastEthernet1/0
P 192.168.2.0/24, 1 successors, FD is 512
        via 192.168.12.3 (512/256), FastEthernet0/1
P 192.168.3.0/24, 1 successors, FD is 5632
          
R1#
*Mar  1 00:23:23.272: %DUAL-5-NBRCHANGE: IP-EIGRP(0) 1: Neighbor 192.168.12.3 (FastEthernet0/1) is down: holding time expired
R1#sh ip eigrp topology 
IP-EIGRP Topology Table for AS(1)/ID(192.168.13.1)

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

P 192.168.100.0/24, 1 successors, FD is 3328
        via 192.168.11.2 (3328/2816), FastEthernet0/0
        via 192.168.13.4 (26112/512), FastEthernet1/0
P 192.168.11.0/24, 1 successors, FD is 512
        via Connected, FastEthernet0/0

Any comments on the details of what is happening ?

Stuart

ReneMolenaar · October 30, 2016, 11:03am

Hi Stuart,

EIGRP will use the FS but afterwards will quickly recalculate. If you want to see this in action, try the debug ip routing command. Here is an example for this topology:

R1#show ip eigrp topology | begin 5.5.5.5
P 5.5.5.5/32, 1 successors, FD is 131072
        via 192.168.13.3 (131072/130816), GigabitEthernet0/2
        via 192.168.14.4 (156416/130816), GigabitEthernet0/3

Once the successor is unreachable, this will happen:

R1#
%DUAL-5-NBRCHANGE: EIGRP-IPv4 1: Neighbor 192.168.13.3 (GigabitEthernet0/2) is down: holding time expired
RT: delete route to 192.168.45.0 via 192.168.13.3, eigrp metric [90/3328]
RT: no routes to 192.168.45.0, delayed flush
RT: delete network route to 192.168.45.0/24
RT: updating eigrp 192.168.45.0/24 (0x0)  :
    via 192.168.14.4 Gi0/3  0 1048578

RT: add 192.168.45.0/24 via 192.168.14.4, eigrp metric [90/28416]
RT: delete route to 5.5.5.5 via 192.168.13.3, eigrp metric [90/131072]
RT: no routes to 5.5.5.5, delayed flush
RT: delete subnet route to 5.5.5.5/32
RT: updating eigrp 5.5.5.5/32 (0x0)  :
    via 192.168.14.4 Gi0/3  0 1048578

RT: add 5.5.5.5/32 via 192.168.14.4, eigrp metric [90/156416]
RT: delete route to 192.168.25.0 via 192.168.13.3, eigrp metric [90/3328]
RT: no routes to 192.168.25.0, delayed flush
RT: delete network route to 192.168.25.0/24
RT: updating eigrp 192.168.25.0/24 (0x0)  :
    via 192.168.12.2 Gi0/1  0 1048578

RT: add 192.168.25.0/24 via 192.168.12.2, eigrp metric [90/5632]
RT: updating eigrp 192.168.25.0/24 (0x0)  :
    via 192.168.14.4 Gi0/3  0 1048578

RT: rib update return code: 19
RT: delete route to 192.168.35.0 via 192.168.13.3, eigrp metric [90/3072]
RT: no routes to 192.168.35.0, delayed flush
RT: delete network route to 192.168.35.0/24
RT: updating eigrp 192.168.35.0/24 (0x0)  :
    via 192.168.12.2 Gi0/1  0 1048578

RT: add 192.168.35.0/24 via 192.168.12.2, eigrp metric [90/5888]
RT: updating eigrp 192.168.35.0/24 (0x0)  :
    via 192.168.14.4 Gi0/3  0 1048578

RT: rib update return code: 19
RT: updating eigrp 192.168.45.0/24 (0x0)  :
    via 192.168.14.4 Gi0/3  0 1048578

RT: updating eigrp 192.168.45.0/24 (0x0)  :
    via 192.168.12.2 Gi0/1  0 1048578

RT: del 192.168.45.0/24 via 192.168.14.4, eigrp metric [90/28416]
RT: add 192.168.45.0/24 via 192.168.12.2, eigrp metric [90/5888]
RT: updating eigrp 5.5.5.5/32 (0x0)  :
    via 192.168.14.4 Gi0/3  0 1048578

RT: updating eigrp 5.5.5.5/32 (0x0)  :
    via 192.168.12.2 Gi0/1  0 1048578

RT: del 5.5.5.5/32 via 192.168.14.4, eigrp metric [90/156416]
RT: add 5.5.5.5/32 via 192.168.12.2, eigrp metric [90/133632]

Above you can see that initially, it starts using R4 but then switches over to R2.

R1#show ip eigrp topology | begin 5.5.5.5
P 5.5.5.5/32, 1 successors, FD is 133632
        via 192.168.12.2 (133632/133120), GigabitEthernet0/1
        via 192.168.14.4 (156416/130816), GigabitEthernet0/3

Hope this helps!

Rene

stuart.w.gall · November 7, 2016, 9:10pm

WOW! Thanks Rene. Awesome debug command.

shantel · December 20, 2016, 10:33pm

19 posts were merged into an existing topic: Introduction to EIGRP

mrafikov1986 · November 8, 2016, 7:20am

Hi Rene,
i think in the second video i have some remark. I think R2 can not will be in topology table of R1. Because he will have the best path to the prefix 5.5.5.0/24 through R1. lets i try to explain my own opinion. I think R2 will have his own best path R2 to R1 = 10, R1 to R3 = 5, R3 to R5 = 5 and last one R5 to prefix = 10, totaly will be 10+5+5+10=30. It is better than 40 (R2 to R5 = 30, R5 to prefix = 10, totaly = 40). I think in topology table router R1 we will not have R2. Because R2 does not advertize prefix 5.5.5.0 to R1. I dont know maybe someone already told about it, but if i am mistaken, please correct me.

shantel · December 20, 2016, 10:34pm

19 posts were merged into an existing topic: Introduction to EIGRP

techhubforhighflyer · January 8, 2017, 3:13pm

How did you get 9 as the feasible distance for R4?