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Beginner

How EIGRP Works with Feasible succesor

I would like to better understand the role of Feasible Successor in EIGRP. I could not find documentation about it.

 

Thank you for your help

4 REPLIES 4
Highlighted
VIP Collaborator

Hello,

The variance command will not affect routes that do not meet the feasability condition: Lets say you have 4 equal metric routes for a destination using EIGRP, and the destination being 10.0.0.1. This is without using variance at all; all 4 routes just happen to all be equal. They ALL meet the feasibility condition because their reported distance (in bold) is less than the feasible distance 323072.



All these routes are in the routing table by default, load balancing as they are equal metric routes. Are there any routes that don't meet the feasability condition? Well let's see here. Lets issue the show ip eigrp topology all-links command and see



P 10.0.0.1/24, 4 successors, FD is 323072, tag is 13979

via 192.168.41.93 (323072/322816), GigabitEthernet1/13

via 192.168.41.97 (323072/322816), GigabitEthernet1/14

via 192.168.41.221 (323072/322816), GigabitEthernet1/15

via 192.168.41.225 (323072/322816), GigabitEthernet1/16



show ip eigrp topology all-links

#Lines omitted

P 10.0.0.1, 4 successors, FD is 323072, tag is 13979, serno 345876
via 192.168.41.93 (323072/322816), GigabitEthernet1/13
via 192.168.41.97 (323072/322816), GigabitEthernet1/14
via 192.168.41.221 (323072/322816), GigabitEthernet1/15
via 192.168.41.225 (323072/322816), GigabitEthernet1/16
via 192 168.38.130 (323328/323072), Port-channel17



Look, the above route in red does not meet the feasability condition as its reported distance is NOT LESS than the feasable distance. In this case if you wanted to add this route to the routing table as an equal metric route with the variance command, you would first need to modify the metric so that it meets the feasibility condition; either by changing bandwidth or delay, or by using an offset list.


Here, you can check others discussions about:
https://community.cisco.com/t5/networking-documents/eigrp-feasible-successor/ta-p/3156640
https://learningnetwork.cisco.com/thread/35690

Regards

Jaderson Pessoa
*** Rate All Helpful Responses ***
Highlighted
VIP Collaborator

Hello,

The variance command will not affect routes that do not meet the feasability condition: Lets say you have 4 equal metric routes for a destination using EIGRP, and the destination being 10.0.0.1. This is without using variance at all; all 4 routes just happen to all be equal. They ALL meet the feasibility condition because their reported distance (in bold) is less than the feasible distance 323072.



All these routes are in the routing table by default, load balancing as they are equal metric routes. Are there any routes that don't meet the feasability condition? Well let's see here. Lets issue the show ip eigrp topology all-links command and see

P 10.0.0.1/24, 4 successors, FD is 323072, tag is 13979

via 192.168.41.93 (323072/322816), GigabitEthernet1/13

via 192.168.41.97 (323072/322816), GigabitEthernet1/14

via 192.168.41.221 (323072/322816), GigabitEthernet1/15

via 192.168.41.225 (323072/322816), GigabitEthernet1/16

show ip eigrp topology all-links

#Lines omitted

P 10.0.0.1, 4 successors, FD is 323072, tag is 13979, serno 345876
via 192.168.41.93 (323072/322816), GigabitEthernet1/13
via 192.168.41.97 (323072/322816), GigabitEthernet1/14
via 192.168.41.221 (323072/322816), GigabitEthernet1/15
via 192.168.41.225 (323072/322816), GigabitEthernet1/16
via 192 168.38.130 (323328/323072), Port-channel17



Look, the above route in red does not meet the feasability condition as its reported distance is NOT LESS than the feasable distance. In this case if you wanted to add this route to the routing table as an equal metric route with the variance command, you would first need to modify the metric so that it meets the feasibility condition; either by changing bandwidth or delay, or by using an offset list.


Here, you can check others discussions about:
https://community.cisco.com/t5/networking-documents/eigrp-feasible-successor/ta-p/3156640
https://learningnetwork.cisco.com/thread/35690

Regards

Jaderson Pessoa
*** Rate All Helpful Responses ***
Highlighted
VIP Collaborator

Hello,

The variance command will not affect routes that do not meet the feasability condition: Lets say you have 4 equal metric routes for a destination using EIGRP, and the destination being 10.0.0.1. This is without using variance at all; all 4 routes just happen to all be equal. They ALL meet the feasibility condition because their reported distance (in bold) is less than the feasible distance 323072.


All these routes are in the routing table by default, load balancing as they are equal metric routes. Are there any routes that don't meet the feasability condition? Well let's see here. Lets issue the show ip eigrp topology all-links command and see


P 10.0.0.1/24, 4 successors, FD is 323072, tag is 13979

via 192.168.41.93 (323072/322816), GigabitEthernet1/13

via 192.168.41.97 (323072/322816), GigabitEthernet1/14

via 192.168.41.221 (323072/322816), GigabitEthernet1/15

via 192.168.41.225 (323072/322816), GigabitEthernet1/16



show ip eigrp topology all-links

#Lines omitted

P 10.0.0.1, 4 successors, FD is 323072, tag is 13979, serno 345876
via 192.168.41.93 (323072/322816), GigabitEthernet1/13
via 192.168.41.97 (323072/322816), GigabitEthernet1/14
via 192.168.41.221 (323072/322816), GigabitEthernet1/15
via 192.168.41.225 (323072/322816), GigabitEthernet1/16
via 192 168.38.130 (323328/323072), Port-channel17



Look, the above route in red does not meet the feasability condition as its reported distance is NOT LESS than the feasable distance. In this case if you wanted to add this route to the routing table as an equal metric route with the variance command, you would first need to modify the metric so that it meets the feasibility condition; either by changing bandwidth or delay, or by using an offset list.

Here, you can check others discussions about:
https://community.cisco.com/t5/networking-documents/eigrp-feasible-successor/ta-p/3156640

 

https://learningnetwork.cisco.com/thread/35690

 

Regards


Regards

Jaderson Pessoa
*** Rate All Helpful Responses ***
Highlighted
VIP Advisor

Hi @John Ventura ,

 

Check this:

Successor and feasible successor

Another two terms that appear often in the EIGRP world are successor and feasible successor. A successor is the route with the best metric to reach a destination. That route is stored in the routing table. A feasible successor is a backup path to reach that same destination that can be used immediately if the successor route fails. These backup routes are stored in the topology table.

For a route to be chosen as a feasible successor, one condition must be met:

  • the neighbor’s advertised distance (AD) for the route must be less than the successor’s feasible distance (FD).

 

The following example explains the concept of a successor and a feasible successor.

successor & feasible successor

R1 has two paths to reach the subnet 10.0.0.0/24. The path through R2 has the best metric (20) and it is stored in the R1’s routing table. The other route, through R3, is a feasible successor route, because the feasiblility condition has been met (R3’s advertised distance of 15 is less than R1’s feasible distance of 20). R1 stores that route in the topology table. This route can be immediately used if the primary route fails.

 

I leave the link with the full explanation:

https://study-ccna.com/eigrp-overview/

 

Also, I commented that there is an interesting event related to EIGRP, in case you have more doubts about this protocol:

https://community.cisco.com/t5/routing/ask-the-expert-eigrp-unveiled/td-p/3811434

 

Remember to mark the correct answers as solved, as this helps other users with similar doubts.

Regards