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EIGRP Feasible successor

Posted on 2011-03-16
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I am having hard time why/how is the feasible successor in EIGRP routing protocol has lesser disatnce than current successor... should it not be greater distance than cuurent successor???
Pl explain.
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Question by:totaram
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8 Comments
 
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Expert Comment

by:602650528
ID: 35155254
Hi totaram,

a feasible successor would NOT have a lesser distance than a successor. A feasible successor is able to take over from a successor as long as it is able to meet the feasibility condition.

Please have a look at this page.

http://en.wikipedia.org/wiki/Enhanced_Interior_Gateway_Routing_Protocol#Successor
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Author Comment

by:totaram
ID: 35157542
Please see the following cisco pg, and read right above fig-3 and explaination of it paragraph below
the the two bullets

http://www.cisco.com/en/US/tech/tk365/technologies_white_paper09186a0080094cb7.shtml#feasibleandreported
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by:602650528
ID: 35158452
hi totaram, what you are mixing up is the terminology. First of all the feasible distance has no direct relationship to the feasible successor. The fact that feasible appear in both names does not mean they are directly related.
The feasible distance is the distance from Router one to network A. If the route is through, router three, the feasible distance is 46277376. That is distance from Router One to Network A. Also feasible distance from Router One to network A, through Router three, is 20307200. Because the distance through Router three is less, Router three becomes the successor. Note that Router three is successor because the distance of the route  from Router three to network A so it is because Router three is there.

Now for any router on any route between Router One and Network A to become a feasile successor, it's advertised distance (called reported distance) to network A must be less than the feasible distance. I guess that is where the confusion came in. This rule is there to avoid routing lop. If e.g Router 4 is so far from Router A , then it is most likely that it's route would pass through Router One hence a loop would be formed. In reality it might be the case but that rule is there to stop loops.


 The reported distance in that literature is actually more known as the advertised distance which is the distance
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Author Comment

by:totaram
ID: 35159858
"Now for any router on any route between Router One and Network A to become a feasile successor, it's advertised distance (called reported distance) to network A must be less than the feasible distance. I guess that is where the confusion came in. This rule is there to avoid routing lop. If e.g Router 4 is so far from Router A , then it is most likely that it's route would pass through Router One hence a loop would be formed. In reality it might be the case but that rule is there to stop loops."

Yes, thats where my confusion lies.. the routing loops will happen happen if router 3 is passing traffic, but it is not passing traffic since its  a faesible successor. Also, if advt distance (reported disatnce) is less than current best path, that router given should become the successor and the second best should be the feasible successor.  The lines that throws me off is:
A feasible successor is a path whose reported distance is less than the feasible distance (current best path).
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by:602650528
ID: 35163157
"Also, if advt distance (reported disatnce) is less than current best path, that router given should become the successor and the second best should be the feasible successor. "

No, successor is chosen based on feasible distance while feasible successor is chosen based on advertised distance. For example, even though the distance between Router Four  and network is shorter (it's same on this case really), what matters to Router One is the overall distance from Router One to network A and the over all distance through Router Three is shorter hence it is the successor and the reason being that the distance between Router One and Three is shorter that between Router One and Four.


 The lines that throws me off is:
A feasible successor is a path whose reported distance is less than the feasible distance (current best path). "

Once again you need to distinguish what parameters is used to determine feasible successor and successor. They are two different parameters you can't place side by side. Successor is chosen using over all distance between Router One and Network A while feasible successor is chosen using advertised distance and that is distance between Router Three/Four to Network A.
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Author Comment

by:totaram
ID: 35167139
Very good... Understood your point Thanks a lot..
I just have a minor question, say if there was a III path to network A, with router five and The route through Router Five has a cost of 4000000 and a reported distance of 307200. Which router will be the feasible successor?? How do we break the tie? Is it lower cost??

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Accepted Solution

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602650528 earned 50 total points
ID: 35168303
A router can have as many feasible successors as long as they fulfil the feasibility condition that their advertised distance is lower than the feasible distance of the successor. Feasible Successor means they are eligible to become a successor if the current successor becomes unavailable. If the current successor becomes unavailable, then Router A will choose the route with the lowest distance (not advertised distance but full distance from Router One to NetworkA), in this case Router Five, among the feasible successors.

If you run the command .. show ip eigrp topology. It would show you the number of successors and feasible successors.

As well you can have more than one successor and that means there are more than one path with equal distance from Route One to Network A. Eigrp and other IGPs would automatically load balance over 4 equal routes so you can have up to 4 successors by default.
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Author Closing Comment

by:totaram
ID: 35177425
Thank you
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