question about EIGRP's feasible successor

shahaij · ‎06-22-2005

Hi,

I read article "Who's Afraid of DUAL-3-SIA?" on http://www.cisco.com/en/US/partner/about/ac123/ac114/ac173/Q2-05/tech_routing.html

there is a sample to explain a new IOS feature.

But I have a question about that sample,

why Router C can't be Router A's feasible successor?

IF C is A's feasible successor, A can declare C as the alternate route as soon as receive B's query.

Am I misunderstanding the concept of feasible successor? can anybody give me some hint?

thank you all!

Regards,

sha

ankurbhasin · ‎06-23-2005

HI Sha,

There is no mention of any values for bandwidth and load in the topology in that link.

What I see that the link between router A to Router B is gigabit which means the bandwidth will be gbps. Now the link between Router A to Router C is only T1 which will be 1.54 MBPS. Much difference.

Now when Eigrp will calculate the metric the best metric will be from router C and that will become the best path. Now it will also check the reported distance from router C. If the reported distance from Router C will be more then the best metric which is been calculated from Router B it will never become feasible succ.

In short to become the feasilbe succ its reported dist should be less then the Feasible distance.

This link will give a better pic.

http://www.cisco.com/warp/customer/103/eigrp-toc.html#eigrpmetrics

HTH

Ankur

shahaij · ‎06-23-2005

Ankur,

Thank you for your advice.

Here is the topology:

routerA

/ |

B-----C------D---|

|10.1.1.0/24

I don't think bandwidth is a problem.

Per RouterA's view, it's best metric(via B) would definitely great than it's reported distance(reported by C).this is beacuse B need to reach 10.1.1.0/24 via C. So C would be A's feasible successor.

A feasible successor is a path whose reported distance is less than the feasible distance (current best path).

Thanks.

Regards,

Sha

ankurbhasin · ‎06-26-2005

Hi Shahaji,

I is not necessary again as the bandwidth between Router C to Router D is not mentioned.

Suppose the bandwidth between C and D is again the same as gigabit ethernet then the reported distance from path C will be same as the best distance from Router B again it depends upon the load which I am taking as 0 right now.

So it all depends upon bandwidth and load.

Regards,

Ankur

shahaij · ‎06-26-2005

Ankur,

Thanks for your reply.

My question is "why Router C can't be Router A's feasible successor? "

So I don't care the bandwidth between C and D.

And I raised the same question to the author of this article -- Russ White. And here is his answer, FYI.

"

C _could_ be A's FS, but we presume it isn't for this article, to illustrate the active process.

"

Thank you anyway.

Sha

ankurbhasin · ‎06-26-2005

Hi Shahaij,

It is not like C cannot be a successor ...it can very well but it depends upen the interface config and load.

I never said it can never be ofcourse it can be but the way it again depends upon the metric which it will calculate using the formula and putting the bandwidth and load. Might be in this case they are not taking or considering C for this article.

Regards,

Ankur