EIGRP Successor/Feasible Successor

I am stuck on some of the feasible successor/Successor stuff in EIGRP. I understand that the Advertised Distance (AD) is ALWAYS < than the Feasble Distance (FD).

I understand that the FD is the overall distance from the originating router to the destination network.
- By this I mean the neighbors AD + the cost from the source to the neighbor

So when a network first converges, it finds at least 1 successor route to a network but my question is how it finds the feasible successor once it finds the successor. From my reading I get the feasiblity requirement states that "if my neighbor's advertised distance is less than my feasible distance, the path will be loop free."

Isn't the AD ALWAYS < than the FD ?? obviously something is amiss here because the AD is used to calculate the FD and is in fact part of the FD.

Also another question regarding this.. is it possible for a feasible succesor to have a lower metric than the successor route?

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Comments

nicklauscombs

do you have the official exam guide? theres a good example of this on page 67

Plazma

I have the guide and have been reading it but i still don't quite understand it.. i keep confusing myself and going in circles

the AD is always < than the FD

the only way to become a feasible successor is to have an AD < the Feasible distance

lildeezul

ok hopefully i can clear it up..

it finds the FS by looking at its topology table.. if the router has two or more routes to the same subnet (subnet x), it will find its Successor route (lowest FD).. after the router has found its successor route, it looks at the other routes found in the topology table to subnet x. if any other routes, (non successor route), has a lower AD than the successor route's FD then that route is a Feasible successor..

read over my examply more than once.. hopefully u will get it

Plazma

so the successor route is picked purely by the metric then correct? And once the successor is picked, it looks in the top. table for routes with a FD < the current successor.. and if there is 1 or more, it labels them as such ?

scheistermeister

Plazma wrote:

so the successor route is picked purely by the metric then correct? And once the successor is picked, it looks in the top. table for routes with a FD < the current successor.. and if there is 1 or more, it labels them as such ?

It would mark it as the feasible successor, correct.

Plazma

that clears that up thanks!

For some reason it wasn't clear to me in the exam cert guide or i over-looked it

APA

Plazma wrote:

so the successor route is picked purely by the metric then correct? And once the successor is picked, it looks in the top. table for routes with a FD < the current successor.. and if there is 1 or more, it labels them as such ?

Remember Administrative distance also plays a part in route selection when more than one route to same destination. Internal EIGRP = 90 External EIGRP = 170

Plazma

Good call!

So i should just say that the successor is found by the complex EIGRP metric along with the Administrative Distance set?

scheistermeister

Plazma wrote:

Good call!

So i should just say that the successor is found by the complex EIGRP metric along with the Administrative Distance set?

The AD is the "believability" of the route. Meaning how must the router trusts the route. So if you are running EIGRP and OSPF the router would pick the EIGRP route (all other things being equal) before the OSPF route because it trusts the routes from EIGRP more.

The metric for EIGRP isn't all the complex. By default it is just Delay and Bandwidth which are both statically set but are changeable.