EIGRP...Feasible successor & successor...

I'm having difficulty understanding how Todd explains the feasible successor and it's relation to how the route is added to the routing table...

Todd explains...
Feasible successor A destination entry is moved from the topology table to the routing table when there is a feasible successor. (I thought it would only be moved to the routing table when it is a successor or if the successor fails?)

Continues...A feasible successor is a path whose reported distance is less than the feasible distance, and is considered a backup route. (Wouldn't this make it a successor? If the reported distance is less than the feasible distance?)

Continues...Only the one with the best metric (the successor) is placed in the routing table. The show ip eigrp topology command will display all the EIGRP feasible successor routes known to a router.

I think I do understand that a feasible successor is a backup route to the successor, but somehow, to me, this explanation makes it seem as though the feasible successor is a better route than the successor....

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Comments

Netstudent

Well the lowest metric route will be put in the routing table and that will be the successor. When populating a routing table, the logic will look for the best route or the lowest metric.

Then if there are more than one path to a "subnet", then the originating router will know who its neighbors are. When the neighbors report their distance to the "subnet"(reported distance), their feasible distance must be less than the originating Router's feasible distance to become a feasible successor.

remember that reported distance is the neighbor's feasible dis.

It's hard to describe without a diagram.

dtlokee

The Feasible Distance is the lowest cost to a destionation. This is calculated by using the reported distance given by a neighbor and the bw/delay of the link to that neighbor. The path with the lowest metric would be the successor, all other paths would become feasible succcessors. Another important concept is the feasibility condition, used to prevent routing loops. This basically says that the Reported Distance (from the neighbor) needs to be lower than the fesiable distance to that destination (this prevents a loop from forming due o an increasing metric)

r_durant

Netstudent wrote:

Well the lowest metric route will be put in the routing table and that will be the successor. When populating a routing table, the logic will look for the best route or the lowest metric.

Then if there are more than one path to a "subnet", then the originating router will know who its neighbors are. When the neighbors report their distance to the "subnet"(reported distance), their feasible distance must be less than the originating Router's feasible distance to become a feasible successor.

remember that reported distance is the neighbor's feasible dis.

It's hard to describe without a diagram.

Ok, so can I look at it like this...
A feasible successor is a path whose reported distance "from the neighbor router(s)" is less than the feasible distance "in the originating router", and is considered a backup route.

Netstudent

yes I believe so...

r_durant

dtlokee wrote:

The Feasible Distance is the lowest cost to a destionation. This is calculated by using the reported distance given by a neighbor and the bw/delay of the link to that neighbor. The path with the lowest metric would be the successor, all other paths would become feasible succcessors. Another important concept is the feasibility condition, used to prevent routing loops. This basically says that the Reported Distance (from the neighbor) needs to be lower than the fesiable distance to that destination (this prevents a loop from forming due o an increasing metric)

So then it becomes kinda like an election...let's say a lower cost to a destination comes in to the originating router, does that becomes the successor? And does the current successor "drop back" to become a feasible successor?

r_durant

Netstudent wrote:

yes I believe so...

Well, I didnt think he was wrong, it seems just how it was worded and how it came over...

Thanks...

Netstudent

Lokee wrong? NAAHHHH!

dtlokee

Netstudent wrote:

Lokee wrong? NAAHHHH!

I have been drinking heavily today, trying to teach my liver a lesson it won't soon forget

Netstudent

let's say a lower cost to a destination comes in to the originating router, does that becomes the successor? And does the current successor "drop back" to become a feasible successor?

I almost positive that the original successor route will stay in the routing table untill it is unreachable. I don;t think that a newer lowest metric route will replace a successor route once the route is in the table.

I have been drinking heavily today, trying to teach my liver a lesson it won't soon forget

Uh oh sounds like some stress relief. I think I may pour me a crown and coke when I get home now that you mention it. Nothing takes the edge off of a long, mentally draining day like a nice cold stiff drink.

EdTheLad

Netstudent wrote:

let's say a lower cost to a destination comes in to the originating router, does that becomes the successor? And does the current successor "drop back" to become a feasible successor?

I almost positive that the original successor route will stay in the routing table untill it is unreachable. I don;t think that a newer lowest metric route will replace a successor route once the route is in the table.

The route with the lowest cost is the successor, the old successor will be removed from the routing table and be seen as a feasible successor in the topology table.Since the new successor has a lower cost than the original successor this new cost will be updated and seen as the new FD that all other FS are calculated against.So depending on what the new FD is, the old FSs may not pass the FS rule and thus may be deleted from the topology table.If the successor was lost and one of the less attractive routes became successor the original FD cost is unchanged.
Hope you can understand what i mean as this is the trick to eigrp.

Netstudent

I stand corrected.

r_durant

Netstudent wrote:

Lokee wrong? NAAHHHH!

Nah...I didn't mean Lokee, I meant Todd!!

Lokee is the Godfather...

r_durant

EdTheLad wrote:

Netstudent wrote:

let's say a lower cost to a destination comes in to the originating router, does that becomes the successor? And does the current successor "drop back" to become a feasible successor?

I almost positive that the original successor route will stay in the routing table untill it is unreachable. I don;t think that a newer lowest metric route will replace a successor route once the route is in the table.

The route with the lowest cost is the successor, the old successor will be removed from the routing table and be seen as a feasible successor in the topology table.Since the new successor has a lower cost than the original successor this new cost will be updated and seen as the new FD that all other FS are calculated against.So depending on what the new FD is, the old FSs may not pass the FS rule and thus may be deleted from the topology table.If the successor was lost and one of the less attractive routes became successor the original FD cost is unchanged.
Hope you can understand what i mean as this is the trick to eigrp.

Ah ok, i gotcha...at least I was thinking along the correct line...