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New Member

EIGRP: Feasible Distance Vs Reported Distance

According to DocID # 16406 

-Feasible distance is the best metric along the Path ( [10,000,000 /BW]+Delay) ]x256

on this particular example Figure 3  Cost for this path is 46277376 through router 4 and 20307200 through Router 3.

And the  Reported Distance for both is 307200.

-Reported Distance is the metric/value between the destination and the first hop next to the source. (Total metric along the path to a destination network as advertised by an upstream neighbor).

In this particular example the Reported Distance is 307200 ( [10,000,000/10,000]+200x256 ).

If this this true,  then the best metric along the path will be : 46533485 instead of 46277376

10,000000/56+2000x256=  46226285 ( the metric between the source and the upstream neighbor)  plus the reported distance 307200  the total metric will be 46533485   instead of  46277376.

It seems like when calculating the best metric from the Source to the destination does not take in consideration the BW of the Reported distance

Thanks.

noto65@excite.com

8 REPLIES
Cisco Employee

EIGRP: Feasible Distance Vs Reported Distance

Hello Noe,

Feasible distance is the best metric along the Path

Not entirely. The Feasible Distance, or FD in short, is the historical minimum of the best metric towards that particular destination (with the history starting from scratch when the route neds to enter the Active state and then returns back to Passive state). This means that the FD may be equal to or lower than the current total best metric towards the destination, including the link to the next hop (or successor in EIGRP parlance).

Also, there is only a single FD for a given destination. Some authors sometime make gross mistakes when they talk about "FD through neighbor A, and FD through neighbor B". What they really have in mind is simply the total distance via the neighbor A or neighbor B, including the cost of the link to these neighbors. However, the FD is not associated with a particular neighbor, only with a particular route.

Also, FD is never sent out or advertised to anyone. It is always kept inside a router and used for internal EIGRP needs (in particular, the feasible condition check) but it it never being shared with any other router.

This is one of the most commonly appearing misconceptions about EIGRP and seems to persist for some reason.

Reported Distance is the metric/value between the destination and the first hop next to the source

Yes, or in other words, the Reported Distance, or RD, as advertised by a particular neighbor, is its current total distance to a particular destination. It is simply the neighbor's own distance to the destination. Our total distance via a particular neighbor is that neighbor's RD plus the metric of the link between us and that neighbor.

10,000000/56+2000x256 = 46226285 ( the metric between the  source and the upstream neighbor)  plus the reported distance 307200   the total metric will be 46533485   instead of  46277376.

This is not correct. You have to remember that EIGRP uses a composite metric that, as a whole, behaves in a non-linear and non-additive way - in particular, the bandwidth is not added to but rather the bandwidth is taken as the minimum of the reported bandwidth and the interface bandwidth. Only the delay is additive.

What EIGRP really does is always advertise the individual metric components (bandwidth, delay, reliability, load) but never the resulting metric itself. Advertising the resulting metric would make it impossible to select the minimal bandwidth and increase the delay. Instead, you always have to look at the metric components and compute the resulting metric yourself (that's why it is absolutely necessary to have the K-values on all EIGRP routers within your AS identical).

So for the route via router 3, the total metric would be:

Bandwidth: min(10000, 128) = 128

Delay: 1000+200 = 1200

Total metric via router 3: (10000000/128 + 1200) * 256 = 20307200

For the route via router 4, the total metric would be:

Bandwidth: min(10000, 56) = 56

Delay: 2000 + 200 = 2200

Total metric via router 4: (10000000/56 + 2200) * 256 = 46277376

These values align with the Cisco document you referenced perfectly.

If the metric in EIGRP was a simple constant assigned to an interface, then your original idea of taking the RD from the neighbor and increasing it by the metric of the link to that neighbor would work perfectly. But EIGRP in particular uses a metric that cannot be simply added to, mostly because of the min() function used for bandwidth. Instead, your perform the necessary operations on the metric components first, and only then you compute the resulting metric.

Does this make sense? Please feel welcome to ask further!

Best regards,

Peter

EIGRP: Feasible Distance Vs Reported Distance

Do you have the response to this question saved in a txt document on your desktop Peter, so you can just copy and paste?

Cisco Employee

EIGRP: Feasible Distance Vs Reported Distance

Hi John,

I apologize - I do not quite get the point. Can you elaborate please? Thanks!

Best regards,

Peter

EIGRP: Feasible Distance Vs Reported Distance

It was a joke. I see you put the same response to this question at least once every 3 weeks it seems. Nothing wrong with the question

Cisco Employee

EIGRP: Feasible Distance Vs Reported Distance

John,

Ah, now I get it. No, I always write it from scratch... Yeah, you see how the superstitions about EIGRP persist? I guess I have to write about them much more frequently

Best regards,

Peter

New Member

Re: EIGRP: Feasible Distance Vs Reported Distance

Hi Peter,

Thank you for your quick response.

One last question, and based your statement: EIGRP uses a composite metric that, as a whole, behaves in a non-linear and non-additive way - in particular, the bandwidth is not added to but rather the bandwidth is taken as the minimum of the reported bandwidth and the interface bandwidth. Only the delay is additive.

Wll I get the same FD values using R2 as source and R1 as destination?

R4 Reported distance: 46226286 ( I hope I got it right ) 10000000/54 +2000 x256

R3 Reported Distance: 20256000 ( I hope I got it right ) 10000000/128+1000 x256

( From R1 to R2 )

So for the route via router 3, the total metric would be: (Feasible Distance)

Bandwidth: min(10000, 128) = 128

Delay: 1000+200 = 1200

Total metric via router 3: (10000000/128 + 1200) * 256 = 20307200

For the route via router 4, the total metric would be:

Bandwidth: min(10000, 56) = 56

Delay: 2000 + 200 = 2200

Total metric via router 4: (10000000/56 + 2200) * 256 = 46277376

Cisco Employee

Re: EIGRP: Feasible Distance Vs Reported Distance

Hello Noe,

Wll I get the same FD values  using R2  as source and R1 as destination?

Let's not call the total distances as FD, okay? The FD is a concept unrelated to this discussion, and as I suggested earlier, there is a difference between the current distance and the FD.

If you want to consider the metric from R2 to R1, it is something that EIGRP would actually never compute because it is a distance from a router to another router, while in reality, EIGRP computes shortest paths towards networks, not towards routers. So the path from R2 to R1 should correctly be computed from R2 to some network directly attached to R1. However, we can work this problem around by claiming that we are computing the distance to a fictive network on R1 whose bandwidth is higher than any other bandwidth in the network (hence it won't influence the BW metric) and whose delay is 0 (hence it won't influence the DLY metric).

So, R4 would report the following metric components:

Bandwidth: 56

Delay: 2000

And R3 would report the following metric components:

Bandwidth: 128

Delay: 1000

R2's distance to R1 via R4 would be:

Bandwidth: min(10000,56) = 56

Delay: 2000+100 = 2199

Metric: (10000000/56 + 2100) * 256 = 46251776

R2's distance to R1 via R3 would be:

Bandwidth: min(10000,128) = 128

Delay: 1000+100 = 1100

Metric: (10000000/128 + 1100) * 256 = 20281600

Best regards,

Peter

New Member

Re: EIGRP: Feasible Distance Vs Reported Distance

Peter

It seems like going from R1 to NetworkA and from NetworkA to NetworkX sitting behind R1 the FD will be the same and that clarifies my original question.

And that is based on example of the previous question/example. (point of origin of the mathematical equation is the same)

Muchas Gracias!!

Noe

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