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Eigrp Metric Calculation

Team,

Can any one tell me EIGRP will take lower Metric or Higher Metric for best path and why?

Regrads

Prashant

4 REPLIES
Silver

Re: Eigrp Metric Calculation

Hi Prashant,

It is very simple - lower metric is better.

EIGRP uses composite metric which is calculated from more parameters: bandwidth, load, delay, reliability.

Don't forgot that bandwith is not cummulative, it is the minimum (slowest) value along the path.

Regarding the delay value, it is cummulative, so it contains the sum of all delays along the path.

There is an equation for the calculation.

There can be situations in which the router will choose the route with worse metric. This arise when you have more options, but only some of the routes do pass the feasibility condition. For more information, please refer to this topic: https://supportforums.cisco.com/message/3994397#3994397 There is a lot of explained, regarding EIGRP.

If you have any questions or want to be more specific, please, don't mind to ask!

Best regards,

Jan

New Member

Eigrp Metric Calculation

Hi Jan,

Thanks for your response and sorry for late reply...

I have properly study the document and still I am trying to digest the things and now there are so many questions are coming in my mind.

Before asking my doubt, kindly clearly me few things

1. What is metric in EIGRP and how we can calculate ?

2. What is the parameter of metric like example msec or micro sec ?

3. If you put the command sh ip eigrp topology.... It will show the best routes path with Successor and FS with the attached with FD and AD value.

Is the FD is the metric values or something else in EIGRP ?

Here we can see if the FD is value less then it will show the Successor path

if FD value is higher then it will show the back path or FS

So pls clear me what is the all of thing and what should I understand if I see the value as a FD and AD.

Now coming my question is Metric value which I asked and you replied me that low metric value is good one.

Can you prove me as a example wise......

Now I have few question on this.....

In practically I don't think is like that

I have seen many times If I increased the bandwidth value then EIGRP will select that path.

Means Higher Bandwidth value is a Primary path and Lower Bandwidth is secondary path.

So As per the formula

Metric= 256*( Bandwidth +Sum of all Delay)

Where Bandwidth = 10^7/ Bandwidth

And Delay in micro sec

So if I increased the Bandwidth it will increased the Metric value.

Then how can you say it will not depend on Bandwidth

Pls clear me if I am wrong......

Let me give one example to understand u....

Let assume I have 3 route which connected one like that way..

R1 is connected to R2 with 100 Mbps link

R1 is connected to R2 with 10 Mbps link

R2 is connected to R3 with 10 Mbps link.

Means R1 will have two link one is 100 Mbps and another is 10 mbps which are connected to R2 and R2 is connected R3 with 10 Mbps link

The lan network of R1 needs to be reach lan of R3.

If you generally configure EIGRP here. I have seen the practically it will choose 100 Mbps link path as a Successor and 10 Mbps link path is FS.

Once the 100 Mbps link goes down then it will take 10 Mbps link as primary.

So can you tell me how it will happen?

Now how can you tell me the lower metric is good way.....

And tell me here what is Delay role here.....

You clear me as a example wise because now I have fully confused about this....

Hope you are clear my question........

Regards

Prashant

Silver

Re: Eigrp Metric Calculation

Hi Prashant,

that is absolutely ok, take your time. The topic regarding EIGRP is very large and it takes more than a while to grap the principles.

So, let's start .

1. What is metric in EIGRP and how we can calculate ?

Metric, in common, is a property of a route, consisting of one or more values, that is used by routing protocol to decide "how good" a route is. It is used when comparing multiple routes to the same destination, to decide which one is better.

Metric in EIGRP consists of:

  1. bandwidth
    =10000000/min(bandwidth) * 256
    where min(bandwidth) is the least bandwidth of all outgoing  interfaces on the route to the destination network represented in kilobits.
  2. delay
    =sum(delay)
    where sum(delay) is the sum of all delays on the route to the destination network represented in tens of microseconds.
  3. load
    dynamically evaluated load of the link, which ranges from 1 to 255 and is multiplied by 256, lower is better
  4. reliablity
    dynamically evaluated, ranges from 1 to 255 and is multiplied by 256, greater value is better
  5. MTU - is not included in the calculation
  6. Hop Count - is not included in the calculation

However, default metric setting uses only bandwidth and delay parameter.

The multiplication by 256 is there for backwards compatibility with IGRP, the EIGRP's predecessor.

2. What is the parameter of metric like example msec or micro sec ?

You can see, that the metric is a combination of multiple quantities, there is no common unit for metric like kbit/s or so, if that was the question...It is just a number.

3. If you put the command sh ip eigrp topology.... It will show the best routes path with Successor and FS with the attached with FD and AD value.

Is the FD is the metric values or something else in EIGRP ?

Yes, the FD refers to metric value. If you mean AD, like advertised distance, that is an term that happened to be in older literature, but now we call it RD - reported distance. That is, because AD is now widely used as Administrative Distance, which describes preference of routing protocols (EIGRP has AD of 90 for internal non-summarized routes, OSPF 110, etc.)

FD - reasible distance is the best metric along a path to a destination network, including the metric to the neighbor advertising that path.

RD(formerly AD) - reported distance is the total metric along a path to a destination network as advertised by an upstream neighbor.

Now coming my question is Metric value which I asked and you replied me that low metric value is good one.

Can you prove me as a example wise......

Now I have few question on this.....

In practically I don't think is like that

I have seen many times If I increased the bandwidth value then EIGRP will select that path.

Means Higher Bandwidth value is a Primary path and Lower Bandwidth is secondary path.

So As per the formula

Metric= 256*( Bandwidth +Sum of all Delay)

Where Bandwidth = 10^7/ Bandwidth

And Delay in micro sec


Pls clear me if I am wrong......

As you stated before, if you have incereased the bandwidth value then EIGRP has selected that path. Therefore, the increase of bandwidth (if increased all along the path) has indeed a positive effect on metric calculation.

So if I increased the Bandwidth it will increased the Metric value.

No, you will in fact decrease the metric value. That is, becasuse - as you mentioned few sentences before Bandwidth = 10^7/ Bandwidth

Therefore, it is the inverted value. The more you increase the bandwidth, the lower value you get from that equation, thus, effectively lowering the final metric.

Then how can you say it will not depend on Bandwidth

Excuse me, but I have never said that it does not depend on bandwidth. I am sorry if I haven't made myself crystal clear.

Let assume I have 3 route which connected one like that way..

R1 is connected to R2 with 100 Mbps link

R1 is connected to R2 with 10 Mbps link

R2 is connected to R3 with 10 Mbps link.

Means  R1 will have two link one is 100 Mbps and another is 10 mbps which are  connected to R2 and R2 is connected R3 with 10 Mbps link

The lan network of R1 needs to be reach lan of R3.

If  you generally configure EIGRP here. I have seen the practically it will  choose 100 Mbps link path as a Successor and 10 Mbps link path is FS.

Once the 100 Mbps link goes down then it will take 10 Mbps link as primary.

I agree and don't agree at the same time. In a configuration like this, the bandwidth of 100Mbps doesn't matter at all because 10Mbps (the minimum of all bandwidths along the path) is used in metric calculations. From the point of EIGRP, both paths are equally good (via 100Mbps and via 10Mbps).

With default setting, the only thing that might matter is the Delay parameter. That is 100 usec for 100Mbps link and 1000usec for 10Mbps link. In this case, the delay will have the final word, because bandwidth used in calculation is the same. Note, that delay is cumulative, so it adds up along the path. Bandwidth is the minimum along the path, therefore 10Mbps.

Let's say you just want to reach R3's 10Mbps Ethernet interface, not its LAN because that would bring another interface (towards the lan) to the calculation.

R1 ---100Mbps link --R2 ---10Mbps link ---R3

bandwidth 10^7Kbps/10 000 Kbps and delay 100usec+1000usec therefore 1100usec

final metric = 1000*256 + (1100/10) * 256 = 256 000 + 28160 = 284160

R1 ---10Mbps link --R2 ---10Mbps link ---R3

bandwidth 10^7Kbps/10 000 Kbps and delay 1000usec+1000usec therefore 2000usec

final metric = 1000*256 + (2000/10) * 256 = 256 000 + 51200 = 307200

Note: delay in the equation needs to be in tens of usec, therefore we need to divide it by 10 first.

Because metric of 284160 is lower than 307200, the path you have suggested is prefered.

Note that bandwidth is the same, only delay matters.

This is the reason why the faster link is used. Not because of the bandwidth parameter, but delay parameter.

If you still don't trust me, I can provide you with example in packet tracer and prove this point to you.

So can you tell me how it will happen?
Now how can you tell me the lower metric is good way.....

That is because bandwidth is not metric. It is used in the calculation of metric as one of the parameters, just like I said before. When you increase the bandwidth parameter on the router, the metric is lowered (because bandwidth in the equation of EIGRP metric is calulated as 10^7/bandwidth)

And tell me here what is Delay role here.....

You clear me as a example wise because now I have fully confused about this....

Hope you are clear my question........

You can see from your previous example, how the delay works.

Hope I made things a little bit clearer now .

Please, if you have any further questions, feel free to ask.

Best regards,

Jan

Re: Eigrp Metric Calculation

EIGRP uses a composite metric. It's actually, the same algorithm as IGRP, except for that the values are multiplied by 256.

EIGRP Metric = 256((10^7\Min_BW) + (Cumulative Delay))

So for a link that has 100Mbps and a delay of 500 you will get the following metric value 153600

There are 5 K values in this metric (K1, K2, K3, K4, and K5)

I know I included the EIGRP Metric algorithm above, but that's not technically the actual algorithm. The actual algorithm is rather long, and

I can remember if off the top of my head. But, the K values are used as plugins to the metric so to speak. So think of it, as one big algorithm, and the K values are values that you plug in to calculate the metric. By default, K1 = 1 and K3 = 1, (K2,K4, and K5 are 0 so they are not really calculated)

Bandwidth, Delay, Reliability, Load, and MTU

MTU is not used in this formula at all, but can be used in certain tie-breaking situations. By default only Bandwidth and Delay are used, and really should be the only ones ever used. The reason I say this, is because, EIGRP, will not dynamically adjust to changes in Reliability, Load, and MTU, like it will Bandwidth and Delay. For instance, if you include Load in the EIGRP Composite Metric calculation, and the load changed, EIGRP will not dynamically adjust the cost of the link to this change.

There are several definitons that you need to be aware of with EIGRP.

Feasible Distance - This is the best cost metric to the destination network, since the last Active > Passive change.

Reported Distance - This is the bst cost metric to the destination network as reported by an EIGRP neighbor.

Successor - This is the best cost metric to the destination network, and will be the one included in the routing table


Feasible Successor - This is a route that has passed the Feasibiliy Condition check. This checks, if the route has a Reported Distance to the destination network, that is lower than the current Feasible Distance to the destination network. If this check passed, the route will be included in the Topology table, but NOT the routing table. If the Successor goes down for any reason, EIGRP will automatically check its Topology table for any Feasible Successors, and promote it to a Successor

So let's say you have a router Router_A with two links (100Mbps and 10Mbps). So, let's say the 100Mbps link is the successor, and the 10Mbps passed the Feasibility Condition, and becomes a Feasible Successor. EIGRP will automatically go to this route, as soon as the Successor goes down.

If you want to see the specific values of your eigrp routes, create a simple eigrp topology and run the following command.

'show ip route x.x.x.x'

This will show you the cumulative delay, and various others values that will help you compute the path yourself.

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