EIGRP Metric Calculation Demystified When it comes to determining ...
EIGRP Metric Calculation Demystified When it comes to determining ...
EIGRP Metric Calculation Demystified When it comes to determining ...
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<strong>EIGRP</strong> <strong>Metric</strong> <strong>Calculation</strong> <strong>Demystified</strong><br />
<strong>When</strong> <strong>it</strong> <strong>comes</strong> <strong>to</strong> <strong>determining</strong> the <strong>EIGRP</strong> metric there seems <strong>to</strong> be many people who are<br />
confused in "how" <strong>it</strong> is actually calculated. I often times hear the following two questions, Can you<br />
just add the individual costs along the path <strong>to</strong> the destination like we do in OSPF? and Do we add the<br />
cost <strong>to</strong> the next-hop router <strong>to</strong> the reported distance (RD) as advertised by the next-hop router? The<br />
answer <strong>to</strong> both of these questions is absolutely and w<strong>it</strong>hout a doubt, No. Why then is there so much<br />
confusion? I believe part of the problem lies in the inaccurate way <strong>it</strong> is referenced by many of the<br />
documentation sources available <strong>to</strong> readers.<br />
For example, in the old Cisco Press book t<strong>it</strong>led, “CCNP Self Study: Building Scalable Cisco<br />
Internetworks (BSCI)”, by Catherine Paquet and Diane Teare, on page 362, <strong>it</strong> states the following in<br />
regard <strong>to</strong> the <strong>EIGRP</strong> metric:<br />
“The lowest-cost route is calculated by adding the cost between the next-hop router and the<br />
destination (referred <strong>to</strong> as the advertised distance [AD]) <strong>to</strong> the cost between the local router and the<br />
next-hop router. (The <strong>to</strong>tal is referred <strong>to</strong> as the feasible distance [FD].)”<br />
Upon first glance this seems correct, but this is clearly an incorrect statement regarding the<br />
<strong>EIGRP</strong> metric. In another example in the same book at the bot<strong>to</strong>m of page 362, <strong>it</strong> gives the following<br />
defin<strong>it</strong>ion for the feasible distance (FD).<br />
“FD or fd (feasible distance) - Equal <strong>to</strong> the sum of the costs of the links <strong>to</strong> reach network (a).”<br />
Again, an apparently inaccurate reference <strong>to</strong> the actual <strong>EIGRP</strong> metric. In add<strong>it</strong>ion, the examples<br />
that follow in the book, clearly show they are “adding” the individual link costs along the path from<br />
source <strong>to</strong> the destination <strong>to</strong> come up w<strong>it</strong>h the <strong>to</strong>tal <strong>EIGRP</strong> metric. An obvious oversight on the part of<br />
the author's resulting in an incorrect <strong>EIGRP</strong> calculation.<br />
Another source for the possible misunderstanding can be found in the new Cisco Press Book<br />
t<strong>it</strong>led “Implementing Cisco IP Routing (ROUTE) Foundation Learning Guide”, by Diane Teare, on<br />
page 62, <strong>it</strong> states the following regarding the <strong>EIGRP</strong> metric:<br />
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<strong>EIGRP</strong> <strong>Metric</strong> <strong>Calculation</strong> <strong>Demystified</strong><br />
”The lowest-cost route is calculated by adding the cost between the next-hop router and the<br />
destination—referred <strong>to</strong> as the advertised distance (AD)—<strong>to</strong> the cost between the local router and the<br />
next-hop router. The sum of these costs is referred <strong>to</strong> as the feasible distance (FD).”<br />
In add<strong>it</strong>ion, on page 71 of the same book, the author states,<br />
”The AD is the <strong>EIGRP</strong> metric for an <strong>EIGRP</strong> neighbor router <strong>to</strong> reach a particular network.<br />
This is the metric between the next-hop neighbor router and the destination network.<br />
The FD is the <strong>EIGRP</strong> metric for this router <strong>to</strong> reach a particular network. This is the sum of the<br />
AD for the particular network learned from an <strong>EIGRP</strong> neighbor, plus the <strong>EIGRP</strong> metric <strong>to</strong> reach that<br />
neighbor (the metric between this router and the next-hop router).”<br />
Notice the author's use of the words “adding” and “sum” in the first reference as well as the use<br />
of “sum” in the second reference. I believe both of these references make an improper choice of the<br />
words <strong>to</strong> describe the calculation of the <strong>EIGRP</strong> metric and gives many readers an erroneous impression<br />
that you can simply “add” the AD of the neighboring router <strong>to</strong> the cost <strong>to</strong> reach the neighboring router.<br />
Resulting in a “sum” equal <strong>to</strong> the <strong>EIGRP</strong> metric. Again, this is incorrect and I will show this in an<br />
example later in the paper.<br />
Please, do not get me wrong, I am not trying <strong>to</strong> disrespect the authors' in any way. In fact, I am<br />
am very fond of both these author's and have several of their books in my own technical library. They<br />
are among my favor<strong>it</strong>e list of authors'. I am merely trying <strong>to</strong> show a possible cause <strong>to</strong> the misguided<br />
and misunderstanding others may have regarding the <strong>EIGRP</strong> metric calculation.<br />
Furthermore, in the following Cisco document on “Enhanced Interior Gateway Routing<br />
Pro<strong>to</strong>col (<strong>EIGRP</strong>)”, Document ID: 16406, <strong>it</strong> gives this defin<strong>it</strong>ion for the feasible distance:<br />
”Feasible distance is the best metric along a path <strong>to</strong> a destination network, including the metric<br />
<strong>to</strong> the neighbor advertising that path. Reported distance is the <strong>to</strong>tal metric along a path <strong>to</strong> a<br />
destination network as advertised by an upstream neighbor.”<br />
Many people confuse the phrase “including the metric” in the above passage <strong>to</strong> mean that you<br />
“add” the metric <strong>to</strong> reach the next-hop neighbor <strong>to</strong> the reported distance (RD) of the upstream neighbor<br />
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<strong>to</strong> arrive at the <strong>to</strong>tal metric along the path. Again, a seemingly misguided statement regarding the<br />
<strong>EIGRP</strong> metric, resulting in confusion and incorrect calculations.<br />
While <strong>it</strong> is true that the feasible distance (FD) is the <strong>to</strong>tal cost along a path from source <strong>to</strong><br />
destination. It is erroneous <strong>to</strong> say the <strong>to</strong>tal metric is equal <strong>to</strong> the “sum” of the individual link costs or <strong>to</strong><br />
say the <strong>to</strong>tal metric is equal <strong>to</strong> the cost <strong>to</strong> reach the next-hop neighbor “plus” the RD of the next-hop<br />
neighbor. You can verify this by using the <strong>EIGRP</strong> metric formula, shown below.<br />
METRIC=256∗[k1∗bandwidthk2∗bandwidth/256−load k3∗delay]∗[k5/reliabil<strong>it</strong>yk4]<br />
Remember the default “k-values” are k1=k3=1 and k2=k4=k5 =0. Looking at the equation and<br />
the “k-values”, if k2=k4=k5 = 0, then mathematically this equation reduces <strong>to</strong> zero. Which we know is<br />
not what happens. Therefore, the <strong>EIGRP</strong> formula must be a “cond<strong>it</strong>ional” formula, a sort of “If-then”<br />
type scenario where if k5=0, then we set the last part of the equation <strong>to</strong> “1”. This results in the<br />
mathematically sound, often reduced <strong>EIGRP</strong> formula shown below.<br />
METRIC=256∗[k1∗bandwidthk3∗delay]<br />
Recall the <strong>EIGRP</strong> formula uses the minimum bandwidth (in kbps) along the path from source <strong>to</strong><br />
destination and the <strong>to</strong>tal cumulative delay (in 10s of microseconds) along this same path from source <strong>to</strong><br />
destination. For example, take the following network:<br />
Figure 1 – <strong>EIGRP</strong> <strong>Metric</strong><br />
The metric (or cost) of each of the individual links can be calculated using the <strong>EIGRP</strong> formula. Shown<br />
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below is a simplified version of the <strong>EIGRP</strong> formula above.<br />
METRIC=256∗BW Delay<br />
where BW = (10^7 / interface BW) and Delay = (<strong>to</strong>tal path delay / 10). Using the network diagram<br />
above and the <strong>EIGRP</strong> formula, will we prove the answer <strong>to</strong> the two opening questions is in fact “No”<br />
and disprove the often misguided, inaccurate statements found in so many publications regarding the<br />
<strong>EIGRP</strong> metric calculation.<br />
Step 1 - calculate the individual link metric for R1-R2.<br />
On the R1 <strong>to</strong> R2 link we are using an interface BW of 1544kbps and a delay of 20000 microseconds.<br />
The metric is calculated as follows:<br />
METRIC=256∗[10,000,000/1,54420,000/10]<br />
METRIC=256∗64762000<br />
METRIC=256∗8476=2169856<br />
Step 2 - calculate the individual link metric for R2-R3.<br />
Likewise, on the R2 <strong>to</strong> R3 serial link we calculate the metric <strong>to</strong> be 2169856.<br />
Step 3 - calculate the individual link metric for R3 <strong>to</strong> the destination network 10.1.1.0/24.<br />
The Fast Ethernet interface on R3 is calculated using an interface BW of 100Mbps and a delay of 100<br />
microseconds. The metric is<br />
Step 4 - adding the individual link metrics.<br />
METRIC=256∗[10,000,000/100,000100/10]<br />
METRIC=256∗10010<br />
METRIC=256∗110=28160<br />
Adding the individual link metrics from the steps above results in a <strong>to</strong>tal <strong>EIGRP</strong> metric of 4367872.<br />
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<strong>EIGRP</strong> <strong>Metric</strong> <strong>Calculation</strong> <strong>Demystified</strong><br />
METRIC=2169856216985628160=4367872<br />
Step 5 - calculate the metric using the <strong>EIGRP</strong> formula.<br />
Using the <strong>EIGRP</strong> formula <strong>to</strong> calculate the <strong>to</strong>tal metric from R1 <strong>to</strong> destination network 10.1.1.0/24<br />
results in metric of 2684416. In the above example the least BW is 1544kbps and the <strong>to</strong>tal cumulative<br />
delay is 20000 + 20000 + 100 = 40100. Using the <strong>EIGRP</strong> formula we get:<br />
METRIC=256∗[10,000,000/1,54440,100 /10]<br />
METRIC=256∗64764010<br />
METRIC=256∗10486=2684416<br />
Comparing the metrics calculated in Step 4 and Step 5, clearly shows these are not the same.<br />
Proving you can not simply “add” the individual link metrics <strong>to</strong> arrive at the <strong>to</strong>tal <strong>EIGRP</strong> metric.<br />
In add<strong>it</strong>ion, we can prove the <strong>to</strong>tal <strong>EIGRP</strong> metric is not equal <strong>to</strong> the cost <strong>to</strong> reach the next-hop<br />
neighbor “plus” the RD as advertised by the next-hop neighbor. Again, we can verify this using the<br />
<strong>EIGRP</strong> formula.<br />
Step 6 - calculate the individual link metric for R1-R2.<br />
As calculated in Step 1 above the cost or metric <strong>to</strong> reach the next-hop neighbor (R2 in our case) is<br />
2169856.<br />
Step 7 - calculate the FD for R2 <strong>to</strong> reach the destination network 10.1.1.0/24.<br />
The cost from R2 <strong>to</strong> destination network 10.1.1.0/24 is 2172416. This is the FD for R2 and is also the<br />
RD that is advertised <strong>to</strong> R1. The minimum BW is 1544kbps and the <strong>to</strong>tal cumulative delay is 20000 +<br />
100 = 20100. Using the <strong>EIGRP</strong> formula we get:<br />
METRIC=256∗[10,000,000/1,54420,100/10]<br />
METRIC=256∗64762010<br />
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<strong>EIGRP</strong> <strong>Metric</strong> <strong>Calculation</strong> <strong>Demystified</strong><br />
METRIC=256∗8486=2172416<br />
Step 8 - adding the RD of the next-hop neighbor <strong>to</strong> the cost <strong>to</strong> reach the next-hop neighbor.<br />
Adding the metric calculated in Step 6 <strong>to</strong> the metric calculated in Step 7 yields a <strong>to</strong>tal metric of<br />
4342272.<br />
METRIC=21698562172416=4342272<br />
Again, adding these two metrics <strong>to</strong>gether clearly shows the “sum” is not the same as the actual<br />
<strong>EIGRP</strong> metric we calculated in Step 5.<br />
So, in conclusion, we just proved that the <strong>to</strong>tal cost or metric in <strong>EIGRP</strong> is not equal <strong>to</strong> the “sum”<br />
of the individual link metrics nor is <strong>it</strong> equal <strong>to</strong> the RD of the next-hop neighbor “plus” the link metric <strong>to</strong><br />
reach the next-hop neighbor. In all calculations of the <strong>EIGRP</strong> metric, you must use the <strong>EIGRP</strong> formula<br />
and calculate the metric from source <strong>to</strong> destination independently. Taking in<strong>to</strong> consideration the<br />
minimum bandwidth along the path and the cumulative delay along the path. Meaning the metric for<br />
R2 <strong>to</strong> the destination network is calculated using the <strong>EIGRP</strong> formula and the minimum bandwidth and<br />
cumulative delay along the path from R2 <strong>to</strong> the destination network. Likewise, the metric for R1 must<br />
also be calculated using the <strong>EIGRP</strong> formula and the minimum bandwidth and cumulative delay along<br />
the path from R1 <strong>to</strong> the destination network. You cannot calculate R1's metric by taking the metric as<br />
seen by R2 and simply add the cost <strong>to</strong> reach R2. As we have shown in the above calculations, this does<br />
not result in the correct <strong>EIGRP</strong> metric calculation.<br />
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