1 2 Previous Next 20 Replies Latest reply: Mar 10, 2012 8:27 PM by Brian McGahan - 4 x CCIE, CCDE # The truth about EIGRP FD Calculation

This is a contradiction I could not get my head around until now. The comment below was taken below from Diane Teare's latest CCNP route books for the 642-902, the same is said across a number other books.

Basically the quote below says the FD is calculated based on the sum of the next hop routers AD and the cost between local router and next hop router. However as you read on through the book it says the metric is calculated using the EIGRP metric calculation. To simplifiy things for now lets just say metric= least cost bw + accumulative delay

CCNP Quote:

Advertised distance and feasible distance—DUAL uses distance information, known as a metric or cost, to select efficient, loop-free paths. The lowest-cost route is calculated by adding the cost between the next-hop router and the destination—referred to as the advertised distance (AD)—to the cost between the local router and the next-hop router. The sum of these costs is referred to as the feasible distance (FD).

The former appears not to work as I don't get the correct FD value by adding local metric to next hop routers AD, unless I also subtract 256. I do however get the correct FD value from the metric formula.

Here is my Topology which is taken from the CCNP Route LAB manual LAB 2-1

(R1)--fa0/0 <-- 100Mbps --> fa0/0-(R3)--loopback3 <--> 10Gbps

(R1)--fa0/0 = 10.1.100.1/24

(R3)--fa0/0 = 10.1.100.3/24

(R3)--Loopback3 = 10.1.3.1/24

I'm going to calculate the FD for 10.1.3.0/24 from R1 perspective using the first scenario FD = AD + Local Router to Next Hop Router metric

R1#sh ip eigrp topology
IP-EIGRP Topology Table for AS(1)/ID(10.1.1.1)

P 10.1.3.0/24, 1 successors, FD is 409600
via 10.1.100.3 (409600/128256), FastEthernet0/0

P 10.1.100.0/24, 1 successors, FD is 281600

via Connected, FastEthernet0/0

Ok so now we calculate the FD by adding the AD for 10.1.3.0/24 via 10.1.100.3 and FD for 10.1.100.0/24 connected local therefore FD = 128256 + 281600 = 409856.

hmm.. 409856 does not match the FD shown by the show ip eigrp topology output as this shows the FD as 409600, there is a difference of 256.

I tried this same calculation on other routes and seem to get a difference of 256. Perhaps the calculation needs to be FD = 128256 + 281600 - 256 =  409600

So now I'll use the metric formula, but first I'll display the metrics needed for this calculation. Note: the min bandwidth is the 100Mbps (10000Kbps) on fa0/0 as shown below and the delay on fa0/0, the delay on loopback 3 at R3 is 5000 microseconds, therefore the cumulative delay is 6000 microseconds as shown directly  below.

R1#sh ip eigrp topology 10.1.3.0/24

IP-EIGRP (AS 1): Topology entry for 10.1.3.0/24

State is Passive, Query origin flag is 1, 1 Successor(s), FD is 409600

Routing Descriptor Blocks:

10.1.100.3 (FastEthernet0/0), from 10.1.100.3, Send flag is 0x0

Composite metric is (409600/128256), Route is Internal

Vector metric:

Minimum bandwidth is 10000 Kbit

Total delay is 6000 microseconds

Reliability is 255/255

Minimum MTU is 1500

Hop count is 1

Outputs below provided as added information to prove the outpu above is true.

R1#sh ip eigrp topology 10.1.100.0/24

IP-EIGRP (AS 1): Topology entry for 10.1.100.0/24

State is Passive, Query origin flag is 1, 1 Successor(s), FD is 281600

Routing Descriptor Blocks:

0.0.0.0 (FastEthernet0/0), from Connected, Send flag is 0x0

Composite metric is (281600/0), Route is Internal

Vector metric:

Minimum bandwidth is 10000 Kbit

Total delay is 1000 microseconds

Reliability is 255/255

Minimum MTU is 1500

Hop count is 0

R3#sh ip eigrp topology 10.1.3.0/24

IP-EIGRP (AS 1): Topology entry for 10.1.3.0/24

State is Passive, Query origin flag is 1, 1 Successor(s), FD is 128256

Routing Descriptor Blocks:

0.0.0.0 (Loopback3), from Connected, Send flag is 0x0

Composite metric is (128256/0), Route is Internal

Vector metric:

Minimum bandwidth is 10000000 Kbit

Total delay is 5000 microseconds

Reliability is 255/255

Minimum MTU is 1514

Hop count is 0

Now to calculate using the metric formula; metric = ((1E7/minbw)+(sum of delays/10ths of microseconds))*256

metric = ((10000000/10000)+(600))*256 = 409600

So the formula matches the FD shown in the topology output.

R1#sh ip eigrp topology 10.1.3.0/24

IP-EIGRP (AS 1): Topology entry for 10.1.3.0/24

State is Passive, Query origin flag is 1, 1 Successor(s), FD is 409600

Routing Descriptor Blocks:

10.1.100.3 (FastEthernet0/0), from 10.1.100.3, Send flag is 0x0

Composite metric is (409600/128256), Route is Internal

Vector metric:

Minimum bandwidth is 10000 Kbit

Total delay is 6000 microseconds

Reliability is 255/255

Minimum MTU is 1500

Hop count is 1

I'm interested in anyone's view on the reasoning behind the said quote above because it does'nt appear to be right if taken literally, it might be true if you subtract 256. Can anyone confirm possible reasons for the difference of 256?

Perhaps the AD is not used in the FD calculation for a given route and is only used as the selector for Feasible Successors etc.. not sure.

The EIGRP formula matches the FD as expected.

thanks DM

• ###### 1. Re: The truth about EIGRP FD Calculation

Router will round down or up values;

https://learningnetwork.cisco.com/docs/DOC-13825

• ###### 2. Re: The truth about EIGRP FD Calculation

Thanks for the link to the EIGRP calculator Martin, this will come in handy

Sorry, but I don't think the router does any rounding up or down as an FD of 409856 does get rounded down to 409600 right?

Thanks,

Dave

• ###### 3. Re: The truth about EIGRP FD Calculation

L0-    6.6.6.0/24 (BW-10000000)

\                                                                          R2 (BW 1544)

\                                                                          /^

----R6--f0/0---(BW- 10000)-- R1- --s0/0.12/21---/

/                                                (1.1.1.0/24)

/

L1-    6.6.7.0 /24 (BW-5000000)

Case 1.

------

Only BW is used in metric calculation. K-values- 1 0 0 0 0 )

R6

--

R6(config)#do sh ip ei to 6.6.6.0/24

IP-EIGRP (AS 1): Topology entry for 6.6.6.0/24

State is Passive, Query origin flag is 1, 1 Successor(s), FD is 256

Routing Descriptor Blocks:

0.0.0.0 (Loopback0), from Connected, Send flag is 0x0

Composite metric is (256/0), Route is Internal

Vector metric:

Minimum bandwidth is 10000000 Kbit

Total delay is 100 microseconds

Reliability is 255/255

Minimum MTU is 1514

Hop count is 0

R6(config)#do sh ip ei to 6.6.7.0/24

IP-EIGRP (AS 1): Topology entry for 6.6.7.0/24

State is Passive, Query origin flag is 1, 1 Successor(s), FD is 512

coming on to R1 first calculate the eigrp metric to reach the next hop.

metric = (10000000/10000) * 256  = 256000

R1(config-router)#do sh ip ei to 10.1.16.0/24

IP-EIGRP (AS 1): Topology entry for 10.1.16.0/24

State is Passive, Query origin flag is 1, 1 Successor(s), FD is 256000

Routing Descriptor Blocks:

0.0.0.0 (FastEthernet0/0), from Connected, Send flag is 0x0

Composite metric is (256000/0), Route is Internal

Vector metric:

Minimum bandwidth is 10000 Kbit

total metric should be RD  + cost to reach neighbor

ie for 6.6.6.0/24------256 + 256000  = 256256

for 6.6.7.0/24   ---- 512 + 256000 =  256512

Or we use the formula [ ((10^7/min bw) + (delay)) * 256 ]

we get  10^7/10000*256   = 256000

verifying-- on CLI

its 256000

R1(config-router)#do sh ip ei to 6.6.6.0/24

IP-EIGRP (AS 1): Topology entry for 6.6.6.0/24

State is Passive, Query origin flag is 1, 1 Successor(s), FD is 256000

Routing Descriptor Blocks:

10.1.16.6 (FastEthernet0/0), from 10.1.16.6, Send flag is 0x0

Composite metric is (256000/256), Route is Internal

Vector metric:

Minimum bandwidth is 10000 Kbit

Total delay is 1100 microseconds

Reliability is 255/255

Minimum MTU is 1500

Hop count is 1

R1(config-router)#do sh ip ei to 6.6.7.0/24

IP-EIGRP (AS 1): Topology entry for 6.6.7.0/24

State is Passive, Query origin flag is 1, 1 Successor(s), FD is 256000

Routing Descriptor Blocks:

10.1.16.6 (FastEthernet0/0), from 10.1.16.6, Send flag is 0x0

Composite metric is (256000/512), Route is Internal

Vector metric:

Minimum bandwidth is 10000 Kbit

Total delay is 6000 microseconds

Reliability is 255/255

Minimum MTU is 1500

Hop count is 1

I guess there's no use to go to R2 and verify things, as the minimum BW will be

1544 for R2 and the metric will be 6467*256=1657856

ie the metric to reach R1 is equal to metric to reach R6.

verifying:-

R2(config-router)#do sh ip ei to 6.6.7.0/24

IP-EIGRP (AS 1): Topology entry for 6.6.7.0/24

State is Passive, Query origin flag is 1, 1 Successor(s), FD is 1657856

Routing Descriptor Blocks:

10.1.12.1 (Serial0/0.21), from 10.1.12.1, Send flag is 0x0

Composite metric is (1657856/256000), Route is Internal

Vector metric:

Minimum bandwidth is 1544 Kbit

R2(config-router)#do sh ip ei to 1.1.1.0/24

IP-EIGRP (AS 1): Topology entry for 1.1.1.0/24

State is Passive, Query origin flag is 1, 1 Successor(s), FD is 1657856

Routing Descriptor Blocks:

10.1.12.1 (Serial0/0.21), from 10.1.12.1, Send flag is 0x0

Composite metric is (1657856/256), Route is Internal

Vector metric:

Minimum bandwidth is 1544 Kbit

So, it don't looks like the FD is equal to metric to neighbor + its best metric to destination.

Case 2:

-------

L0-    6.6.6.0/24 (DLY- 10)

\                                                                                   R2 (DLY 2000)

\                                                                                      /^

----R6--f0/0---(DLY 1000)-- R1- --s0/0.12/21---/

/                                                (1.1.1.0/24)

/

L1-    6.6.7.0 /24 (DLY-100)

Delay is NOT in tens of microsec

K values-  0 0 1 0 0

here we can say that the metric to reach the neighbor + the RD by the neighbor is equal to FD, as the delay is cumulative, and there is no MIN BW factor here.

CASE-3-- K values-- 10100

same as your post but not identical

When i learned the metric I had the same question, but i couldn't find any reason anywhere.

even if you see debugs like following:

The debug

*Mar  1 05:23:54.194: IP-EIGRP(Default-IP-Routing-Table:1): Int 12.1.1.0/24 M 2067456 - 1657856 409600 SM 409600 - 256000 153600

where metric M equals 2067456  (which is sum of min.BW*256  = 1657856), (cumulative delay * 256 = 409600)

same for the Source Metric  SM - 409600  that = 256(min BW) + 256 *(total delay)

409600      =         256000      +   153600

That too only support the formula based metric calculation.

Regards

• ###### 4. Re: The truth about EIGRP FD Calculation

Thanks lp4nb for confirming you see the same issue and Martin for responding.. I tried this on another route calculation and the difference appears to be 256.

By difference I mean minus 256 and I'm not sure why yet.

thanks Dmorrow

• ###### 5. Re: The truth about EIGRP FD Calculation

I had tried earlier the difference changes. :-)

and what Martin indicated in post 3 I thought the same way about it, but couldn't decipher which one  to round off, the  RD or the local metric to Neighbor, or the one when we use (10^7/BW).

The problem is this thing is propritary and I think that is the reason it is not documented in detail. Or I'm missing sth.

Regards

P.S though, I still can't comment on the contradiction (original Question).

• ###### 6. Re: The truth about EIGRP FD Calculation

Yup, I guess you are right lp4nb EIGRP is proprietary and Martin could actually be on the money regarding the rounding off.

Thank you both for your help. If I find anything else I'll be sure to update this discussion.

• ###### 7. Re: The truth about EIGRP FD Calculation

You are correct, there is definitely a  misunderstanding in "how" the EIGRP is truly calculated.  In fact, I wrote a little PDF document describing this same issue some time ago, but thought it needed editing before I posted it for others.  You can read it here:

https://learningnetwork.cisco.com/docs/DOC-13825

Regarding the "rounding", yes Cisco routers will round down or "truncate" the decimal portion on the calculations.  For example, take a serial link with a bandwidth of 1544 and delay of 20000 microseconds.  Using the formula without rounding yields the following EIGRP metric.

METRIC = 256 * [(10,000,000/1544) + (20000/10)]

METRIC = 256 * [(6476.68) + (2000)]

METRIC = 256 * (8476.68)

METRIC = 2,170,031.09

when in fact the true EIGRP metric is 2,169,856.  Indicating the router rounded down or "truncated" the decimal portion and used 6476 rather than 6476.68.  You can verify this on any Cisco router as well.

Hope this helps.

Brian

• ###### 8. Re: The truth about EIGRP FD Calculation

Thanks Brian this is most definately a big help, that is a really good document.

The examples you provided showing the two different calculations provide a vastly different metric value, which as you suggest disproves the descriptions of how the FD value is calculated.

Clearly the formula provides an accurate FD value.

btw I like your EIGRP Calculator.

I'm going to test something and post this in a few minutes, I'm not sure if this will disprove the rounding theory but I'll give it a crack.

• ###### 9. Re: The truth about EIGRP FD Calculation

Looking at 3 topologies below. Each Topology has varying link speeds and therefore provide different EIGRP FD metric values. The test was to prove if the Cisco documentation referenced in this conversation is correct about it's FD calculation.

In short the Cisco document suggests that an FD value of a given network can be calculated by simply adding the RD advertised by the next hop router and the metric value between the local router and that next hop router.

I believe what Cisco and it's authors suggest has merit but is only partially accurate. For every calculation performed in each of the 3 topology tests below there is always a difference of 256000. Therefore if you use the Cisco documented calculation as mentioned previously it's value is always 256000 higher than EIGRP FD reported value shown by the show ip eigrp topology command and consequently the formula always matches the FD.

Therefore I conclude that for a given network if you add the EIGRP AD and the metric of local router to next hop router then subtract 256000 you will match the value of the FD reported by the show ip eigrp topology command.

In addition to this I don't see this as a rounding up/down as the number is always exact subtraction of 256000.

Perhaps the Cisco documentation should be amended to include a subtraction of 256000 at some point down the track. I welcome anyone to re-check these calculations or perform your own calculation & confirm if you are seeing the same result or not.

Thanks to all for responding to this discussion, I learned some valuable information.

Topology 1:

(R1)-fa0/0 <--100Mbps--> fa0/0-(R2)-fa0/1 <--100Mbps--> fa0/1(R3)s0/0 <--64Kbps--> s0/1-(R4)-loopback1(10Gbps)

R1-fa0/0 = 10.1.1.0/30

R2-fa0/1 = 10.1.1.4/30

R3-s0/0 = 10.1.1.8/30

R4-loopback = 10.100.1.0/30

Test: Calculating EIGRP FD Metric for network 10.100.1.0/30 from R1, by adding EIGRP Metric of Local Router to EIGRP RD advertised by Next Hop Router.

Route Path: R1-->R2-->R3-->R4-Loopback1(10.100.1.0/30)

R1#sh int fa0/0
FastEthernet0/0 is up, line protocol is up
Hardware is Gt96k FE, address is c205.26d4.0000 (bia c205.26d4.0000)
Internet address is 10.1.1.1/30
MTU 1500 bytes, BW 100000 Kbit/sec, DLY 100 usec,

R2#sh int fa0/1
FastEthernet0/1 is up, line protocol is up
Hardware is Gt96k FE, address is c206.26d4.0001 (bia c206.26d4.0001)
Internet address is 10.1.1.5/30
MTU 1500 bytes, BW 100000 Kbit/sec, DLY 100 usec,

R3#sh int s0/0
Serial0/0 is up, line protocol is up
Hardware is GT96K Serial
Internet address is 10.1.1.13/30
MTU 1500 bytes, BW 64 Kbit/sec, DLY 20000 usec,

R4#sh int lo1
Loopback1 is up, line protocol is up
Hardware is Loopback
Internet address is 10.100.1.1/30
MTU 1514 bytes, BW 8000000 Kbit/sec, DLY 5000 usec,

R1#sh ip eigrp topology 10.100.1.0/30
IP-EIGRP (AS 100): Topology entry for 10.100.1.0/30
State is Passive, Query origin flag is 1, 1 Successor(s), FD is 40645120
Routing Descriptor Blocks:
10.1.1.2 (FastEthernet0/0), from 10.1.1.2, Send flag is 0x0
Composite metric is (40645120/40642560), Route is Internal
Vector metric:
Minimum bandwidth is 64 Kbit
Total delay is 25200 microseconds
Reliability is 255/255
Minimum MTU is 1500
Hop count is 3

R1#sh ip eigrp top 10.1.1.0/30
IP-EIGRP (AS 100): Topology entry for 10.1.1.0/30
State is Passive, Query origin flag is 1, 1 Successor(s), FD is 28160
Routing Descriptor Blocks:
0.0.0.0 (FastEthernet0/0), from Connected, Send flag is 0x0
Composite metric is (28160/0), Route is Internal
Vector metric:
Minimum bandwidth is 100000 Kbit
Total delay is 100 microseconds
Reliability is 255/255
Minimum MTU is 1500
Hop count is 0

Calculating topology 1 metric for 10.100.1.0/30 by adding the Local EIGRP Metric and next hop router EIGRP RD
10.100.1.0 FD = 28160 + 40642560 = 40670720

The FD reported on R1 by show ip eigrp topology 10.100.1.0/30 = 40645120, therefore difference between the two value's = 256000

------------------------------------------------------------

Topology 2:

(R1)-fa0/0 <--10Mbps--> fa0/0-(R2)-fa0/1 <--100Mbps--> fa0/1(R3)s0/0 <--64Kbps--> s0/1-(R4)-loopback1(10Gbps)

IP addresses are same as Topology 1

Test: Calculating EIGRP FD Metric for network 10.100.1.0/30 from R1, by adding EIGRP Metric of Local Router to EIGRP RD advertised by Next Hop Router.

Route Path: R1-->R2-->R3-->R4-Loopback1(10.100.1.0/30)

R1#sh int fa0/0
FastEthernet0/0 is up, line protocol is up
Hardware is Gt96k FE, address is c205.26d4.0000 (bia c205.26d4.0000)
Internet address is 10.1.1.1/30
MTU 1500 bytes, BW 10000 Kbit/sec, DLY 100 usec,

R2#sh int fa0/1
FastEthernet0/1 is up, line protocol is up
Hardware is Gt96k FE, address is c206.26d4.0001 (bia c206.26d4.0001)
Internet address is 10.1.1.5/30
MTU 1500 bytes, BW 100000 Kbit/sec, DLY 100 usec,

R3#sh int s0/0
Serial0/0 is up, line protocol is up
Hardware is GT96K Serial
Internet address is 10.1.1.13/30
MTU 1500 bytes, BW 64 Kbit/sec, DLY 20000 usec,

R4#sh int lo1
Loopback1 is up, line protocol is up
Hardware is Loopback
Internet address is 10.100.1.1/30
MTU 1514 bytes, BW 8000000 Kbit/sec, DLY 5000 usec,

R1#sh ip eigrp top 10.100.1.0/30
IP-EIGRP (AS 100): Topology entry for 10.100.1.0/30
State is Passive, Query origin flag is 1, 1 Successor(s), FD is 40645120
Routing Descriptor Blocks:
10.1.1.2 (FastEthernet0/0), from 10.1.1.2, Send flag is 0x0
Composite metric is (40645120/40642560), Route is Internal
Vector metric:
Minimum bandwidth is 64 Kbit
Total delay is 25200 microseconds
Reliability is 255/255
Minimum MTU is 1500
Hop count is 3

R1#sh ip eigrp top 10.1.1.0/30
IP-EIGRP (AS 100): Topology entry for 10.1.1.0/30
State is Passive, Query origin flag is 1, 1 Successor(s), FD is 258560
Routing Descriptor Blocks:
0.0.0.0 (FastEthernet0/0), from Connected, Send flag is 0x0
Composite metric is (258560/0), Route is Internal
Vector metric:
Minimum bandwidth is 10000 Kbit
Total delay is 100 microseconds
Reliability is 255/255
Minimum MTU is 1500
Hop count is 0

Calculating topology 1 metric for 10.100.1.0/30 by adding the Local EIGRP Metric and next hop router EIGRP RD
10.100.1.0 FD = 258560 + 40642560 = 40901120

The FD reported on R1 by show ip eigrp topology 10.100.1.0/30 = 40645120, therefore difference between the two value's = 256000

-------------------------------------------------------------------

Topology 3

(R1)-fa0/0 <--100Mbps--> fa0/0-(R2)-fa0/1 <--1Mbps--> fa0/1(R3)s0/0 <--512Kbps--> s0/1-(R4)-loopback1(10Gbps)

IP addresses are same as Topology 1

Test: Calculating EIGRP FD Metric for network 10.100.1.0/30 from R1, by adding EIGRP Metric of Local Router to EIGRP RD advertised by Next Hop Router.

Route Path: R1-->R2-->R3-->R4-Loopback1(10.100.1.0/30)

R1#sh int fa0/0
FastEthernet0/0 is up, line protocol is up
Hardware is Gt96k FE, address is c205.26d4.0000 (bia c205.26d4.0000)
Internet address is 10.1.1.1/30
MTU 1500 bytes, BW 100000 Kbit/sec, DLY 100 usec,

R2#sh int fa0/1
FastEthernet0/1 is up, line protocol is up
Hardware is Gt96k FE, address is c206.26d4.0001 (bia c206.26d4.0001)
Internet address is 10.1.1.5/30
MTU 1500 bytes, BW 1000 Kbit/sec, DLY 100 usec,

R3#sh int S0/0
Serial0/0 is up, line protocol is up
Hardware is GT96K Serial
Internet address is 10.1.1.13/30
MTU 1500 bytes, BW 512 Kbit/sec, DLY 20000 usec,

R4#sh int lo1
Loopback1 is up, line protocol is up
Hardware is Loopback
Internet address is 10.100.1.1/30
MTU 1514 bytes, BW 8000000 Kbit/sec, DLY 5000 usec,

R1#sh ip eigrp topology 10.100.1.0/30
IP-EIGRP (AS 100): Topology entry for 10.100.1.0/30
State is Passive, Query origin flag is 1, 1 Successor(s), FD is 5645056
Routing Descriptor Blocks:
10.1.1.2 (FastEthernet0/0), from 10.1.1.2, Send flag is 0x0
Composite metric is (5645056/5642496), Route is Internal
Vector metric:
Minimum bandwidth is 512 Kbit
Total delay is 25200 microseconds
Reliability is 255/255
Minimum MTU is 1500
Hop count is 3

R1#sh ip eigrp top 10.1.1.0/30
IP-EIGRP (AS 100): Topology entry for 10.1.1.0/30
State is Passive, Query origin flag is 1, 1 Successor(s), FD is 28160
Routing Descriptor Blocks:
0.0.0.0 (FastEthernet0/0), from Connected, Send flag is 0x0
Composite metric is (28160/0), Route is Internal
Vector metric:
Minimum bandwidth is 100000 Kbit
Total delay is 100 microseconds
Reliability is 255/255
Minimum MTU is 1500
Hop count is 0

Calculating topology 1 metric for 10.100.1.0/30 by adding the Local EIGRP Metric and next hop router EIGRP RD
10.100.1.0 FD = 28160 + 5642496 = 5670656

The FD reported on R1 by show ip eigrp topology 10.100.1.0/30 = 5645056, therefore difference between the two value's = 256000

• ###### 10. Re: The truth about EIGRP FD Calculation

Dmorrow,

Very nice labbing.  However, I do not get the same 256,000 difference in all three topologies.  Lets take your first topology.

R2 to destination network = 40642560

R1 to R2 metric = 28160

adding these two together

40642560

00028160

40670720

However, the correct metric = 40645120

the difference is

40670720

40645120

00025600

difference = 25,600

Take topology #2.

R2 to destination network = 40642560

R1 to R2 metric = 258560

adding these two together

40642560

00258560

40901120

However, the correct metric = 40645120

the difference is

40901120

40645120

00256000

difference = 256,000

Take topology #3.

R2 to destination network = 5642496

R1 to R2 metric = 28160

adding these two together

5642496

0028160

5670656

However, the correct metric = 5645056

the difference is

5670656

5645056

0025600

difference of 25,600

So you do not have the same 256,000 in all cases.  Again from my paper, the only way to calculate the correct EIGRP metric is by using the formula.

Regarding the rounding issue, this occurs in the router when you determine the BW.  Recall from the formula,

BW = (10,000,000 / interface bandwidth)

If we have an interface bandwidth of say T1 (1544kbps) with a default delay of 20,000 microseonds.  We would get the following metric (no rounding).

BW = (10,000,000 / 1544) = 6476.68394

Delay = 20,000 / 10 = 2000

Metric = 256 (BW + Delay)

Metric = 256 (8476.68394)

Metric = 2170031.09

but this is incorrect.  The correct metric is

BW = (10,000,000 / 1544) = 6476  <---rounded down

Delay = 20,000 / 10 = 2000

Metric = 256 (BW + Delay)

Metric = 256 (8476)

Metric = 2169856

This was really fun and I am glad you learned some valuable information.

Brian

• ###### 11. Re: The truth about EIGRP FD Calculation

Brian is right, again; (nothing new here, right ?)

Strange but Metric is not done the way CCNP books say it does or the way we interpret it.

i did some testing on my own, will post results later today;

• ###### 12. Re: The truth about EIGRP FD Calculation

What was I smoking? thanks Brian you are right. I clearly can't count and thankfully I'm not an accountant for a large corporate

The 10^7/1544Kbps+total-delay results in a non integer value, so I would totally understand the router rounding this type of value given that we know the FD and AD are represented as integer values only.

The router rounding on those integer values 25,600 and 256,000, I'm yet to get that.. Take toplogy #2 for example;

40901120 would be rounded down to 40645120 that just doesn't make sense to me.

There is a pattern with the 256 and decimal place shift corresponds with the length of the FD metric.. not sure what that means if anything

With you having corrected those calculations I'd attempted earlier, I agree that the EIGRP metric formula is the only accurate FD calculation and the simple calculation suggested in the Cisco material has no merit yet.. well it isn't obvious right? Well this is why a number of us have raised this issue in the first place, hence the discussion in play and an attempt to find sanity behind it.

I can see you have already done a great deal of good work on this subject, so thank you for sharing those findings.

Has been fun and a pleasure so far!

Cheers Dave

• ###### 13. Re: The truth about EIGRP FD Calculation

I think you may be confused on the "rounding".  The rounding happens within the EIGRP calculation when you try and find the value for BW.  Recall:

BW = (10,000,000 / interface bandwidth)

It is in this calculation that you will often find a decimal portion such as in T1 and fractional T1 circuits (10^7 / 1544) or (10^7 / 64)  these produce non integer values.

In regards to the Topology #2 example, I do not recall anyone mentioning that the value 40901120 is rounded down to 40645120.

The fact that your examples show a pattern regarding 256 times some multiple of ten, is purely coincidental.  Take for instance this topology:

R1 <---1544kbps---> R2 <---64kbps---> R3 <---100Mbps---> 10.1.1.0/24

Delays:
R1 to R2 = 20,000 microseconds
R2 to R3 = 20,000 microseconds
R3 to 10.1.1.0/24 = 100 microseconds

The individual link metrics are as follows:

R1 to R2 = 2169856
R2 to R3 = 40512000
R3 to 10.1.1.0/24 = 28160

I will leave the math to you to verify.  The advertised metric to the destination network 10.1.1.0/24 from R2 is its FD to the destination network.  In this case, we can calculate a Metric of 40514560 using the EIGRP formula.

Now if we take the case in the book where it says you "add" the advertised distance (AD) of the upstream neighbor to the cost to reach the next-hop neighbor.  We get the following metric:

Metric = (metric of R1 to R2) + (R2's AD) = 2169856 + 40514560 = 42,684,416

However, the true metric is 41,026,560.  For a difference of

42,684,416 - 41,026,560 = 1,657,856.

So, clearly not related to 256 times some multiple of ten you were getting.

Hope this helps.

Brian

• ###### 14. Re: The truth about EIGRP FD Calculation

Its just that the Routers don't do any floating point math when calculating the EIGRP metric.

Regards

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