EIGRP Feasible Distance Query !

[Vaibhava Varma]

Dear All

I was reading some documents recently and got confused by below statement regarding the FD Calculation

"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)."

What this implies is that

FD = [Cost between the local router and the next-hop router] + (AD)

I think FD is rather calculated as below which even the IOS output confirms

FD=256* (( 10^7/Lowest BW along path in Kbps) + ( Cumulative Delay in microsec/10) )

Can someone please double confirm me on this.

Regards

Varma

[r.shummoogum]

IMHO: Both statements are correct. The latter one is a manual way of calculating whereas the former one is how it is done in a real scenario as there is no way for a router to know the min B/W unless advertised by a neighbor chain.

[cadet alain]

Hi,

Both assertions are true, the AD is calculated as you mentioned for each attached prefix and the FD for not attached prefixes and sent as AD to neighbours.

when a router receives an update for a non attached prefix with an AD it calculates its metric to the neighbour advertising the route and then add the AD to get the FD.

Regards.

Alain.

[Vaibhava Varma]

Hi Alain and r.shummoogum

Thanks for sharing your views. I am still confused to understand how both assertions are true as I am not getting the same FD Value using both the assertions. The Cisco IOS is showing the FD Value as per Assertion 2

FD=256* (( 10^7/Lowest BW along path in Kbps) + ( Cumulative Delay in microsec/10) )

I used this scenario as below

R3---S1/0-----S1/0--R1---S1/1----S1/0--R2

The Link between R1 and R2 is configured as 192.168.2.0/30 and when I check in the EIGRP Topology Table of R1 it shows metric as 2169856 per above formula where BW=1544 and Delay=20000

R1#sh ip eigrp topology 192.168.2.0

IP-EIGRP (AS 100): Topology entry for 192.168.2.0/24

  State is Passive, Query origin flag is 1, 1 Successor(s), FD is 2169856

  Routing Descriptor Blocks:

  0.0.0.0 (Null0), from 0.0.0.0, Send flag is 0x0

      Composite metric is (2169856/0), Route is Internal

      Vector metric:

        Minimum bandwidth is 1544 Kbit

        Total delay is 20000 microseconds

        Reliability is 255/255

        Load is 1/255

        Minimum MTU is 1500

        Hop count is 0

R1#

Next when I check on R3 the EIGRP Topology Table shows as 2681856 per above formula where BW=1544 and Delay=40000. I am not seeing here the FD based on Assertion 1 where

FD = [Cost between the local router and the next-hop router] + (AD)

If the above assertion would have been used then

FD=2169856 ( I belive cost to the next-hop router would be caluclated taking BW=1544 and Delay=20000) + 2169856 =4339712

But its not so. Can you please help me to understand the difference for same.

R3#sh ip eigrp topology 192.168.2.0

IP-EIGRP (AS 100): Topology entry for 192.168.2.0/24

  State is Passive, Query origin flag is 1, 1 Successor(s), FD is 2681856

  Routing Descriptor Blocks:

  192.168.1.1 (Serial1/0), from 192.168.1.1, Send flag is 0x0

      Composite metric is (2681856/2169856), Route is Internal

      Vector metric:

        Minimum bandwidth is 1544 Kbit

        Total delay is 40000 microseconds

        Reliability is 255/255

        Load is 1/255

        Minimum MTU is 1500

        Hop count is 1

R3#

Regards

Varma

[cadet alain]

Hi,

I explained myself badly.

the prefix 192.168.2.0 on R1 has a FD of 2169856 per the formula and an AD of 0 as it is directly connected to R1 then when advertised to R3 it is the AD of the prefix.Now when R3 receives this prefix it compares its bandwith to the min bandwidth advertised by R1 for this prefix and sees its the same, it sees the dealy advertised by R1 and so to derive the FD it uses the formula with the bandwidth still to 1544 and add its own delay to the advertised delay and so derives the FD.

I think this is more clearer now.

Regards.

Alain.

[Vaibhava Varma]

Hi Alain

So this implies that FD is calculated independent of AD and will always use assertion 2 and not the assertion 1 listed below

FD = [Cost between the local router and the next-hop router] + (AD)

Regards

Varma

[r.shummoogum]

FD = [Cost between the local router and the next-hop router] + (AD)

FD= X + (AD)

In other words:

X = 256 *delay(in tens of usec) if Bandwidth between next hop is not the minimum bandwidth.

So in your case the delta is 512000 === 256 * 2000

[Vaibhava Varma]

Hi

I understand the delta from above explaination but when I tried to check it in lab using below setup FD still does not match Assertion 2 as explained above but only Assertion 1

R1--S1/0-----S1/0--R2---F0/0-----F0/0--R3---F0/1----F0/0--R4

The EIGRP Topology table on R1 still shows the FD for 192.168.2.0 to be calculated using assertion 2

FD=256* (( 10^7/Lowest BW along path in Kbps) + ( Cumulative Delay in microsec/10) )

ie

FD =256 * [ (10^7/1544) + (20000+100+100)/10]

     =256*[6476+2020]

     =256* 8496

     =2174976

R1#sh ip eigrp topology 192.168.2.0

IP-EIGRP (AS 100): Topology entry for 192.168.2.0/24

  State is Passive, Query origin flag is 1, 1 Successor(s), FD is 2174976

  Routing Descriptor Blocks:

  192.168.0.1 (Serial1/0), from 192.168.0.1, Send flag is 0x0

      Composite metric is (2174976/30720), Route is Internal

      Vector metric:

        Minimum bandwidth is 1544 Kbit

        Total delay is 20200 microseconds

        Reliability is 255/255

        Load is 1/255

        Minimum MTU is 1500

        Hop count is 2

R1#

Here Assertion 1 per above explaination does not matches the FD being shown on IOS

FD = [Cost between the local router and the next-hop router] + (AD)

FD= X + (AD)

FD =256 * [ (10^7/1544) + (20000)/10] + 30720

     =256*[6476+2000] + 30720

     =256* 8476 + 30720

     =2200576

Regards

Varma

[r.shummoogum]

Feasible distance is the best metric along a path to a destination network, including the metric to the neighbor

advertising that path.

I think you can scrap the following definition: It is a wording issue

FD = [Cost between the local router and the next-hop router] + (AD)

[dslice · Accepted solution]

Where did you read this? They sure confused matters in two ways. First, you are absolutely right that the metric calculation is based on the values the calculating router has in his table for the prefix, using min bandwidth and cumulative delay. This doe not directly factor in what the neighbor has as his metric (though the neighbors reported delay is added to your interface delay to get cumulative and his reported bandwidth is compared to the interface bandwidth to get min bandwidth.). You don't take his calculated value and add it to yours or anything like that.

The second way it confuses things is by using the wrong phrase, thus the wrong acronym. It's not really known as Advertised Distance (AD). It's called the Reported Distance (RD). By calling it AD, the author leads to confusion between this attribute and the Admin Distance. Very bad wording.

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