Advertised Distance(AD) vs Feasible Distance(FD)

Sa'ad

Hello Mates,

confused with the difference between AD and FD the new BSCI book gives a some what confusing definition that sounds the same for both.

It says AD is the cost between the next hop router and the destination.

And FD is the sum. Then that would mean to me they're both the same thing.

I'm confused. Please someone help me understand this. Thanks in Advance.

EdTheLad

FD is the local cost to the destination network.To reach a destination you route packets via
a next-hop router i.e. down stream router or in eigrp terminology,the successor.The downstream router also has a local cost i.e. FD to reach the destination.When a downstream router advertises its FD to an upstream router,this advertised FD is known as the AD on the upstream router.
So basically to get to network x my local cost is 5 i.e. my FD is 5, my next hop router B who advertises this network is closer to the destination x and has a FD of 3.When an update arrives from B with a cost of 3 this is know locally as the AD,the cost configured on
the interface connected to B is 2,so adding the AD and the cost of the link to B i get the FD of 5.
A little long winded but i think you should get the idea.

NightShade1

In addition to what our mate said above:


This is simple

FD=fiasable distance

AD=advertise distance

AD = distance from the neighbor router to the destiny

FD = AD+the cost between your router and his neighbor

is simple as that
Simple example

R1

---

R2

---

R3

---

R4

Cost from R1 to R2 is 4

Cost from R2 to R4 is umm 8 so AD is 8 cause is the cost from the neighbor of R1 to destiny

FD = 4+8=12

in this example i just gave you a uniroute from R1 to R4 but normally there will be many routes with many differets FD, the router pick the one with lower FD and put it in the routing table.
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Sa'ad

Man u guys are the best, especially I loved it how nightshade1 has it illustrated for me. Thanks a bunch fellows.

NightShade1

You are welcome buddy.

If you google a bit you will see better graphics and examples of this.

optimus

You studying the new book also eh? I am on the lab on page 148 now. I set the lab up. Looking at all the debugging information for EIGRP when a route change occurs is going to take me a little time to understand. The next chapter OSPF I dove into a little bit next. I hope a new Exam Cram book or something comes out so I can see what kind of questions they are looking to ask.

Good luck with your studies.

Pari

Thanks a ton EdTheLad & NightShade1. i was very confused before i read you post.

Danielh22185

I'm Curious what new book you are studying...