Hi Ricardo,
If link between R1-R2 fails as you explained on your book and R1 would try to choose a new next hop toward 11.234.0.0/24. It will first try to choose the path with the best metric, which is via R4 (3584) but since R4 Reported distance is not less than the FD for 11.234.0.0/24 (R1 perspective) it will not suffice the feasibility condition and it won't use it right?
Exactly right. What would happen is this: Because this path appears to be the best available after the R1-R2 link goes down, but R4 does not meet the Feasibility Condition using R1's current FD, R1 will go Active and send out Queries. It will not start using R3 right away.
The Query sent by R1 will contain the R1's distance through its old successor, R2, after the topology change took place - but because the link has failed, this distance is in fact infinity. The Query will be received by R3 and R4. R3 will not be affected by this Query - it does not use R1 to reach the destination - so it will respond right away with its existing distance of 1280. R4 is, however, affected by this Query. The distance in this Query causes R4 to lose its current successor (from R4's viewpoint, R1 does not meet the Feasibility Condition anymore), and because it has no other feasible successors to promote to a successor role, it will formally go Active - but because it has no other neighbors at all, it will return to Passive immediately, knowing that the path has effectively been lost. R4 will therefore respond with infinity, and remove the network from its routing and topology table.
R1 will collect all Replies, reset its Feasible Distance to infinity, and because the resulting shortest path is now available through R3, R1 will start using R3 as the next hop to the network while also setting FD to R1's new true distance of 5120. Because R1's true distance has changed from the infinity to 5120, R1 will send an Update, at which point R4 will learn about this network again from R1.
Now, if FD was not an historical value it will cause a routing loop because the RD from R4 (3072) will be less than the CD via R3 (5120) and now the FD (supposing that the FD was not an historical value). Is that right?
Yes - if FD was not a historical but rather a current distance then with regard to loop prevention, it would become entirely lost. EIGRP would effectively degrade to RIP. If the R1-R2 link was lost which would cause the distance to go to infinity then just about any neighboring router would be fit to be chosen. R1 could in that case indeed choose R4 as the next hop, because at the very moment R1-R2 link failed and no one except R1 knows about this, R4 still has no clue, and its distance information at that point in time is outdated. This is exactly what naive routing protocols like RIP do: "Does the neighbor appear to provide a shorter path? If yes, let's use it! And - oh... does that distance information from the neighbor already take the changed topology into account? There's no way to know, so don't bother." - and you end up having transient routing loops.
So, the purpose of keeping the FD as an historical value is to make sure that if we lose our best metric next-hop router to a destination the only way to know if there is no routing loop is to compare the RD from the neighbors against the FD (which is/was the lowest metric to a network at some stable point in time).
Exactly.
Let me try to use a different example. Imagine that you are a reseller of precious goods - paintings, sculptures, art in general. You have your network of peer resellers from which you can purchase these goods, and to which you can resell them afterwards. Each of them offers a particular piece for a different starting price (Reported Distance), and depending on the level of your business relations with that partner, there is a price increase for purchasing that particular piece from this partner (link cost - please note that for the purpose of this example, the profit margins are additive constant costs, not percentages). The starting prices and profit margins may change over time.
What you absolutely want to avoid is purchasing an item you have already purchased and resold at some point in the past, and resell it in such a way it gets back to you again. If you just purchase it and won't be able to resell it again, you will generate a loss, not even a balanced revenue, because now you're buying the same piece possibly for more money than for which you have sold it the last time. And if the commodity goes in circles round and round, its price grows continuously, so that it may happen that it becomes so expensive you cannot afford to buy it at all.
As an example, consider this output: A, B, C, D are four resellers, both have 1000 bucks as their starting capital, and each one of them uses a profit margin of 50 bucks. Reseller A has a commodity that costs 200 bucks in the beginning, and it sells it to B for 250 bucks. B resells it to C for 300 bucks, C resells it to D for 350 bucks, D resells it to A for 400 bucks, and so one. Notice how each of the resellers is continuously poorer than before when the commodity gets back to him after the full round before the reseller can sell it again, and around line 22, some of the resellers go broke as their balance becomes negative:
Price = 200, A:B:C:D = 1000:1000:1000:1000
1: A sells to B for 250, end state: A:B:C:D = 1250:750:1000:1000
2: B sells to C for 300, end state: A:B:C:D = 1250:1050:700:1000
3: C sells to D for 350, end state: A:B:C:D = 1250:1050:1050:650
4: D sells to A for 400, end state: A:B:C:D = 850:1050:1050:1050
5: A sells to B for 450, end state: A:B:C:D = 1300:600:1050:1050
6: B sells to C for 500, end state: A:B:C:D = 1300:1100:550:1050
7: C sells to D for 550, end state: A:B:C:D = 1300:1100:1100:500
8: D sells to A for 600, end state: A:B:C:D = 700:1100:1100:1100
9: A sells to B for 650, end state: A:B:C:D = 1350:450:1100:1100
10: B sells to C for 700, end state: A:B:C:D = 1350:1150:400:1100
11: C sells to D for 750, end state: A:B:C:D = 1350:1150:1150:350
12: D sells to A for 800, end state: A:B:C:D = 550:1150:1150:1150
13: A sells to B for 850, end state: A:B:C:D = 1400:300:1150:1150
14: B sells to C for 900, end state: A:B:C:D = 1400:1200:250:1150
15: C sells to D for 950, end state: A:B:C:D = 1400:1200:1200:200
16: D sells to A for 1000, end state: A:B:C:D = 400:1200:1200:1200
17: A sells to B for 1050, end state: A:B:C:D = 1450:150:1200:1200
18: B sells to C for 1100, end state: A:B:C:D = 1450:1250:100:1200
19: C sells to D for 1150, end state: A:B:C:D = 1450:1250:1250:50
20: D sells to A for 1200, end state: A:B:C:D = 250:1250:1250:1250
21: A sells to B for 1250, end state: A:B:C:D = 1500:0:1250:1250
22: B sells to C for 1300, end state: A:B:C:D = 1500:1300:-50:1250
23: C sells to D for 1350, end state: A:B:C:D = 1500:1300:1300:-100
So how can you make sure this does not happen? Imagine that the least historical starting selling cost for which you have been able to sell a particular piece by Picasso was 10K bucks (the Feasible Distance). The claimed prices can change, of course, and at the present time, your two neighbor resellers X and Y state that a similar Picasso can be purchased from them for a starting price of, say, 8K bucks and 12K bucks, respectively, while the profit margin is 10K for X and 1K for Y, so the total prices would be 18K for X and 13K for Y. These total prices would become the starting selling costs for you should you decide to purchase that piece from any of those neighbors.
The reseller Y with the total price of 13K seems to be perfectly nice, but its own price of 12K seems suspicious. There's a real possibility - though not a guarantee! - that at some point in the past, it may have bought the piece from you when you offered it for 10K starting cost (you never offered it for less) and with a profit margin of 2K, then he waited speculatively, and now he's making a seemingly perfect offer to you. Are going to you like this? I don't think so.
The reseller X, on the other hand, could never have purchased the piece from you, as its current cost is 8K and you have never, ever, offered it for less than 10K. Wherever he got that piece, one thing is for certain - it's not from you.
And that's how the Feasible Distance works ;)
As always, feel welcome to ask further!
Best regards,
Peter
