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If ISP (A) can be routed to ISP (B), and ISP (B) can be routed to ISP (C) and vice versa, then why ISP (A) and ISP (C) can't be routed through ISP (B) to each other?

The packets are routed like this from ISP (A) to ISP (C):

A---D---E---F---C (it goes to a far away locations before getting back)

Semphie94
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Because ISPs choose the most cheapest paths for them. If routing traffic to the ISP (C) for ISP (A) is cheaper through ISP (D) (which is probably a big company with really big links, which sells the traffic in significant volumes and because of that have lower prices) than by ISP (B) (which can be a pretty small ISP with small resources, so they traffic price is higher) they will do it.

Basically that depends on agreements between companies, which are based on price.

Actually, you CAN route the traffic through ISP (B), but for that you need to set up a proxy/VPN server on ISP (B) and create the chain by yourself.

Evengard
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  • Makes sense, but still if ISP (A) can route to a server in ISP (B) Directly, then it should do the same to ISP (C). because ISP (B) has a direct path to ISP (C), so why can't ISP (A) uses ISP (B) to cross the bridge to (C)? instead of going overseas to a far away countries then back with a high latency/ping. – Semphie94 Aug 20 '15 at 12:41
  • Money. :) They have indeed route to ISP (B) probably because they just... Don't have other route to it - so it cames as the only one possible for them. But when they have multiple routes to ISP (C) they will choose the cheapest. – Evengard Aug 20 '15 at 14:07
  • Great answer. But why ISP (B) chose a direct path to (C) if they can do the same as ISP (A)? – Semphie94 Aug 20 '15 at 14:21
  • Imagine this - ISP (B) is connected to ISP (A) and ISP (C), and only through ISP (A) and ISP (C) they are getting their internet - for example, to distribute it for their clients. They don't want to waste their links on something other than distributing internet to their clients, so they deny the transit traffic from ISP (A) to ISP (C), but they are using ISP (A) and ISP (C) connections as a mean to reach the Internet as a whole. – Evengard Aug 20 '15 at 14:40
  • They are using two connections because, well, using one of them is too risky - what if ISP (A) goes down, if they are connected only to ISP (A) they will loose all internet access - not a good thing for an ISP! - and while they have two connections, they can use them to route traffic across both of them - because some destinations will be cheaper via ISP (A), and others - via ISP (C). – Evengard Aug 20 '15 at 14:43
  • OMG. I don't understand a thing... In other words, you're saying that ISP (A) has the ability to route their traffic to (B)---(C), but they configured their router to choose a far away ISP when connecting to (C) because it's cheaper that way? And ISP (A) can connect to ISP (B) directly because they offered a cheaper route to them? And ISP (B) agreed on the price of (C) route even though it was expensive? Is that how it works then? – Semphie94 Aug 20 '15 at 15:05
  • That's a little messed up, lol. ISP (A) and ISP (C) are both offering connection to ISP (B). ISP (B) on their router denies transit traffic. Imagine now that there is an ISP (X) and an ISP (Y). For ISP (B), going to ISP (X) through ISP (A) is cheaper, than through ISP (C). But when it wants to go to ISP (Y) it is where ISP (C) is cheaper than ISP (A). So they are using both, picking one of them for different destinations. – Evengard Aug 20 '15 at 15:10
  • Duuuuuh... I never been super confused in my life like now. Please make it more simpler than that, my brain doesn't function very well atm. So if (A) already has (B). And (A) wants to go to (C), it'll pick whatever route because (A) doesn't want (B) because (A) refused to buy (B) route to open the gate to go to (C)? Please tell me this is true. I can't take this anymore. – Semphie94 Aug 20 '15 at 15:29
  • Yeah. That can be like that. Real life is more complicated, but that's indeed one of possibilities. – Evengard Aug 20 '15 at 15:33
  • I hope you're not exaggerating. Thank you so much for wasting your time here for me anyways. It really appreciated! – Semphie94 Aug 20 '15 at 15:56
  • OMG! I understand now. I misunderstood something here: ISP (B) has a peering with A and C. And ISP (A) can't use (B) route to go to (C) because (B) is not a transit provider!! lol – Semphie94 Aug 20 '15 at 16:56
  • That's why (A) uses Transit provider to connect to (C) because there's no peering between (A) and (C)!! – Semphie94 Aug 20 '15 at 16:57
  • YEAH! That's it! – Evengard Aug 20 '15 at 17:00
  • Wow. I feel so ecstatic now! – Semphie94 Aug 20 '15 at 17:07