I have two transit providers, A and B. The business has chosen to make provider A more attractive due to billing concerns (we get attractive bulk pricing from this global provider). This was done by a combination of AS pretending for ingress and local preference. As expected a bulk of the traffic goes through A unless it has an issue.

There is an obscure ISP that we need to reach which happens to take a substantially better route through provider B. While I can certainly alter the traffic on egress, on ingress BGP will just "do it's thing" and the prepended route will look less attractive meaning return traffic will go through provider A.

My goal is to have most of the traffic go through provider A (preferred) except for this one AS.

How can I engineer around this problem?

Edit: I apologize I did not provide the router model as I preferred to frame this as a generic routing question. The router is a Juniper MX204. The part I'm really stuck on is how can I ensure return traffic, only for this ASN goes through provider B and not A.

  • There are several things you can do to change the preference for a prefix, but it depends on the router model and software, but you have not given us that. – Ron Maupin Jul 9 '20 at 21:05
  • @RonMaupin - I edited the question. Can you reinstate it please. I apologize for not providing clarity, I'm a bit new here. – N0CNija923 Jul 9 '20 at 21:24
  • "how can I ensure return traffic, only for this ASN goes through provider B and not A." You have no control over what other ASes do on the Internet. That is the point of an Autonomous System, it is autonomous. You do not want other ASes to tell you how to run your AS, and other ASes do not want you to tell them how to run their AS. You may be able to get the cooperation of your directly connected ISPs by entering into business agreements with them, but beyond that, you have no control. – Ron Maupin Jul 9 '20 at 21:27
  • Did any answer help you? If so, you should accept the answer so that the question doesn't keep popping up forever, looking for an answer. Alternatively, you can post and accept your own answer. – Ron Maupin Dec 17 '20 at 19:07

Short answer: you can't.

You are asking to route traffic not by the destination address, but by the source address. In other words, you want traffic to take a particular path based on its source address (i.e., sourced from obscure ISP). That's not how routing normally works.

It would be technically possible to make this happen using policy-based routing, but that would require the cooperation of every ISP along the path. Practically speaking, that would never happen.

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