I do work for a small regional ISP and having some difficulties figuring out how to properly advertise a multi-homed customer's network to our peers. All routers are Juniper M/MX.
Customer X has their own ASN and IP addresses. say AS 65530
Customer X has an internet connection from ISP A and advertises their prefix via BGP to ISP A
Customer X has an internet connection from ISP B and advertises their prefix, with a couple of prepends, via BGP to ISP B (ISP I am doing work for)
ISP B is BGP peered with several tier 1&2 carriers. Lets narrow it to 2 for this question. Level3 (AS3356) and Sprint (AS1239)
In order to properly route traffic to Customer X, ISP B has to accept their prefix via the direct connection to them, as well as from Level3 and Sprint. When ISP B receives Customer X's prefix with a shorter AS-path from Level3 or Sprint, ISP B's router does not advertise it back to the respective Tier1 (to prevent loops of course).
So how do I make sure the Tier1 ISPs know that ISP B is a valid path to Customer X's prefix with Customer X still advertising directly?
ISP B could filter Customer X's prefix from their Tier1 peers, then the routers would advertise the prefix learned directly from the customer. But then if the customers link to ISP B fails, ISP B would have no other route to Customer X, so ISP B's other customers could not communicate with Customer X.
I don't recall coming across such a scenario in any of my training.
Any advice? I hope I was clear enough in my description. I can add a drawing if necessary.
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