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Consider the following situation where we have a customer that is connected to two ISPs for redundancy enter image description here

Let's say that this customer has its own IP address space that it wants to advertise to those two ISPs and the internet using BGP. These customer-owned IP blocks are provided by the RIRs, right?

So how exactly would the ISPs ensure that the IPs that are being advertised to them were actually registered and are valid?

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As Ron has said, "very little". Some will do no validation at all. Others may only require a piece of paper - a signed "letter of authority". Ultimately, they have little option other than "blind trust". No matter how many databases they consult, they are trusting that you are who you say you are, and the address space you present is yours to handle.

(Having been the person doing this. I require that LOA - actually, legal required it. And I check WHOIS, and other various registries for ownership. But back then, few were actively stealing address space.)

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  • Another vote for LOA requirement and a check on whois tools for validation as well as requring the whois info to be updated and correct, not 'These IPs used to belong to so and so but they said we can use them and then went out of business so here we are'.
    – FrameHowitzer
    Commented Jun 14, 2023 at 18:08
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You may be surprised to learn that there is very little validation, and it varies wildly from one ISP to another.

RFC 7454 describes best practices for filtering prefixes received by peers. Lots of ISPs follow the recommendations, but many do not.

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