By knowing what IP ranges the smaller ISPs own. If a small A connects to bit B, and B knows that A has IPs 123.234.., then it should not accepts routes that route other IPs to A.
It's not that simple, an ISP that has multiple peering partners doesn't just accept traffic for its own IPs, but also for those of its peering partners (and their peering partners, recursively).
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By knowing what IP ranges the smaller ISPs own. If a small A connects to bit B, and B knows that A has IPs 123.234.., then it should not accepts routes that route other IPs to A.
It's not that simple, an ISP that has multiple peering partners doesn't just accept traffic for its own IPs, but also for those of its peering partners (and their peering partners, recursively).
and it knows A has those ips by...
:)