They generally do for their direct end-customers who announces their own address space. The problem is when peering with other Tier 1's and large ISPs, with mu

They generally do for their direct end-customers who announces their own address space. The problem is when peering with other Tier 1's and large ISPs, with multiple downstream customers. How do they keep and update a list of the other ISPs customers and the routes they should expect from them? Once someone manages to get a bogus route into a large ISPs table it tends to propagate further.