Let me offer an example

Assuming we have a basic hosting topology ( 2 BGP routers, a couple of ISPs, virtualization, etc ) and 2 /24 to go with it.

Some customer has some virtual machines in my virtualization with public IP's.

Now, the problem is :

The first BGP router has 2 ISP's, the 2nd has different 2 ISP's.

My client complains that he cannot connect to some of his machines which are using a different ISP which I do not have in the datacenter.

It is quite weird, because he can telnet to his hosts but he cannot send data through the port (oracle tnsping) .

The only fix I found was statically routing his destinations through the secondary BGP router.

But it lasts for a week then he calls me again and I have to remove the static routes and that fixes the problem for another week/month.

This looks like some issues between the ISP's or some asynchronous routing, but where?

Any feedback is valued, thank you.