BGP circular routing

Hi,

I guess this is not related to bgpd, but I hope there are skilled
network admins here who can give me advice.

I have a problem with circular routing on a site which talks BGP with
two upstream providers, with traffic to site which has static default
route over third ISP:

--> ISP1 --> ISP3 -->
SITEA SITEB
<-- ISP2 <-- ISP3 <--

I tried to prepend self / neighbor to ISP2 - no change (ISP1 has best
routes for 99% of the prefixes, including to SITEB). I contacted ISP2,
they said the problem is with ISP3. I contacted ISP3, they said ISP2
announces my prefix (they're my LIR) so the best route is over them. I
contacted ISP2 again, they said they prepended my prefix to ISP3, but
situation is the same.

Is it OK for ISP2 (my LIR) to announce and prepend my prefix? I thought
I should be in control of that.

Is there anything I can do about the situation?

Thank you in advance,

--
Before enlightenment - chop wood, draw water.
After enlightenment - chop wood, draw water.

Marko Cupać
https://www.mimar.rs/