I have a strange question because this is not very common. Lets asume the following scenario to simplify it:

net A -- RTA --- RT1-ISP1 --- IP Cloud --- RT2-ISP1 -- RTB -- net b --- RT1-ISP2 --- IP cloud --- RT2-ISP2 --

I'm having trouble with this, is RTA connected to both RT1ISP1 and RT1ISP2 and likewise RTB to RT2ISP1 and RT2ISP2? Also is there any connection between ISP1 and ISP2?

RTA and RTB are in differents ASs. ISP1 is preferred via BGP (local preference). Sometimes we lost reachability between net A and net B but we can stiil see the routes trough ISP1, so the BGP can not converge to ISP2. The problem is because the ISP1 have troubles with their IP-MPLS network, so, sometime they lost traffic.

Do you now about some trick that can be used to know that the path to net b is not good despite the route is still in RTA?

Actually, this is quite common that an ISP has connectivity problems but still advertises routes in BGP, its very annoying!

I was thinking to force the loopback from ISP1 to be learned in RTA to some route learned from RTB, but how. I think that if I make a tunnel interface it would still depend from BGP to work and BGP depend form tunnel, so it would not work.

Either tunnelling or perhaps multihop bgp .. I've not done this so cant offer you any config but I'm thinking that providing you can establish some sort of connection between RTA and RTB (and in my experience tunnel interfaces dont always notice when an intermediate link drops which is why I suggested BGP which will definitely notice) then if ISP1 has problems the packets will stop and the connection will drop forcing ISP2 to be used instead. What I've not thought about is that you will need to make sure that however you configure this the connection above will need to be forced to run over the ISP1 link only and it must fail when ISP1 fails otherwise if it reverts to ISP2 in the event of a failure and your forcing traffic out on ISP1 then you'll have no connectivity.. off top of my head you'll want to use addresses (eg P2Ps) that ISP1 sees but are not known to ISP2.

Any idea that do not imply to change ISP1 nor use ISP2 as default?

Ditching ISP1 or at least complaining heavily to them would be the sensible thing, they're affecting your business's ability to function.. Steve

Thanks in Advance, -as