Hi,
Impossible. If You have only default route from provider B, all traffic will go to existing routes, except illegal traffic without destination. For example, port scanning.
BGP isn't for load balancing, it's for traffic management. You'll need to plan what traffic destination to be sent to which provider. If no, then one provider will be better to almost all the world.
Anyway, You'll need to ask second provider for real routes. It can be full table, or something smaller. For example, Your country inside routes from direct peers between local providers. And also You'll need to ask provider to announce Your routes to these links too. Some providers give possibilities to control outgoing routes through route tags, You can ask provider for it. Some routing paths can be assimetric, but mostly it's better to keep it simetric. Anyway, it will be nat a real balancing.
No one will give You universal recipe for traffic and route management. It's individual an depends from Your providers, Your need, Your possibilities, Your wishes.
Kind Regards,
UgisD.
