What is the exact consensus protocol Ripple uses?
If the Ripple network splits in half, then each half will validate ledgers independently, potentially with conflicting transactions. If all servers are properly configured, they will not accept any of these ledgers as fully validated, since none of them will have sufficient validations. It's possible both sides will think they're in the minority (if the split is close to 50/50).

This reflects a basic Ripple design principle -- don't tell people they can rely on results if the results are unreliable. If conditions make reliable operation impossible, it is preferable not to operate than to give people results they cannot rely on. Ripple is designed to detect these kinds of conditions.

When the network rejoins, servers will see the other ledger chain. They will gradually avalanche to the chain with more validations until one chain or the other has a super-majority of trusted validators. At that time, the network is back in agreement and everyone can again rely on the results of transactions.