In a bad actor sits at the only cross road to split the network in halves (theoretically, this is unrealistic in practice due to internet) that is called an "eclipse attack". In Bitcoin both sides will know that because the Hashrate would seem unrealistic for both sides.

Also, the Bitcoin gossip network fulfills the "Continuous Byzantine Agreement". If everyone signs the message that they forward - malicious actors cannot fake messages. This so called "Dolev Strong Protocol". This makes malicious actors easy to identify and ignore. Honest actors establish p2p channels only with other honest actors.