If you are referring to the nodes, you start with your base. There are nodes close to each base that allows for a somewhat easy task of capturing, but that really depends on how the server admin sets up the nodes.
As I mentioned before, you could play the same map but each time you play it, the objectives could change. Just because a node is close to your base doesn't mean it is connected to other nodes, so capturing it will be futile. But, the next time you play it, the node you dismissed in the previous game is now connected and requires you to capture it. What makes Onslaught fun is the objective to capture all (or some) of the nodes before you can destroy their base.
It really depends on how the server admin sets up the nodes. The game does have a default setting for the connections, but by no means is it considered an extremely easy task. Sure, a team can capture nodes closest to their base, but that doesn't mean the connections are valid. Fighting doesn't migrate to the middle, per say, since a team can disable any node to cut off the connection.