This is a bit 'dangerous' question in that it assumes that the Roman frontier policy was somehow consistent, or conducted under specific instructions from above, such as the Senate or Emperor. In reality, there were almost as many different methods of dealing with 'barbarian' tribes as there were tribes (or rather, political units - towns, states, kingdoms etc.) in all of Europe, Africa, and Middle East that the Romans came in contact with. More often than not, the decisions about individual alliances or treaties would be made based on the judgement (or sometimes, on the whim) of the war general or provincial proconsul or whoever was the supreme authority in the field. I can think of some instances in the Roman imperial history where the Senate or the Emperor demanded to deal with the locals on the frontiers with specific methods, but this was always limited to a certain period in time or geographical region. Obviously, frontier situations in, say, the Scottish frontier and the Sahara frontier would be widely different, and thus required different methods of dealing with the locals.

There are, however, a couple of well-established Roman institutions linked to making alliances as treaties, such as the deditio (i.e. unconditional surrender of the enemy to Roman power, which gave the Romans the right to dictate the conditions of the ''alliance''), religious practices, and, of course, the Romans categorized their subject states and allies (note that Romans considered those tribes and nations that they were allied or in any dependent relationship with as belonging to their their imperium) into different groups, in terms of the rights and responsibilities they had in relation to Rome. Also, in lot of the frontier situations, treaties and alliances were sort of made with individual Roman generals/officials rather than with the state of Rome; say, one proconsul could draft a treaty with the condition that the local tribe didn't need to pay taxes to Rome, but then when the next year came along the new proconsul could, if it suited him, consider the treaty nulled because it was between the tribe and his predecessor and start levying taxes.

So, Roman foreign policy is extremely murky and complicated business. Lot of the time it's difficult to see any consistency or reasoning why some tribes would be required to perform a deditio when their neighbors were not. I am only very well informed on the Roman imperial frontier politics in 2nd century BC Spain, so, we're talking of the very earliest developments of Roman imperialism. Would be nice if someone who knows about frontier situations during the high Empire could chip in? And, I'm not sure if OP's referring to Roman Empire as an imperial state, or specifically to the era of the principate as opposed to the Republican era. But, if you're interested in the Roman dealings with Spanish tribes during the Republican expansionism, or some specific aspects such as deditio, religious dimensions of alliances etc. I can do my best to answer? I'm not sure what exactly what OP's means with 'HOW the agreements' were drawn up?