Trade Agreements effects - Please Explain?

Does anyone have a good understanding in regard to Trade Agreements and their effect?

Trade Routes are setup automatically by the game. Roads increase efficiency of movement and consquently increase Income. Ports allow for Sea Trade. Building a Trader, and the subsequent upgrade buildings, further increases Income.

I still have yet to explore Diplomancy in the game. I was perusing this thread, Trade - How Exactly? . In it is conflicting and incomplete information re Trade Agreements.

Can anyone clarify Trade Agreements and their effects?

Thank you.

I'm fairly sure that initially trade is only done between cities with a port that you own (that are in close proximity). A trade agreement with another faction means you can trade with ports that they own. Note that each level or port affects how much trade fleets you can have per province, I think the max is three (which is good, no more MTW style trading with every port on the map).

Attaching trade agreements to ceasefires is a good tactic... I managed to get a ceasefire with the creeks that gave me trade rights, and a regular tribute of 1,000 denarii for 6 turns! (in exchange for some map info as well)

Trade agreements give you the ability to trade with that nation. Well duh, but since each of your cities has a limited number of fleets it can send out based on the size of it's port it pays to have the most options. If war disrupts trade to one city I believe the AI will adjust and switch to other options.