A IDEAL FEDERAL SYSTEM : mutal trust and agreement to live together.
The above two aspects are essential for institution and practice of federalism and govts. should agree to some rules of power sharing and trust each other that each will abide by the agreement.
The balance of power between the central and the state govt varies from one federation to another.This balance mainly depends upon the historical context in which the federations were formed .

Two routes of formation of federeations:
COMING TOGETHER FEDERATION VS HOLDING TOGETHER FEDERATION
1 when independent states come together on their own to form a bigger unit-by pooling their sovereignity and retaining identity they can increase their security.
1.When a large country decides to divide its power between the constituent states and the national govt.
2.US, Switzerland, Australia
2. India, Spain , Belgium
3.All constituents states have equal power and are strong vis-à-vis the federal govt.
3.Central govt. tends to be more powerful vis-à-vis states. Very often contituent units have unequal powers.Some units are granted equal powers.