There are 3 ways of advertising default route in BGP.
Method 1: Using network 0.0.0.0 command.
It requires only that the route 0.0.0.0 is present in the Interior Gateway Protocol (IGP) routing table. This is the preferred approach.
Method 2: Using default-information originate command.
It requires explicit redistribution of the route 0.0.0.0. This protects against someone accidentally redistributing a default route in BGP which could potentially be disastrous.
Method 3: Using neighbor default-originate command.
This method does not require the presence of the 0.0.0.0/0 network in the routing table of the advertising router.