What are the two advertising methods for default BGP routes?
1, peer xxxx default-route-advertise is mandatory advertising, and the default route will be advertised regardless if the local has the default, and it is only for a single neighbor.
2, import-route xxx(import the default route of other IGP protocols) advertises the default route with the help of default-route import (or network 0.0.0.0). In fact, it is not mandatory advertising. When import-route command imports the route into BGP, BGP will advertise the default route.