Bug 2229563

Description of problem:
FRR should have a configuration option that allows BGP receive routing preferred to default routing.

Currently, FRR defines the route selection as below.
~~~
Route Selection
The route selection process used by FRR’s BGP implementation uses the following decision criterion, starting at the top of the list and going towards the bottom until one of the factors can be used.

Weight check

Prefer higher local weight routes to lower routes.

Local preference check

Prefer higher local preference routes to lower.

If bgp bestpath aigp is enabled, and both paths that are compared have AIGP attribute, BGP uses AIGP tie-breaking unless both of the paths have the AIGP metric attribute. This means that the AIGP attribute is not evaluated during the best path selection process between two paths when one path does not have the AIGP attribute.

Local route check

Prefer local routes (statics, aggregates, redistributed) to received routes.
~~~
ref. https://docs.frrouting.org/en/latest/bgp.html

In IPv6 environment, default routing is provided by IPv6 router advertisement.
However, some environments don't prefer to use default router than BGP routing.
For such environment, FRR should have an option to configure BGP routing is preferred than default routing.