BGP why do we need an IGP for BGP to work

When learning about BGP the fact that an Interior Gateway Protocol (IGP) or static routing is needed to make sure BGP functions correctly can be confusing.

Because of iBGP split horizon we require a full mesh of iBGP neighbor adjacencies. This means that every iBGP routers has to become a neighbor with every other iBGP router within the AS.

How do these iBGP routers reach each other? by using an IGP. It's also common to use loopback interfaces for the neighbor adjacency. Loopback interfaces only go down when you shut them. When a physical interface goes down, the IGP can find another path to the loopback interface.

Links

https://networklessons.com/bgp/internal-bgp-border-gateway-protocol-explained/