Why IBGP does needs a full mesh

Question

Why IBGP does needs a full mesh?

Answer 1

BGP employs the AS_PATH attribute for loop detection. When a router sees its own AS number in a BGP advertisement, the advertisement is dropped down. IBGP routers have similar AS number thus the AS number can’t be employed for loop detection. IBGP neighbors will not promote prefixes learned from one IBGP neighbor to the other IBGP neighbor; thus, a full mesh is needed.