Border Gateway Protocol (BGP) refers to a gateway protocol that enables the internet to exchange routing information between autonomous systems (AS). As networks interact with each other, they need a way to communicate. This is accomplished through peering. BGP makes peering possible.
What routing protocols do ISPs use?
BGP is always used as the routing protocol of choice between different ISPs, which is known as external BGP. Large ISPs also use BGP as the core routing protocol within their own networks, which is called internal BGP.