1. The single biggest reason to consider using BGP between an enterprise and an ISP is to influence the choice of best path(best route).
2. Running BGP could mean that enterprise router requires significant memory and more processing power on the route . The design might also require other enterprise routers besides the Internet-connected routes to know the BGP routes, requiring additional routers to have significant CPU and memory resources. Although you can configure BGP to choose one route over another ussing PAs, the advantage of choosing one path over another might not be significant . Alternative , you could ask the ISP to advertise only a default route with BGP.
3. single-homed (1 link per ISP , 1 ISP)
with single-homed designs, only one possible next-hop router exists for any and all routes for destinations in the Internet. As a result , no matter what you do with BGP, all learned routes would list the same outgoing interface for every route , which minimizes the benefits of using BGP.
Dual-homed (2+links per ISP , 1 ISP)
The second link gives the enterprise a chiose. 1: To prefer one Internet connection over another for all destinations. 2:To treat both Internet connections as equal. To make one route be preferred , that static default route would be assigned a batter administrative distance(AD) than the other route.
To make a decision to use one path instead of another , an enterprise router must know about at least some of those routes . Exchanging BGP information for such a large number of routes consumes bandwidth . It also consumes memory in the routers and requires some processing to choose the best routes.
Single-multihomed (1link per ISP , 2+ ISPs)
Dual-multihomed (2+ links per ISP , 2+ ISPs)
4. To help reduce the memory requirement of receiving full BGP updates (BGP updates that include all routes), some ISPs give you three basic options for what routes the ISP advertises:
Default route only: The ISP advertises a default route with BGP , but no other routes.
Full updates: The ISP sends you the entire BGP table.
Partial updates: The ISP sends you routes for prefixs that might be batter reached through that ISP, but not all routes, plus a default route(to use as needed instead of the purposefully omitted routes)
with partial updates , the ISP advertises routes for prefixes that truly are batter reached through a particular link.