Hello Bruce
ISPs of all levels that administrate infrastructure that supports the Internet are assigned specific AS numbers, and they use them as they see fit within their network. Generally speaking, Autonomous Systems within ISPs are clustered together geographically, to a certain extent. You will have a network segmented into various different sections each with its own AS, and they’re interconnected using eBGP. Something like this, but to a much bigger scale:
But geography is not the only thing that affects the way these network sections are clustered. Network traffic patterns, interconnections with other ISPs, and interconnections with customers also play a role.
Now for a private enterprise, where private AS numbers are used, how you distribute your network depends on what you want to do. Typically, in all but the largest private networks, an enterprise will employ a private BGP AS at the connection to the Internet, where an AS is defined, and an eBGP connection is established with the ISP. If you have more than one branch, typically, you would have a different AS number at each branch, but you would still connect to the ISP, so you wouldn’t have direct interaction between the BGP Autonomous Systems.
The accompanying technology used often defines how BGP will be used. For example, MPLS uses MultiProtocol BGP configurations to function. Similarly, you can use BGP with DMVPN in either an iBGP or eBGP arrangement, where each one has its pros and cons.
The specific lesson was created to show how BGP behaves in various situations. As such, it’s rare that you would configure a BGP setup similar to the one in the lesson.
So you see, you can configure all branches to have the same AS, or different AS’es, depending on what you want to do and what accompanying protocols and features are being used.
First of all, if you want to use BGP between the sites, they must be directly connected somehow. But there is usually some infrastructure like MPLS or DMVPN between them. But if they were to be directly connected, then if they use the same AS, by definition, iBGP would be used. If you used a different AS, by definition again, eBGP would be used.
I hope this has been helpful!
Laz