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Where do local ISPs' core networks connect to?

Do they connect to the Internet Backbone owned by AT&T and Sprint (among others) or do they connect to an IXP (Internet eXchange Point)?

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It depends. Ask the ISP. –  Jenny D Feb 6 at 7:52
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1 Answer

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Where do local ISPs core networks connect to? Do they connect to the Internet Backbone owned by AT&T and Sprint (among others) or do they connect to an IXP (Internet eXchange Point)?

Yes. They connect either to the "internet backbone" or to a local IXP. Or they connect to neither. Or even connect to both.

Which one they connect to will depend (to some degree) on the specific ISP in question, though the vast majority of local ISPs connect to an IXP, because that's what an IXP is - a point where different ISPs connect to exchange traffic between their networks.

Regarding the internet backbone... you are misinformed as to what it actually is and who "owns" it, but suffice to say, it doesn't really exist in the form that you seem to think. It's really just a mish-mash of Tier 1 Networks (and Tier 2 Networks).

Tier 1 networks are commonly defined as networks "that can reach every other network on the Internet without purchasing IP transit or paying settlements," and Tier 2 networks are defined as "networks that peer with some networks, but still purchase IP transit or pay settlements to reach at least some portion of the Internet."

There's a somewhat useful image at the Wikipedia page on Tier 1 Networks, that illustrates all the various ways the ISPs and assorted Tier # networks can connect with each other. (Pasted below)

As you can see, some Tier 1 Networks also have ISP capabilities and would therefore be "connected to the internet backbone" as you put it. Some connect to IXPs, some only connect to other ISPs within their Network Tier, and some even connect to both the "internet backbone" and to an IXP. So it's a fair bit more complicated than connecting to the internet backbone, or an IXP. It can be either, both, or even none.

enter image description here

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Is the "Internet backbone" a physical network or a logical/virtual network? Does this belong to the "Core Layer" of the Cisco hierarchical model? –  chris Feb 6 at 9:45
    
The model and hierarchy you're trying to apply isn't really applicable here. "The internet," and even the "internet backbone" are networks of networks. So, in most cases, these networked networks will contain an access layer, a distribution layer and a core layer, as well as both physical and logical/virtual components. The model you're referencing is meant to be applied to a single network, not to a network of networks like the internet, or the internet backbone. ... It might be constructive for you to tell us what you're actually asking or trying to figure out or do here. –  HopelessN00b Feb 6 at 9:58
    
It is safe to say that the Internet backbone is the exchange of traffic between Tier1, Tier2, Tier3 networks and IXP? "It might be constructive for you to tell us what you're actually asking or trying to figure out or do here" - I am learning BGP. –  chris Feb 6 at 10:29
    
@chris I would say the "internet backbone" is the Tier 1 and Tier 2 Networks themselves. You might find objections to including the Tier 3 Networks in that definition. As to "learning BGP"... I'm not sure how this helps that, but whatever works for you, I guess. –  HopelessN00b Feb 6 at 10:44
    
I do thank you for your answer, are you ccnp or ccie? –  chris Feb 6 at 10:45
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