Its not about how many peers you have, it is about who you peer with. The old-school definition of Tier 1 is that you can communicate with 100% of the Internet, ie: have "full routes" without paying for transit. Transit is defined as receiving routes to networks other than those within the AS used in the BGP session. Peers are simply networks with which you have BGP sessions, but not all BGP sessions are peers. Some networks have BGP sessions with their customers. Thus, a network with lots of customers who require BGP sessions will appear to have lots of peers. However, these are not settlement-free peering arrangements, but are sessions where the network is providing transit (full routes) to its customer, and do not add value in the form of additional routes to the network in question. ATDN appears to have BGP sessions with all the major networks. If none of those sessions are paid transit agreements, then yes, they are Tier 1. However, there is no qualitative difference between a network who fits the strict Tier 1 definition and a network who pays those networks for transit, or for the peering sessions themselves.
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