Please tell me if my understanding here is correct:
- Tier 1s sell transit but never buy
- Tier 2s sell transit but also have to buy transmit
- Tier 3s buy transit but never sell
- Tier 2s and Tier 3s peer at IXPs, but only advertise their own prefixes (and those of their customers).
- Tier 1s have private peerings with each other, sharing their full BGP tables.
- Tier 2s have private peerings with each other, sharing their full BGP tables.
- Tier 3s and Tier2s buy transmit from Tier 2s and Tier 1s (respectively) advertising only their own prefixes (and that of their customers), but they receive full BGP tables from the transmit provider.
Is this about right?
If so, I have a few further questions:
- Do large Tier1/Tier2 carriers operate different BGP AS's for the local/regional services?
- Example: AT&T is a Tier 1, but they also sell DIA and business/residential broadband. Would AT&T operate a separate "Tier 3"-type network in each local market, where they peer with other local providers at IXPs, but only advertise the local "Tier 3" routes instead of their national/global prefixes?
- Do Tier 2s advertise their transit-customers prefixes at IXPs, or strictly their own prefixes & those of their Enterprise clients?
- Is there anything that prevents Tier 3 ISPs from buying transit directly from Tier 1s?
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