ISPs, tiers, transiting, and the Internet

The Internet has a hierarchical structure. Within its structure, we have Tier 1, Tier 2, and Tier 3 networks. The following diagram shows an example of how these interconnect:

There are rules as to how traffic can be exchanged from one Internet Service Provider (ISP) to another. Specifically, they have to do with how each tier ISP handles transit traffic.

• *Tier 1 networks must route all traffic that they receive, to any other connected network.
• *Tier 2 networks must route all traffic that they receive, to any other connected network, except from one Tier 1 network to another Tier 1 network.
• *Tier 3 networks must route all traffic that they receive to any other connected network, except from one Tier 2 or 1 network to another Tier 2 or 1 network.

So, if you have a Tier 3 network (your ISP for example), and it is connected to two or more Tier 2 networks, then your ISP should not be responsible for routing traffic from one Tier 2 network to another Tier 2 network. This will overload the ISP’s network with traffic it was never designed to carry.

The interconnection between ISPs is called ISP Peering.