While an ISP provides connectivity to its customers, it also provides connectivity to customers of other ISPs. In doing this, an ISP must be able to ensure the appropriate use of its resources.
For example, Figure 6 shows three ISPs and three customers. ISP 1, ISP 2, and ISP 3 are directly connected to one another through a physical link and a corresponding EBGP session (represented here by a single line). Customer 1 is connected to ISP 1 through a physical link and corresponding EBGP session. Customer 2 is similarly connected to ISP 2, and Customer 3 is similarly connected to ISP 3. Each ISP provides transit service to its own customers. Figure 6 illustrates how the ISP permits traffic to transit across its backbone from its own customers or to its own customers.
Figure 6: Transit Service
Each ISP provides nontransit service to other ISPs. For example, Figure 7 shows that ISP 1 does not permit traffic between ISP 2 and ISP 3 to cross its backbone. If ISP 1 permits such traffic, it squanders its own resources with no benefit to its customers or itself.
Figure 7: Nontransit Service
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