The goal of network policies is to show the various ways Tier 1 ISPs reach a network. Each policy represents a different path the Tier 1 ISPs take to reach a network.
For example, we might have a route with this path in the database: We then truncate this path to:
We’ll use this example policy for AS206924. The diagram is:
And the prefixes in this policy are:
This tells us that the Tier 1 ISP AS2914 (NTT Communications) sees a route for the both prefixes through AS20473 (Choopa LLC) and that AS1299 (Telia Company) can reach these prefixes through AS2914 across the same AS path.
These paths might not be the same physical links, Understanding the layout of Tier 1 carriers is not the scope of this guide.
However, this does mean that if someone is singlehomed (meaning that is their only ISP) to Telia, They will pass though Telia, only.
If someone is singlehomed to AS3356 then based on the diagram above, the data could come into AS206924 either via Telia or NTT. Geography likely is the factor that decides what carrier Level 3 passes the packets to.
It is also worth pointing out that these diagrams only show you inbound traffic. Outbound traffic policy is not displayed in this diagram and could vary a lot.
Last updated: 3rd April 2019