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Autonomous System Topology

Santino01

Hi everybody,

to my knowledge an autonomous system (AS) is defined as an entity that shares a common routing policy. It can be an ISP (Tier1, Tier2, Tier3), an enterprise or an university. The ASes exchange traffic in a hierarchical order. Smaller ASes are customers of larger ASes. The largest ASes are the tier1 ISPs, while the the smallest are typically non-ISPs like enterprises.

So if the enterprises and universities are typically at the bottom of the hierarchy do they ever have a direct connection/edge to an other enterprise? Or is there always an ISP in between that establishes the connection?

Does an enterprise always need an ISP to get connected to the internet? Or are there large enterprises that provide connectivity by themselves and sell connectivity to small ISPs?

I would appreciate any advice!

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Comments

EdTheLad

How do you expect other enterprises to know how to route to your network? How do you expect devices on the internet know how to get to your network? The answer to those questions might answer your questions!

Santino01

Thanks for your response. Unfortunately I don´t have a complete understanding of these issues. I think an AS is advertising BGP routes that are reachable through it. A provider will announce all possible routes to its customers in order to generate more traffic and thus money. I don´t see how this is answering my question.

EdTheLad

Santino01 wrote: »

I think an AS is advertising BGP routes that are reachable through it.

Your first question was asking if it's possible that a university is connected directly to another enterprise network. You have inferred that a university is an AS and running BGP. The next questions to ask yourself is, does the university have a public or private AS number? Will the university allow transit traffic?