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Can one AS unite several ISPs, can/does an ISP own several ASes? No matter what I read nobody really clarifies this relationship or at least doesn't writes it down. Or is ISP and AS just a synonym? Is there a special reference in the rfc towards this topic?

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An Internet Service Provider is a business entity. An Autonomous System is a construct of the BGP protocol. An ISP can own several AS's or only one.

An autonomous system is

a set of routers under a single technical administration [RFC 1930]

So it's unlikely (but not impossible) for two or more companies to share an AS.

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    It is also possible for a company to own an AS without being an ISP.
    – JFL
    Jun 11, 2020 at 11:53
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    A company can have a public IP network and an AS number and purchase transit service from a transit provider and announce its network to the Internet. You can consider it as an ISP in a way but it provide this service only to itself, not to other company.
    – JFL
    Jun 11, 2020 at 12:05
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    Google, Facebook, Amazon, etc.. all have their own ASes (many of them actually) but they do not provide Internet connectivity to customers, they are not considered ISPs.
    – JFL
    Jun 11, 2020 at 12:07
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    Any organization with a large network can have an AS. For example, my client is a government agency and we have our own AS.
    – Ron Trunk
    Jun 11, 2020 at 12:20
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    An ISP isn't anything official, and has nothing to do with the BGP protocol. If all the ISPs went out of business, there would still be ASes.
    – Ron Trunk
    Jun 11, 2020 at 12:22
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An Autonomous System is a set of routing prefixes advertised via BGP.

While that can match an ISP, providers can also advertise multiple ASes - e.g. from mergers, acquisitions or from customers. On the other hands, customers can have multiple ISPs advertise their AS for redundancy, route optimization, or anycasting.

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An ISP is an organisation that provides internet access to others. An AS is a network that participates in the Internet on an "autonomous" basis.

What is or is not an ISP can get somewhat fuzzy, for example Google do sell internet service in some regions, but it's far from their main buisiness. JANET provide inernet access to a bunch of educational/research institutions in the UK, but my understanding is they get most of their funding directly from the government rather than being paid directly for Internet service. Then there are hosting providers who obviously provide internet to their customer's servers, but generally not to their homes and buisiness premisis

A very small ISP that only has a single upstream connection to the internet may not have an AS number at all. In general though, all but the tiniest of ISPs will have their own AS as this gives them more autonomy.

Equally a buisiness that is not an ISP may have thier own AS to give them more autonomy and allow them to route traffic through multiple ISPs.

There are a number of reasons a buisiness may have more than one AS number, including.

  1. Network islands. For example, a hosting provider I use has a presense in the USA and Europe, but they do not have a private transatlantic link. So their USA and Europe networks are seperate.
  2. Mergers and Acquisitions, when a company is taken over it may be difficult to fully merge the networks. Or it may be considered desirable to keep it's operations seperate for buisiness reasons.
  3. Different routing policy, if a company runs both a local ISP and a global transit network, they may have a more restrictive peering policy for their global transit network than for their local ISP, or an ISP may want to offer a "discount" option that uses the cheapest path to a destination and a "premium" option that uses the (hopefully) fastest path.
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If your organization is having more than two ISP connect ed to network for internet access and you wants to confiagure EBGP routing protocols then AS number comes into picture

AS number mean autonomous systems number organization should purchase one AS number if not you need to buy from concern entity and isp will have one AS number. In EBGP routing protocols will communicate different AS numbers..

In EBGP configuration different AS number is mapped with each othe to enable communication from isp to organisation..

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