I am currently studying the basics of BGP and I am very early into this topic.

So with EIGRP, I was told that an AS is basically a collection of routers/a routing domain that is under a single administrative control and has its own independent policies. With BGP, the explanation of an AS appears to be the same yet my book mentions not to confuse a BGP AS with an EIGRP AS.

How do these two exactly differ, please? If I understand this correctly, all autonomous systems have an unique number assigned to them. An EIGRP AS is a collection of routers while a BGP AS is a collection of networks that belong to a specific organization?

Thank in advance for your help

1 Answer 1


BGP AS numbers are assigned by global authorities because they are used on the public Internet. (There are some private BGP AS numbers that can be used any way you like inside your own network, but not on the public Internet.) A company participating in routing on the public Internet will need to have at least one BGP AS, and it may have more than one. That company has complete control of everything inside a BGP AS that is assigned to it.

IGPs, such as EIGRP or OSPF have an AS concept, but that is within a company. Such IGP ASes are independent route advertisement domains. You can, for example, have multiple EIGRP ASes in your company that you control, but each can have different route advertisement policies that you configure. The AS numbers chosen for IGP ASes are ones that a company where the AS exists decides, not a global authority.

There are methods to communicate routing information between IGP ASes, including the commonly used redistribution router commands, but the route advertisement policies in one IGP AS can be completely different that that of a different IGP AS within the same company.

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