Types
Autonomous Systems can be grouped into three categories, depending on their connectivity and operating policy.
A multihomed Autonomous System is an AS that maintains connections to more than one other AS. This allows the AS to remain connected to the Internet in the event of a complete failure of one of their connections. However, this type of AS would not allow traffic from one AS to pass through on its way to another AS.
A stub Autonomous System refers to an AS that is connected to only one other AS. This may be an apparent waste of an AS number if the network's routing policy is the same as its upstream AS's. However, the stub AS may in fact have peering with other Autonomous Systems that is not reflected in public route-view servers. Specific examples include private interconnections in the financial and transportation sectors.
A transit Autonomous System is an AS that provides connections through itself to other networks. That is, network A can use network B, the transit AS, to connect to network C. ISPs are always transit ASs, because they provide connections from one network to another. The ISP is considered to be 'selling transit service' to the customer network, thus the term transit AS.
Read more about this topic: Autonomous System (Internet)
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