📖 What is Autonomous System (AS)?
An Autonomous System (AS) is a collection of IP networks and routers under the control of a single administrative entity, such as an ISP. Each AS is identified by a unique, globally recognized Autonomous System Number (ASN).
"This is the fundamental unit of routing for BGP, which is the protocol used to route traffic between different ASes across the internet."
📚 Certification: CompTIA Network+ Certification Exam (N10-009)
🔑 What are the Key Concepts of Autonomous System (AS)?
- ▸ Autonomous System Numbers (ASNs) are unique identifiers assigned by IANA or Regional Internet Registries to ensure global routing uniqueness across the internet.
- ▸ Border Gateway Protocol (BGP) serves as the primary Exterior Gateway Protocol (EGP) used to exchange routing information between different autonomous systems.
- ▸ Internal BGP (iBGP) manages routing within a single AS, while External BGP (eBGP) handles the exchange of reachability information between separate ASes.
- ▸ Routing policies in an AS are often based on business agreements and security requirements rather than simple technical metrics like hop count.
- ▸ Multihoming allows an AS to connect to multiple ISPs, providing redundancy and load balancing by controlling how traffic enters and leaves the network.
🎯 How does Autonomous System (AS) appear on the N10-009 Exam?
You may be asked to identify the correct protocol for routing traffic between two different internet service providers. The correct answer will be BGP, as it is the only protocol designed for inter-AS communication.
A scenario might describe a large corporation that requires redundant internet connections from two different providers. You will need to recognize that the corporation must operate as its own AS to manage this multihoming.
Expect questions that require you to distinguish between an Interior Gateway Protocol (IGP) like OSPF and an Exterior Gateway Protocol (EGP) like BGP based on whether the routing occurs within or between ASes.
❓ Frequently Asked Questions
Why can't we just use OSPF or EIGRP to route the entire internet?
IGPs are designed for speed and convergence within a single administrative domain. They cannot scale to the size of the internet and lack the policy-based routing controls required for inter-AS business agreements.
What is the practical benefit of a company obtaining its own ASN?
Having an ASN allows a company to implement multihoming. This means they can connect to multiple ISPs simultaneously, ensuring that if one provider fails, traffic can automatically reroute through another AS.