Which of the following statements about an autonomous system is FALSE?

Question

Options

• AAn AS is a set of routers under a single technical administration.
• BAn AS efficiently advertises all IGP known routes to other ASs.
• CAn AS presents a consistent routing policy to other autonomous systems.
• DAn AS has a 2-byte or 4-byte AS number assigned.

• A
  6% (2)
• B
  91% (32)
• C
  3% (1)

Explanation

Option B is false because an AS does not advertise all IGP-known routes to other ASes. IGP routes (learned via OSPF, EIGRP, IS-IS, etc.) are strictly internal - the AS uses BGP to share only selected routes with external ASes, typically aggregated or filtered according to its routing policy. Flooding all internal routes externally would be a scalability and security disaster.

Why the distractors are true:

• A is true by RFC definition - an AS is literally "a set of routers under a single technical administration."
• C is true - presenting a consistent routing policy to the outside world is one of the core purposes of the AS boundary abstraction.
• D is true - original AS numbers were 2-byte (0–65535); RFC 4893 extended this to 4-byte (32-bit) to accommodate growth.

Memory tip: Think of an AS like a company building with security badges. Employees (routers) share full internal floor maps (IGP routes) inside, but when communicating with partner companies (other ASes via BGP), you only share the lobby directory - not every internal hallway. "IGP stays inside, BGP selects what leaves."

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