Which routing protocol uses an autonomous system number?

N10-009 By islever Editorial Last updated

Short answer

C BGP

Question

Which of the following routing protocols uses an autonomous system number?
  1. IS-IS
  2. OSPF
  3. BGP Correct Answer
  4. EIGRP

Key Takeaways

The question tests your understanding of routing protocol identifiers; the trap is that EIGRP configuration also asks for an AS number, but only BGP relies on ASNs as an integral part of its inter-domain path-vector routing and AS-path loop prevention.

In CompTIA Network+ N10-009, BGP is the routing protocol that uses autonomous system numbers (ASNs) to exchange routes between organizations and ISPs. The community consensus strongly favors BGP, despite some confusion about EIGRP also requiring an AS number.

Common mistake: Choosing EIGRP because the `router eigrp` command takes an autonomous system number; however, that AS number is just a local routing-domain or process identifier, whereas BGP's entire operation and route-selection process are dependent on ASNs for inter-AS communication.

Expert Analysis

Why the Answer Is Correct

BGP (Border Gateway Protocol) is the routing protocol that uses autonomous system numbers (ASNs) to identify each autonomous system. It is an exterior gateway protocol that exchanges routing information between ASes, and the AS-path attribute is used for loop prevention and policy-based route selection. Comment [1] correctly cites CompTIA's explanation that ASNs are allocated to ISPs and that BGP handles communications between separate autonomous systems.

Why the Other Options Are Wrong

IS-IS and OSPF are link-state interior gateway protocols; they use areas, levels, or process IDs rather than ASNs. EIGRP does require an AS number in its configuration (router eigrp AS-number), but that number is only used to identify a single routing domain and is not the same as BGP's internet-wide ASN adjacency and path-vector mechanism. Comment [4] points out this nuance and still selects BGP as the correct answer for the exam context.

Community Comment Notes

The voting shows 78% for BGP and 22% for EIGRP, reflecting the common ambiguity. Comment [2] raised the EIGRP question, but later comments including [3] and [4] reaffirm BGP as the intended CompTIA answer. On N10-009, if the question asks which routing protocol uses an autonomous system number among these options, BGP is the official answer because ASNs are fundamentally tied to BGP and the internet routing architecture.

Official Reference

These links open the official vendor documentation in a new tab.

Exam Strategy

Remember that BGP is the only exterior gateway protocol in the list, and ASNs are used to identify independent networks on the internet. When EIGRP appears as an option, recall that although its configuration uses an AS number, BGP is the protocol where the ASN is a core part of the protocol itself—so choose BGP unless the question explicitly mentions EIGRP redistribution or a non-internet context.

Community Votes

C
78%
D
22%

78% of anonymous learners picked answer C. Votes are pick records left by other test-takers — they are not the verified answer.

Discussion 5 contributions

Compiled from public community threads and reader submissions, lightly edited for clarity. Handles and vote tallies are shown as recorded, and comments reflect the views of individual contributors.

From public community threads 5

  1. Nanox 👍 3 Answered C
    C. BGP. According to CompTIA: "Autonomous system numbers (ASN) are allocated to ISPs by IANA via the various regional registries. BGP is a type of exterior gateway protocol (in fact, it replaced a protocol named EGP) for communications between routers in separate autonomous systems. When BGP is used within an AS, it is referred to as Interior BGP (IBGP), and when implemented between autonomous systems, it is referred to as Exterior BGP (EBGP)." Refeerance CompTIA Network + Study Guide.
  2. Philco 👍 2 Answered D
    Yes, EIGRP (Enhanced Interior Gateway Routing Protocol) does use an autonomous system number (ASN) to identify the collection of routers that share routing information within a specific routing domain; according to :-Generative AI So Does BGP: Yes, BGP (Border Gateway Protocol) utilizes an Autonomous System Number (ASN) to identify each network So , what is the right answer???? Is there anybody who is willing to stick his/her neck out for this?
  3. HeatSquad77 👍 2 Answered C
    C. BGP BGP (Border Gateway Protocol) is the routing protocol that uses an Autonomous System Number (ASN). This is used to identify unique networks or groups of networks in different organizations and is critical for inter-domain routing between different ASes on the internet
  4. 93831b0 👍 1 Answered C
    EIGRP uses autonomous system number, but it is mainly used for identification and redistribution purposes and not for core path selection like BGP.
  5. Fosterboi 👍 1 Answered C
    I looked up other sources and I consistently found BGP to be the answer.

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