Which routing protocol uses an autonomous system number?
Which of the following routing protocols uses an autonomous system number?
Community Votes
78% of anonymous learners picked answer C. Votes are pick records left by other test-takers — they are not the verified answer.
Community Insight
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.
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.
Community Discussion (5 comments)
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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
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.
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