A path vector protocol is a routing protocol in which each route announcement carries the path it has taken through routing domains, not just the destination. The standard example is the Border Gateway Protocol (BGP), which RFC 4271 describes as “an inter-Autonomous System routing protocol.” So yes, BGP is a path vector protocol: it advertises a destination prefix together with the list of Autonomous Systems (ASes) the route information has passed through.
The definition in plain terms
A router running a path vector protocol learns two things about each route: that a network prefix is reachable, and which sequence or set of ASes the announcement crossed on the way. It can then use that path when applying routing policy and choosing among competing routes.
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A terminology note: “path vector” is the explanatory label commonly applied to BGP’s behavior. RFC 4271’s own terms are “route,” “path attributes,” and “AS_PATH.” The RFC’s Abstract also states the core job: “The primary function of a BGP speaking system is to exchange network reachability information with other BGP systems.”
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Routes pair prefixes with path attributes
A BGP route associates a set of destinations, expressed as IP prefixes in Network Layer Reachability Information (NLRI), with attributes that describe a path to them. Speakers send these routes to peers in UPDATE messages. BGP runs over TCP and listens on port 179.
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AS_PATH is the path
AS_PATH is a well-known mandatory attribute. It identifies the ASes through which the routing information in an UPDATE has passed, and it can contain AS_SEQUENCE and AS_SET segments.
- When a speaker advertises a route to an external peer, it updates AS_PATH.
- When it advertises to an internal peer, the base specification says it must not modify AS_PATH.
Three route stores
BGP keeps received, selected, and outgoing routes separately: Adj-RIBs-In (received), Loc-RIB (selected for local use), and Adj-RIBs-Out (to be advertised).
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Why carrying the path matters
RFC 4271 explains that reachability information plus the list of ASes is enough to build a graph of AS connectivity. From that graph, routing loops can be pruned and some AS-level policy decisions enforced. A concrete rule follows from this: in route selection, a route whose AS_PATH contains the local AS cannot be considered better than another route, because using it could cause a loop.
This is not a guarantee that path data prevents every routing problem. The RFC describes the loop-related criterion and the decision process; it does not promise loop-free results under every configuration or extension.
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Path vector does not mean “shortest path wins”
BGP runs a decision process that evaluates route preference, picks routes for local use, and selects which routes to advertise onward. Policy influences which routes are considered or propagated. The path is evidence a router can use, not a rule that the shortest AS path always wins.
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RFC 4271, A Border Gateway Protocol 4 (BGP-4) by Y. Rekhter, T. Li, and S. Hares, was published in January 2006 as an IETF Standards Track document. It obsoletes RFC 1771 and has since been updated by later RFCs. It is the base specification, and the RFC itself notes that extensions can modify behavior. For a specific feature or current operational rule, check the relevant extension rather than assuming the 2006 text is the whole story.
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Key takeaways
- BGP exchanges reachability information between ASes.
- Each route ties a destination prefix to attributes describing a path, notably AS_PATH.
- That path lets speakers reason about AS connectivity, loops, and policy.
- Route choice is policy-driven through a decision process.
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