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Routing & Switching · Intermediate

Path-Vector Routing Protocol

A routing protocol class that advertises the full sequence of autonomous systems a route has traversed, used for loop prevention.

Definition

Path-vector protocols such as BGP carry an AS path list with each route. A router rejects any route whose AS path already contains its own AS number, which prevents inter-domain loops. Path selection is policy driven using attributes rather than a single metric.

Example

A BGP router in AS 65001 discards an update whose AS_PATH is 65002 65001 64500.

Exam tip

BGP is the only path-vector protocol on most exams; know that AS_PATH is its loop-prevention mechanism.

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