Picture 1: Network Topology with 5 eBGP Neighbors Let’s check how the AS_PATH attribute is built when a prefix 70.36.1.0/24 originated on the router AS1 is sent in a BGP update message and received by the router AS5 (Picture 1). So the AS path lists all the ASes that need to be traversed to reach the location where the prefix that the path is attached to is advertised from. When a BGP router sends out an update to a neighbor in a different AS (i.e., an external or eBGP neighbor), it adds its own AS number to the front (left side) of the AS path. With a simple check of a routing table, network operators can determine the source and target ASes and all transit ASes through which the packet moves on its way to the destination. BGP AS PATH Attribute for Network Path VisibilityīGP routing information provides full Internet path visibility. This is one of the attributes that is evaluated in the BGP best path selection process. The set of ASes along the path between two Autonomous Systems on the Internet is called BGP AS_PATH. Each AS is represented by a unique number called an AS number (ASN). An AS can be an ISP, a university or the entire corporate network. BGP’s role is to exchange routing and reachability information between autonomous systems (ASes) on the Internet. Border Gateway Protocol (BGP) is a core routing protocol used by most of the Internet Service Providers (ISPs).
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