Reading BGP AS Paths During Troubleshooting
How to read AS_PATH, understand prepending and avoid assuming that the shortest path is always the selected route.
AS_PATH tells you where an advertisement has been
When a BGP route crosses autonomous systems, each eBGP speaker normally adds its ASN to AS_PATH. The result gives operators a visible record of the autonomous systems the advertisement traversed.
That makes AS_PATH valuable during transit issues and route leaks. If a prefix that normally arrives through one provider suddenly shows a different sequence, the path can reveal where policy or propagation changed.
Shortest path is only part of best-path selection
A common shortcut is to say BGP chooses the shortest AS path. That is incomplete. Major implementations evaluate attributes in an ordered decision process, and locally controlled attributes such as weight or local preference can be considered before AS_PATH length.
This matters during troubleshooting. If a router prefers a longer AS path, inspect the full set of attributes and local policy before assuming BGP is behaving incorrectly.
Prepending is a hint, not a command
AS-path prepending repeats an ASN in an advertisement to make that path look longer. Operators use it to influence inbound traffic from networks that consider AS_PATH length.
The remote network remains in control. Its local preference, communities or traffic-engineering policy may outweigh your prepend. That is why prepending should be validated with real route views rather than assumed to work universally.
Loop prevention and your own ASN
BGP normally rejects an incoming route if its AS_PATH already contains the local ASN. This is a core inter-AS loop-prevention mechanism.
There are advanced cases where allowas-in or similar features intentionally relax that rule, but those designs need careful controls. If you see your ASN unexpectedly in a path, investigate before overriding loop protection.
A practical incident workflow
Start with the prefix and confirm what route each edge router receives. Capture next hop, AS_PATH, local preference, MED, communities and whether the route is eBGP or iBGP learned. Compare the good and bad paths side by side.
Then trace the policy that modifies those attributes. A parsed AS path is useful context, but most difficult BGP incidents are solved by understanding policy, not by counting AS numbers alone.