Competing Origins: When Two ASes Announce the Same Prefix, Who Do You Believe?

Originally published at https://blog.pathvector.dev/protocol-lab-bgp-03/ — part of the free Protocol Lab series.

This post is part of Protocol Lab, a free, hands-on series for learning networking protocols by building and breaking them in a container lab. All the lab material — topologies, configs, and scripts — lives in the repo: github.com/pathvector-studio/protocol-lab.

In this lab, you will make two different ASNs announce the same prefix and observe how an upstream router sees the competing origins. The theme is simple but foundational: seeing a prefix in BGP is not the same as knowing who is allowed to originate it.

Reading guide: rfc-notes/bgp-rfc4271-lab03.md

Expected time: 45–60 minutes.

Competing Origins: When Two ASes Announce the Same Prefix, Who Do You Believe?

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