The order in which BGP compares route attributes impacts routing decisions, especially because some attributes like MED are nontransitive and require special handling. These key points summarize this behavior:

• MED nontransitivity:

  • BGP compares the MED attribute only among routes received from the same neighboring autonomous system (AS). If route A is better than B, and B is better than C, route A is not necessarily better than C unless all three routes are from the same AS.

• Grouping for MED comparison:

  • Before evaluating the MED attribute, BGP groups routes by their neighboring AS. Within each group, BGP selects the route with the lowest MED value as the group’s best route.

• Iterative best path selection:

  • Once a best-path is chosen from each group, BGP resumes the standard best-path selection process among these group winners to determine the overall best route.