The issue with the BGP adjacency between R1 and R2 failing to establish is likely due to a mismatch in the Autonomous System (AS) numbers configured for the BGP session. In BGP, both routers need to have the correct remote AS number configured to form an adjacency. If R1 has the wrong remote AS number for R2, changing it to the correct one (which is 6500 in this case) will resolve the issue.