As shown in the figure, MPLS/BGP IPVPN is deployed in the network. The MP-BGP neighbor relationship is established between PE1 and PE2 through the LoopbackD interface. After the configuration is completed, it is found that CE1 and CE2 cannot learn routes from each other. Use the command on PE2 to query neighbor information as shown in the figure. What is the most likely cause of the problem?