The diagnostic output explicitly states the root cause: the local router has only the inet6-unicast address family configured for this peer, while the remote router advertised inet-unicast and inet-vpn-unicast in its OPEN message, and BGP requires at least one common negotiated address family (NLRI type) between two peers for a session to remain established; with zero overlap between the two sides' configured families, the session is torn down immediately after the OPEN exchange, which is precisely why the peer state shows Active rather than Established. This confirms the mismatched-address-family statement directly and unambiguously from the exhibit's own Down reason and Detail reason fields. The diagnostic tool's own embedded Suggestion text goes further, explicitly advising the administrator to additionally verify that the local-address configured for the peer matches the remote router's configured neighbor address and vice versa — a secondary, commonly encountered condition in which mismatched or effectively duplicated/incorrect address bindings between the two sides' neighbor statements can independently prevent a session from completing correctly, even once the family mismatch is resolved. This is why address-configuration consistency between the peers is flagged as a second concern worth checking in this scenario. Nothing in the exhibit references NTP or time synchronization at all, nor does anything in the output reference authentication, an MD5 mismatch, or any related failure signature; both of those conditions would present through entirely different diagnostic messages than the one shown here. Reference topics: Junos Enterprise Routing – BGP, Address Family Negotiation and the show bgp diagnostics neighbor Command.