A company added a new private VIF to a new Direct Connect connection, but the VIF's state is DOWN even though the physical connection shows UP and RUNNING in the console. The customer router shows an ARP entry for the VLAN interface toward AWS. What is a likely cause for the private VIF being DOWN?

A private VIF being in a DOWN state while the physical connection is UP and RUNNING, and ARP entries are present, strongly suggests a BGP peering issue. BGP uses TCP port 179 to establish and maintain peering sessions. If TCP port 179 is blocked on the customer's Direct Connect router, the BGP session cannot be established, leading to the VIF being DOWN. ICMP blocking would prevent pingability but not necessarily BGP establishment. An incorrect 802.1Q VLAN ID would prevent ARP from resolving and the interface from coming up at all. 802.1ad (QinQ) is a different standard for double-tagging VLANs and is not the default or expected configuration for a standard private VIF.