An administrator has noticed an issue in a freshly deployed VMware Cloud Foundation (VCF) environment where the BGP neighborship between the Tier-0 gateway and a physical router remains in the Idle…

Question

Options

• AAutonomous System number mismatch.
• BDistributed Firewall blocking traffic.
• CGeneve tunnel down.
• DOverlay MTU too low.

• A
  83% (20)
• B
  4% (1)
• C
  8% (2)
• D
  4% (1)

Explanation

In the context of VMware Cloud Foundation (VCF), particularly versions 5.x and the architectural advancements in VCF 9.0, the establishment of North-South routing via the NSX Tier-0 Gateway is a critical post-deployment or bring-up task. The Tier-0 gateway uses Border Gateway Protocol (BGP) to peer with physical Top-of-Rack (ToR) switches to exchange reachability information for the overlay networks. When a BGP session is reported in the "Idle" state, it indicates that the BGP Finite State Machine (FSM) is at its first stage and is not yet attempting a TCP connection, or it has encountered an error that forced it back to this state. According to VMware VCF documentation and NSX troubleshooting guides, if the administrator can successfully ping between the Tier-0 uplink IP and the physical router interface, Layer 3 reachability is confirmed. This eliminates issues related to physical cabling, VLAN tagging on the trunk ports, or basic IP interface configuration. The primary reason a BGP session remains Idle despite successful ICMP reachability is a configuration mismatch. Specifically, an Autonomous System (AS) number mismatch is the most frequent culprit. BGP requires that the "Remote AS" configured on the Tier-0 gateway matches the "Local AS" of the physical peer. If the SDDC Manager automated workflow or the manual configuration in NSX Manager contains a typo in these values, the protocol handshake will fail While a Distributed Firewall (DFW) could technically block port 179, it is not common in a "freshly deployed" environment for the default rules to block the Edge Node's control plane traffic. Geneve tunnels and MTU issues (Option C and D) typically affect the data plane--causing packet loss for encapsulated guest VM traffic--but they do not prevent the BGP control plane (running over standard TCP) from moving beyond the Idle state. Therefore, verifying the AS numbers in the VCF Planning and Preparation Workbook against the physical switch configuration is the verified resolution path.

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