An administrator is troubleshooting BGP flapping in a VMware Cloud Foundation (VCF) 9 environment. A Tier-0 Gateway is running in Active/Active mode with two Edge nodes. BFD is enabled on the eBGP…

Question

Options

• ABFD timers are mismatched between Tier-0 Gateway and the upstream routers.
• BThe MTU does not match on the end-to-end between Tier-0 Gateway and upstream routers.
• CBFD is configured in echo mode on the upstream routers.
• DThe Edge nodes are undersized and are experiencing high contention on CPU and drops BFD

• A
  5% (2)
• B
  78% (29)
• C
  14% (5)
• D
  3% (1)

Explanation

In a VMware Cloud Foundation (VCF) environment, particularly with the high-performance requirements of North-South routing, BGP and BFD (Bidirectional Forwarding Detection) are used in tandem to ensure rapid failure detection. A common but subtle issue in fresh or modified environments is an MTU (Maximum Transmission Unit) mismatch on the physical or virtual When BGP establishes a neighborship, it initially exchanges small keepalive packets. These small packets easily pass through interfaces even if there is an MTU mismatch (e.g., the Edge is set to 9000 bytes but a physical switch in the path is limited to 1500 bytes). However, once the BGP state reaches "Established," the routers begin exchanging full routing tables. These BGP Update packets are often large and will be fragmented or dropped if they exceed the MTU of any hop in the path. The symptom described--where the session is stable for a few minutes (during the initial handshake) and then flaps--is the hallmark of an MTU issue. The "Detect Time Expired" diagnostic in BFD occurs because the BGP hold timer expires when it fails to receive the large update packets, or the BFD packets themselves are delayed/lost due to the congestion caused by retrying large, failed transmissions. According to VMware NSX troubleshooting documentation, if pings (small packets) succeed but the BGP session fails specifically when traffic load or route counts increase, the MTU should be the first setting verified. VCF 9.0 and 5.x designs mandate consistent MTU settings (typically 9000 MTU for the overlay and at least 1500+ for the uplinks) across the entire path, including the virtual switch (VDS), the Edge VM vNICs, and the physical ToR switches. A mismatch here prevents the completion of the BGP state machine's full synchronization, leading to the cyclic "flapping" observed by the

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