3V0-25.25: Advanced VMware Cloud Foundation 9.0 Networking

By Broadcom · Question Mode

QuestionQ35

VerifiedUnknown / not found in official Broadcom sources

An administrator is troubleshooting BGP flapping in a VMware Cloud Foundation 9 environment. A Tier-0 Gateway operates in Active/Active mode with two Edge nodes. BFD is enabled on the eBGP sessions to the upstream routers. Each Edge node uses a separate uplink IP for BGP. Following network maintenance, one BGP session begins flapping every few minutes, while the other BGP sessions remain stable.

On the affected Edge node, the get bfd-sessions command displays:

State: Down -  
  
Diag: Detect Time Expired -

Symptoms:

The upstream router also reports the BFD session as Down with Control Detection Time Expired.

There are no interface errors or packet loss for normal traffic, and clearing the BFD session temporarily restores it—but it flaps again after a few minutes.

What is the root cause?

Explanation

A BFD Control Detection Time Expired diagnostic occurs when the receive detection interval expires before the expected BFD control packets arrive. In a single affected eBGP peer relationship, incompatible BFD timer settings between the Tier-0 Gateway and its upstream router can cause the session to declare down repeatedly even though ordinary traffic remains stable. Broadcom guidance identifies an expired BFD receive interval as this diagnostic and notes that overly aggressive BFD timer settings can cause BFD flapping.

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