BGP flapping when the number of prefixes advertised increases

Products

• VMware NSX

Issue/Introduction

• NSX alarms indicate "BGP Down"
• The NSX Edge BGP state is flapping Up/Down
• The problem started after increasing the number of network prefixes advertised to NSX from physical

Environment

VMware NSX-T 3.x
VMware NSX 4.x

Cause

In this scenario BGP only has a problem when a larger number of prefixes is introduced. A larger number of prefixes to advertise will result in larger BGP packet size. An MTU misconfiguration on the datapath between Edge and TOR is the most common reason for this issue.

Resolution

Validate that MTU on T0 uplink, vDS and physical network is all consistent. Inconsistent configuration can result in dropped packets and BGP flapping.
For further information on MTU, see KB Understanding MTU and MSS Calculations Across Complex, Layered Infrastructures.

Additional Information

Troubleshooting NSX BGP