DHCP relay fails to provide DHCP lease to VMs.

Products

• VMware NSX

Issue/Introduction

• VMs connected to overlay segments relying on DHCP relay do not complete DHCP lease process.
• PXE VMs may fail to boot correctly due to no DHCP lease ACK received.
• Packet captures show the VM receives a valid DHCP Discover, Offer and Request while the DHCP server sends a full Discover, Offer, Request and Acknowledgement. This however does not get forwarded from the DHCP relay to the client VM.
• The Offer and the Acknowledgement observed from the DHCP server have differing subnet mask but Transaction ID and DHCP server Identifier matches.
• Some IPs work while others do not with nothing else changing in configuration (connected segment or DHCP server).
  
  
  Offer
  
  
  
  ACK
  
  
  
  Note: The preceding log excerpts are only examples. Date, time, and environmental variables may vary depending on your environment.
  
  

Environment

VMware NSX
VMware NSX-T Data Center

Cause

This can occur when the DHCP server is misconfigured (external to NSX) with a duplicated IP subnet in two different ranges. For instance if two scopes exist, one for a /24 and one for an IP in a /28. If an IP that exists in both is offered the DHCP process can fail. There should not be overlap within the DHCP server.

Resolution

Review the DHCP configuration and ensure no duplicate subnet present.