DHCP server packet dropped, RPF check failed

With the Internal DHCP Server the devices in the LAN get correctly the IP address, instead with an  External DHCP there are Dropped Packets: DHCP server packet dropped, RPF check failed

Here an example:

Resolution for SonicOS 7.X

This release includes significant user interface changes and many new features that are different from the SonicOS 6.5 and earlier firmware. The below resolution is for customers using SonicOS 7.X firmware.

• In Device | Diagnostics | Network Path check if the interface for the External DHCP Server is correct.
• If it is not correct add a static route from Policy | Rules and Policies | Routing Rules as shown below:
  
  

Resolution for SonicOS 6.5

This release includes significant user interface changes and many new features that are different from the SonicOS 6.2 and earlier firmware. The below resolution is for customers using SonicOS 6.5 firmware.

• In System | Diagnostics | Find Network Path check if the interface for the External DHCP Server is correct.
• If it is not correct add a static route from Network | Routing as shown below: