Azure Internal(Private) Load Balancer does not seem to process/forward the IP Fragmented packets received over UDP Transport

Question

Hi Experts,

We have observed a behavior wrt the Azure Internal Load Balancer where it seems like the LB is not accepting and forwarding the IP fragmented packets (to the backend VMs) when received over UDP transport. Apparently the Load balancer does not even forward the first fragment ahead. Is there a solution/workaround to achieve this?

FWIW, the same IP fragmented packets when received over TCP Transport do seem to be processed and passed ahead by the Load Balancer.

Answer 1

Hello @Harneet Singh , Welcome to the Microsoft Q&A platform.

As documented in the limitations for Azure Load Balancer here Forwarding IP fragments isn't supported on load-balancing rules. IP fragmentation of UDP and TCP packets isn't supported on load-balancing rules. HA ports load-balancing rules can be used to forward existing IP fragments. You can go through this documentation for additional details on HA ports. If a packet is already fragmented, it's forwarded based on the two-tuple distribution mode when enabled on HA ports load-balancing rules.
Hope this helps! Please let me know if you have any additional questions. Thank you!