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What's your physical network MTU ?

We recently had issues with VMs failing to get "large" (> 1400 bytes) chunks of metadata on physical networks configured with a 1500 bytes MTU.

Increasing the physical layer's MTU to > 1550 fixed the issues.

Another option is to lower the MTU inside your VMs to 1450 bytes. This is generally done through through DHCP (dhcp option 26), but as Windows hosts ignore this, you'll have to hardcode this value in your images.

This is all caused by the GRE overhead.