Jumbo Frames

Jumbo Frames, and why you might not want to use them

Question: How do I optimize NFS copy speed on 1Gb home network?

My Answer:

Jumbo frames can certainly help, but don’t expect it to magically give you 100% increase in throughput. Check out my post here in another question that discusses a way to test network throughput without bringing disk i/o into the picture. This will give you a baseline to see if there are perhaps other factors not related to the network.

If you do plan on enabling Jumbo Frames, as some here have suggested, please be aware of some limitations and requirements.

1) All devices along the path must support and have jumbo frames enabled. This includes:

  • Both the sending and receiving nic
  • Both switch ports the devices are connected to
  • All L2 switch ports along the path from sender to receiver (switch-to-switch connections)
  • All L3 interfaces along the path, if applicable

2) Make absolutely sure that all devices in the same vlan support and are configured for jumbo frames.

Reason for #2: Consider two PCs setup in the same vlan, one configured for 9000 byte frames, and the other for 1500 byte frames. The PC1, configured for standard frames, will be able to send packets successfully to PC2. However, the reply that PC2 send will all be 9000 byte frames. Since PC1 is configured for standard frames, it will be unable to process the jumbo frames, and they will be dropped. In many cases, the TCP window will continue to be cut in half and resent until they are able to communicate. To fix this issue, you’ll need to insert an L3 device (router) between the two PCs so they are able to communicate.