Jumbo Frames

New in version 2.6.0.

By default, DPDK ports are configured with standard Ethernet MTU (1500B). To enable Jumbo Frames support for a DPDK port, change the Interface’s mtu_request attribute to a sufficiently large value. For example, to add a DPDK physical port with an MTU of 9000, run:

$ ovs-vsctl add-port br0 dpdk-p0 -- set Interface dpdk-p0 type=dpdk \
      options:dpdk-devargs=0000:01:00.0 mtu_request=9000

Similarly, to change the MTU of an existing port to 6200, run:

$ ovs-vsctl set Interface dpdk-p0 mtu_request=6200

Some additional configuration is needed to take advantage of jumbo frames with vHost User ports:

  • Mergeable buffers must be enabled for vHost User ports, as demonstrated in the QEMU command line snippet below:

    -netdev type=vhost-user,id=mynet1,chardev=char0,vhostforce \
    -device virtio-net-pci,mac=00:00:00:00:00:01,netdev=mynet1,mrg_rxbuf=on
  • Where virtio devices are bound to the Linux kernel driver in a guest environment (i.e. interfaces are not bound to an in-guest DPDK driver), the MTU of those logical network interfaces must also be increased to a sufficiently large value. This avoids segmentation of Jumbo Frames received in the guest. Note that ‘MTU’ refers to the length of the IP packet only, and not that of the entire frame.

    To calculate the exact MTU of a standard IPv4 frame, subtract the L2 header and CRC lengths (i.e. 18B) from the max supported frame size. So, to set the MTU for a 9018B Jumbo Frame:

    $ ip link set eth1 mtu 9000

When Jumbo Frames are enabled, the size of a DPDK port’s mbuf segments are increased, such that a full Jumbo Frame of a specific size may be accommodated within a single mbuf segment.

Jumbo frame support has been validated against 9728B frames, which is the largest frame size supported by Fortville NIC using the DPDK i40e driver, but larger frames and other DPDK NIC drivers may be supported. These cases are common for use cases involving East-West traffic only.