Understanding jumbo frames

What jumbo frames are

Jumbo frames are packets that are longer than the standard Ethernet (IEEE 802.3) frame size of 1,518 bytes. The frame size definition for jumbo frames is vendor-specific because jumbo frames are not part of the IEEE standard. The most commonly used jumbo frame sizes are 9,018 bytes and higher.

Advantage of jumbo frames

Because the data is transferred in longer frames than standard Ethernet, jumbo frames reduce the CPU overhead by reducing the number of frames processed.

Limitation of jumbo frames

On the filer, jumbo frames are supported on all copper or fiber Gigabit Ethernet Controllers and interfaces numbered II or higher. The interfaces must be operating at 1,000 Mbps.

Frame size and MTU size definitions

The two commonly used terms to define the size of a jumbo frame are frame size and Maximum Transmission Unit (MTU) size. The definitions of these terms are as follows:

Frame size:

The frame size of an Ethernet frame is the sum of the Ethernet header (14 bytes), the payload (IP datagram, up to 9,000 bytes), and the Frame Check Sequence (FCS) field (4 bytes).

MTU size:

The MTU size specifies the maximum number of bytes of data (the payload) that can be encapsulated in an Ethernet frame. For example, the MTU size of an Ethernet frame (RFC 894) is 1,500 bytes.

Filer frame size and MTU size definition

The default frame size defined for a jumbo frame on the filer is 9,018 bytes, and the maximum supported size is 9,210 bytes. These sizes correspond to MTU sizes of 9,000 and 9,192, respectively.

Ways to set up jumbo frames on the filer

You can set up jumbo frames on the filer in the following two ways:

setup command--The setup command prompts you for jumbo frames if you have an interface that supports jumbo frames on your filer.
ifconfig command--If your system is already running Data ONTAP, you can enable jumbo frames by setting the MTU size on an interface. For information about how to set up the MTU size, see Setting the MTU on an interface.

Network infrastructure requirements

Before enabling jumbo frames on the filer, the following network infrastructure requirements (as appropriate) must be satisfied:

The switch ports must also have jumbo frames enabled.
If the filer and the client are on different subnets, the next-hop router must be configured for jumbo frames.

Client configuration recommendations

You should consider the following client configuration recommendations to gain performance benefits of jumbo frames:

You must configure jumbo frames on the client. To find out how to configure jumbo frames on your client, see the network adapter documentation for your client.
You should enlarge the client's TCP window size. The minimum value for the client's window size should be 2 * (MTU size - 40), and the maximum value can be the highest value your system allows. Typically, the maximum value you can set for your client's TCP window is 65535.
If the filer is configured to support jumbo frames and the client is not, the communication between the filer and client occurs at the client's frame size.
You must ensure that the User Datagram Protocol (UDP) clients are configured with the same MTU size as the filer.
The UDP clients do not communicate their MTU. Therefore, the filer and the client should be configured with the same MTU size, or the receiver might not receive packets being sent.
You must consider the per-route MTU if the filer and the client are on different subnets.
If the filer and the client (both configured to use jumbo frames) are on different subnets and an intermediate subnet does not support jumbo frames, the advantages of using jumbo frames are lost. In the case of TCP clients, jumbo frames are not used at all. In the case of UDP clients, IP fragmentation occurs and leads to performance degradation.