Understanding frame size, MTU size, and jumbo frames

When to change the default frame size

The standard Ethernet (IEEE 802.3) frame size is 1,518 bytes. You might want to change the default frame size of a network interface on your filer if your filer includes the following types of network interfaces.

Gigabit Ethernet Controllers.

Changing the default frame size for any copper or fiber Gigabit Ethernet Controller numbered II or higher, as well as the Gigabit Ethernet infrastructure to which it connects, can significantly increase performance depending upon the activity.

FDDI interfaces.

You can set the frame size on an FDDI interface when the network infrastructure does not support the default FDDI frame size.

The default frame size can be changed by reconfiguring the Maximum Transmission Unit (MTU) value on filers and clients.

Frame size and MTU size definitions

Two commonly used terms to describe frame characteristics are frame size and MTU size:

Frame size:

The frame size of a standard Ethernet frame (defined by RFC 894) is the sum of the Ethernet header (14 bytes), the payload (IP datagram, usually 1,500 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 a standard Ethernet frame is 1,500 bytes; this is the default for Network Appliance filers. However, a jumbo frame, with an MTU size of 9,018 bytes or more, can also be configured.

About jumbo frames

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.

Because jumbo frames are larger than standard frames, fewer frames are needed and therefore CPU processing overhead is reduced.

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.

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 a filer

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

During initial setup, the setup command prompts you to configure jumbo frames if you have an interface that supports jumbo frames on your filer.
If your system is already running, you can enable jumbo frames by setting the MTU size on an interface. For information about how to set up the MTU size, see Configuring network interfaces.

Network infrastructure requirements

Before enabling jumbo frames on the filer, clients and intermediate routers on the network must have jumbo frames enabled. In particular, the following network infrastructure requirements (as appropriate) must be satisfied:

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

Client configuration guidelines

Follow these guidelines in configuring clients to work with jumbo frames:

You need to configure jumbo frames on the client as well as on the filer.
Find out how to configure jumbo frames on your client by checking the network adapter documentation for your client.
Enlarge the client's TCP window size.
The minimum value for the client's window size should be two times the MTU size, minus 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.
Ensure that the User Datagram Protocol (UDP) clients are configured with the same MTU size as the filer.
UDP clients do not communicate their MTU size. Therefore, the filer and the client should be configured with the same MTU size, or the filer might send packets that the clients cannot receive.
Check the MTU of any intermediate subnets 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 intermediate router fragments the IP packets and the advantages of using jumbo frames are lost.