How to enable Jumbo frame support on Juniper ScreenOS firewalls

Jumbo frames are Ethernet frames with more than 1500 bytes of payload. Conventionally, jumbo frames can carry up to 9000 bytes of payload, but variations exist and some care must be taken using the term. Many Gigabit Ethernet switches and Gigabit Ethernet network interface cards support jumbo frames. Some Fast Ethernet switches and Fast Ethernet network interface cards also support jumbo frames.

Jumbo frame support is available in ScreenOS 6.0.0 and above.Jumbo frames are supported on the ISG-1000 and ISG-2000 devices without IDP.

Jumbo frames are also supported on the NS-5000 series running MGT2 and SPM2 cards and the NS-5000 series running MGT3 and SPM3 cards.

Limitation: DI and IDP are not supported in jumbo frame mode.

Enabling Jumbo Frame Mode: To enable jumbo frame mode, use the following command:

set envar max-frame-size=

Set size to any value between 1514 through 9830, inclusive. For example, set size to 7500, as in this example: set envar max-frame-size=7500. Note that there are no spaces before or after the equal sign (=).

Important: When you enable jumbo frames and restart the security device, only interfaces on the 4-port SFP IO card and the management Ethernet interface become active. This scenario is shown in the example below.

Showing Max Frame Size Setting: To show the max-frame-size setting, use the following command:

get envar

Disabling Jumbo Frame Mode: To disable jumbo frames support and return the device to the normal maximum frame size (1514 bytes), use the following command:

unset envar max-frame-size

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