By default, the maximum transmission unit (MTU) size for packets sent on a Layer 3 interface is 1500 bytes. This size can be manually set to any size from 512 to 1500 bytes on a per-interface basis. Some configurations require Ethernet frames with an MTU value greater than 1500 bytes. These are called jumbo frames.
To use jumbo frames on a firewall you must specifically enable jumbo frames at the global level. When this is enabled, the default MTU size for all Layer 3 interfaces is set to a value of 9192 bytes. This default value can then be set to any value in the range of 512 to 9216 bytes.
After setting a global jumbo frame size it becomes the default value for all Layer 3 interfaces that have not explicitly had an MTU value set at the interface configuration level. This can become a problem if you only want to exchange jumbo frames on some interfaces. In these situations, you must set the MTU value at every Layer 3 interface that you do not want to use the default value.
The following procedure describes how to enable jumbo frames on a firewall, set the default MTU value for all Layer 3 interfaces and to then set a different value for a specific interface.
Enable Jumbo Frames and Set MTU Values
Enable jumbo frames and set a default global MTU value. Select Device > Setup > Session and edit the Session Settings section. Select Enable Jumbo Frame. Enter a value for Global MTU. The default value is 9192. The range of acceptable values is: 512 - 9216. Click OK. A message is displayed that informs you that enabling or disabling Jumbo Frame mode requires a reboot and that Layer 3 interfaces inherit the Global MTU value. Click Yes. A message is displayed to inform you that Jumbo Frame support has been enabled and reminds you that a device reboot is required for this change to be activated. Click OK. Click Commit.
Set the MTU value for a Layer 3 interface and reboot the firewall. The value set for the interface overrides the global MTU value. Select Network > Interfaces. Select an interface of the Layer3 Interface type. Select Advanced > Other Info. Enter a value for MTU. The default value is 9192. The range of acceptable values is: 512 - 9216. Click OK. Click Commit. Select Device > Setup > Operations and select Reboot Device.

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