Interface Mtu Packet Size - Fortinet FortiGate Series Administration Manual

System Network

Interface MTU packet size

FortiGate Version 4.0 MR1 Administration Guide
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UDP Echo Use UDP echo to detect the server. Select this option of the server is configured to
provide UDP echo services. In some cases a server may be configured to reply to
UDP echo requests but not to reply ICMP pings.
UDP echo uses UDP packets on port number 7 to send a text string to the server
and expects an echo reply from the server. The echo reply just echos back the
same text to confirm that the server can respond to UDP requests.
Spillover Set the spillover threshold to limit the amount of bandwidth processed by the
Interface. The Spillover Thresholds range is 0-2097000 KBps.
Threshold
The FortiGate unit sends all ECMP-routed sessions to the lowest numbered
interface until the bandwidth being processed by this interface reaches its spillover
threshold. The FortiGate unit then spills additional sessions over to the next lowest
numbered interface.
For more information, including the order in which interfaces are selected, see
"ECMP route failover and load balancing" on page
Note: For more information about TCP and UDP echo, see
To improve network performance, you can change the maximum transmission unit (MTU)
of the packets that the FortiGate unit transmits. Ideally, the MTU should be the same as
the smallest MTU of all the networks between the FortiGate unit and the destination of the
packets. If the packets that the FortiGate unit sends are larger than the smallest MTU,
they are broken up or fragmented, which slows down transmission. You can easily
experiment by lowering the MTU to find an MTU size for optimum network performance.
Select interfaces on some FortiGate models support frames larger than the traditional
1 500 bytes. Contact Fortinet Customer Support for the maximum frame sizes your
FortiGate unit supports.
To be able to send larger frames over a route, all Ethernet devices on that route must
support that larger frame size, otherwise your larger frames will not be recognized and are
dropped.
If you have standard size and larger size frame traffic on the same interface, routing alone
cannot route them to different routes based only on frame size. However you can use
VLANs to make sure the larger frame traffic is routed over network devices that support
that larger size. VLANs will inherit the MTU size from the parent interface. You will need to
configure the VLAN to include both ends of the route as well as all switches and routers
along the route. For more information on VLAN configurations, see the
guide.
To change the MTU size of the packets leaving an interface
1 Go to System > Network > Interface.
2 Choose a physical interface and select Edit.
3 Below Administrative Access, select Override default MTU value (1 500).
4 Set the MTU size.
If you select an MTU size larger than your FortiGate unit supports, an error message
will indicate this. In this situation, try a smaller MTU size until the value is supported.
Note: If you change the MTU, you need to reboot the FortiGate unit to update the MTU
value of VLAN subinterfaces on the modified interface.
Configuring interfaces
344.
RFC 862.
VLAN and VDOM
195