Fortinet FortiGate 100 - Manual (Page 113)

Network configuration Configuring the external interface for PPPoE
FortiGate-100 Installation and Configuration Guide
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4 Select Connect to DHCP server to automatically connect to a DHCP server.
If you do not select Connect to DHCP server, the FortiGate unit will not connect to a
DHCP server. You can deselect this option if you are configuring the FortiGate unit
offline.
5 Select OK.
The FortiGate unit attempts to contact a DHCP server from the external interface to
set the external IP address, netmask, and default gateway IP address. When the
FortiGate unit gets this information from the DHCP server, the new addresses and
netmask are displayed in the IP address and Netmask fields.
Configuring the external interface for PPPoE
Use the following procedure to configure the external interface to use PPPoE. This
configuration is required if your ISP uses PPPoE to assign the IP address of the
external interface.
1 Go to System > Network > Interface.
2 For the external interface, select Modify .
3 Set Addressing mode to PPPoE and select OK to change to PPPoE mode.
4 Enter your PPPoE account user name and password.
5 Select OK.
The FortiGate unit attempts to contact the PPPoE server to set the external IP
address, netmask, and default gateway IP address. When the FortiGate unit gets this
information from the PPPoE server, the new addresses and netmask are displayed in
the external IP address and netmask fields. If the PPPoE connection with your ISP is
dropped, the FortiGate unit automatically attempts to re-establish the connection.
6 Select Connect to PPPoE server to automatically connect to the PPPoE server.
7 Select OK.
Changing the external interface MTU size to improve network performance
To improve the performance of your Internet connection, you can adjust the maximum
transmission unit (MTU) of the packets that the FortiGate unit transmits from its
external interface. Ideally, you want this MTU to be the same as the smallest MTU of
all the networks between the FortiGate unit and the Internet. If the packets that the
FortiGate unit sends are larger, they get broken up or fragmented, which slows down
transmission speeds.
Trial and error is the only sure way of finding the optimal MTU, but there are some
guidelines that can help. For example, the MTU of many PPP connections is 576, so if
you connect to the Internet via PPP or PPPoE, you might want to set the MTU size to
576. DSL modems also have small MTU sizes. Most Ethernet networks have an MTU
of 1500.
Note: If you connect to your ISP using DHCP to obtain an IP address for the external interface,
you cannot set the MTU below 576 bytes due to DHCP communication standards.