Alcatel-Lucent 7750 SR OS Interface Configuration Manual page 306

Configuration Commands
config>port>sonet-sdh>path
config>port>tdm>ds1>channel-group
config>port>tdm>ds3
config>port>tdm>e1>channel-group
config>port>tdm>e3
Description This command configures the maximum payload MTU size for an Ethernet port or PPP-enabled port
or sub-port and Frame Relay-enabled port or subport. The Ethernet port level MTU parameter
indirectly defines the largest physical packet the port can transmit or the far-end Ethernet port can
receive. Packets received larger than the MTU will be discarded. Packets that cannot be fragmented
at egress and exceed the MTU are discarded.
The value specified for the MTU includes the destination MAC address, source MAC address, the
Ethertype or Length field and the complete Ethernet payload. The MTU value does not include the
preamble, start of frame delimiter or the trailing CRC.
PoS channels use the MTU to define the largest PPP payload a PoS frame may contain. A significant
difference between SONET/SDH PoS channel and Ethernet physical MTU values the overhead
considered part of the framing method and the overhead considered to be part of the application using
the frame. In Ethernet, the preamble, start of frame delimiter and the CRC are considered part of the
framing overhead and not part of the frame payload. For a PoS channel, the HDLC framing overhead
is not included in the physical MTU; only the PPP and PPP payload are included. If the port mode or
encapsulation type is changed, the MTU assumes the default values of the new mode or
encapsulation type.
The no form of this command restores the default values.
Default
The default MTU value depends on the (sub-)port type, mode and encapsulation and are listed in the
following table:
Page 306 7750 SR OS Interface Configuration Guide