Interface Configuration
7750 SR Interface Configuration Guide Page 365
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 dis-
carded.
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 differ-
ence 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 follow-
ing table:
Parameters mtu-bytes — Sets the maximum allowable size of the MTU, expressed as an integer.
Values 512 — 9212config>port>sonet-sdh>path512 — 9208
config>port>tdm>ds3 512 — 9208
config>port>tdm>ds1>channel-group512 — 9208
Type Mode Encap Type Default
(Bytes)
10/100, Gig, or 10GigE Access null 1514
10/100, Gig, or 10GigE Access dot1q 1518
10/100, Gig, or 10GigE Access q-in-q 1522
SONET/SDH or TDM Access mpls 1506
SONET/SDH or TDM Access bcp-null 1518
SONET/SDH or TDM Access bcp-dot1q 1522
SONET/SDH or TDM Access ipcp 1502
SONET/SDH or TDM Access frame-relay 1578
ATM, SONET/SDH or TDM Access atm 1524
10/100 or 100FX Ethernet Network null 1514
10/100 or 100FX Ethernet Network dot1q 1518
SONET/SDH Network ppp-auto 1524
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