LevelOne GTL-2691 - Manual (Page 1005)

C HAPTER 31 | Interface Commands
Interface Configuration
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R ELATED C OMMANDS
negotiation (1002)
capabilities (999)
switchport mtu This command configures the maximum transfer unit (MTU) allowed for
layer 2 packets crossing a Gigabit or 10 Gigabit Ethernet port or trunk. Use
the no form to restore the default setting.
S YNTAX
switchport mtu size
no switchport mtu
size - Specifies the maximum transfer unit (or frame size) for a
Gigabit and 10 Gigabit Ethernet port or trunk. (Range: 1500-9216
bytes)
D EFAULT S ETTING
1518 bytes
C OMMAND M ODE
Interface Configuration (Ethernet, Port Channel)
C OMMAND U SAGE
◆ Use the jumbo frame command to enable or disable jumbo frames for
all Gigabit and 10 Gigabit Ethernet ports. To set the MTU for a specific
interface, enable jumbo frames and use this command to specify the
required size of the MTU.
◆ The comparison of packet size against the configured port MTU
considers only the incoming packet size, and is not affected by the fact
that an ingress port is a tagged port or a QinQ ingress port. In other
words, any additional size (for example, a tagged field of 4 bytes added
by the chip) will not be considered when comparing the egress packet’s
size against the configured MTU.
◆ When pinging the switch from an external device, information added for
the Ethernet header can increase the packet size by at least 42 bytes
for an untagged packet, and 46 bytes for a tagged packet. If the
adjusted frame size exceeds the configured port MTU, the switch will
not respond to the ping message.
◆ For other traffic types, calculation of overall frame size is basically the
same, including the additional header fields SA(6) + DA(6) + Type(2) +
VLAN-Tag(4) (for tagged packets, for untaqged packets, the 4-byte
field will not be added by switch), and the payload. This should all be
less than the configured port MTU, including the CRC at the end of the
frame.
◆ For QinQ, the overall frame size is still calculated as described above,
and does not add the length of the second tag to the frame.