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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.
Syntax
Default Setting
1518 bytes
Command Mode
Interface Configuration (Ethernet, Port Channel)
Command Usage
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switchport mtu size
no switchport mtu
size - Specifies the maximum transfer unit (or frame size) for a Gigabit or
10 Gigabit Ethernet port or trunk. (Range: 1500-12288 bytes)
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.
The port MTU size can be displayed with the show
command.
– 397 –
Chapter 11
| Interface Commands
Interface Configuration
show interfaces status
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