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Catalyst 3750 Switch Command Reference
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Chapter 2 Catalyst 3750 Switch Cisco IOS Commands
system mtu
command. Therefore, if you use TFTP to configure a new switch by using a backup configuration file
and want the system MTU to be other than the default, you must explicitly configure the system mtu
and system mtu jumbo settings on the new switch and then reload the switch.
Gigabit Ethernet ports operating at 1000 Mb/s are not affected by the system mtu command, and
10/100-Mb/s ports are not affected by the system mtu jumbo command.
You can use the system mtu routing command to configure the MTU size on routed ports.
Note You cannot configure a routing MTU size that exceeds the system MTU size. If you change the system
MTU size to a value smaller than the currently configured routing MTU size, the configuration change
is accepted, but not applied until the next switch reset. When the configuration change takes effect, the
routing MTU size defaults to the new system MTU size.
If you enter a value that is outside the range for the specific type of switch, the value is not accepted.
Note The switch does not support setting the MTU on a per-interface basis.
The size of frames that can be received by the switch CPU is limited to 1998 bytes, regardless of the
value entered with the system mtu command. Although forwarded or routed frames are usually not
received by the CPU, some packets (for example, control traffic, SNMP, Telnet, and routing protocols)
are sent to the CPU.
Because the switch does not fragment packets, it drops:
• switched packets larger than the packet size supported on the egress interface
• routed packets larger than the routing MTU value
For example, if the system mtu value is 1998 bytes and the system mtu jumbo value is 5000 bytes,
packets up to 5000 bytes can be received on interfaces operating at 1000 Mb/s. However, although a
packet larger than 1998 bytes can be received on an interface operating at 1000 Mb/s, if its destination
interface is operating at 10 or 100 Mb/s, the packet is dropped.
Examples This example shows how to set the maximum jumbo packet size for Gigabit Ethernet ports operating at
1000 Mb/s or greater to 1800 bytes:
Switch(config)# system mtu jumbo 1800
Switch(config)# exit
Switch# reload
You can verify your setting by entering the show system mtu privileged EXEC command.
Related Commands Command Description
show system mtu Displays the packet size set for Fast Ethernet, Gigabit
Ethernet, and routed ports.
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