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Catalyst 3750 Metro Switch Command Reference
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Chapter 2 Catalyst 3750 Metro Switch Cisco IOS Commands
system mtu
Usage Guidelines When you use this command to change the system MTU or jumbo MTU, or to set an alternate MTU size,
you must reset the switch before the new configuration takes effect.The system mtu routing command
does not require a switch reset to take effect.
Note The system MTU setting is saved in the switch environmental variable in NVRAM and becomes
effective when the switch reloads. Unlike the system MTU routing configuration, the MTU settings you
enter with the system mtu and system mtu jumbo commands are not saved in the switch configuration
file, even if you enter the copy running-config startup-config privileged EXEC 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 are not affected by the system mtu command; Fast Ethernet ports are not affected
by the system mtu jumbo command. If you do not configure the system mtu jumbo command, the
setting of the system mtu command applies to all Gigabit Ethernet interfaces.
You can use the system mtu routing command 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.
Beginning with Cisco IOS Release 12.2(55)SE, you can define an alternate MTU size by using the
system mtu alternate bytes command and apply it to specified interfaces by using the system mtu
alternate interface interface-id command. You can define only one alternate MTU size per switch, but
you can apply it to multiple interfaces. The alternate MTU cannot be greater than the jumbo MTU size
set on the switch. When you apply an alternate MTU size to an interface, frames received on the interface
that are greater than the alternate MTU size are dropped.
You can apply the alternate MTU size to Fast Ethernet or Gigabit Ethernet interfaces, but if the alternate
MTU size is greater than 1998 bytes, you cannot apply it to Fast Ethernet interfaces. The alternate MTU
size has no effect on the routing MTU size.
The size of frames that can be received by the switch CPU is limited to 1998 bytes, no matter what value
was entered with the system mtu command. Although frames that are forwarded or routed typically are
not received by the CPU, in some cases packets are sent to the CPU, such as traffic sent to control traffic,
SNMP, Telnet, or routing protocols.
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
Examples This example shows how to set the maximum packet size for Gigabit Ethernet ports to 1800 bytes:
Switch(config)# system mtu jumbo 1800
Switch(config)# exit
Switch# reload
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