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Cisco ME 3400 Ethernet Access Switch Command Reference
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Chapter 2 Cisco ME 3400 Ethernet Access 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. The MTU settings you enter with the system mtu commands are not
saved in the switch IOS 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 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. 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 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.
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, 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
12.2(25)SEG The system mtu bytes range was changed to 1500 to 1998. The routing
bytes keywords were added.
12.2(55)SE The alternate {bytes | interface interface-id} keywords were introduced.
Release Modification
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