Cisco ME 3400 Command Reference Manual page 663

Chapter 2 Cisco ME 3400 Ethernet Access Switch Cisco IOS Commands
Usage Guidelines When you use this command to change the system MTU or jumbo 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.
The system MTU setting is saved in the switch environmental variable in NVRAM and becomes
Note
effective when the switch reloads. The MTU settings you enter with the system mtu and system mtu
jumbo 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 Mbps are not affected by the system mtu command, and
10/100-Mbps 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.
You cannot configure a routing MTU size that exceeds the system MTU size. If you change the system
Note
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:
•
•
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 Mbps. However, although a
packet larger than 1998 bytes can be received on an interface operating at 1000 Mbps, if its destination
interface is operating at 10 or 100 Mbps, the packet is dropped.
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
You can verify your setting by entering the show system mtu privileged EXEC command.
OL-9640-07
switched packets larger than the packet size supported on the egress interface
routed packets larger than the routing MTU value
Cisco ME 3400 Ethernet Access Switch Command Reference
system mtu
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