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Catalyst 3750-E and 3560-E Switch Software Configuration Guide
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Chapter11 Configuring Interface Char acteristics
Configuring the System MTU
Configuring the System MTU
The default maximum transmission unit (MTU) size for frames received and se nt on all interfaces on the
switch or switch stack is 1500 bytes. You can change the MTU size to support switched jumbo frames
on all Gigabit Ethernet and 10-Gigabit Ethernet interfaces and to support routed frames on all route d
ports.
•The system jumbo MTU value applies to switched packets on the Gigabit Ethernet and 10-Gigabit
Ethernet ports of the switch or switch stack. Use the system mtu jumbo bytes global configuration
command to specify the system jumbo MTU value.
•The system routing MTU value applies only to routed packets on all r outed ports of the switch or
switch stack. Use the system mtu routing bytes global configuration command to specify the
system routing MTU value.
When configuring the system MTU values, follow these guidelines:
•The switch does not support the MTU on a per-interface basis.
•You can enter the system mtu bytes global configuration command on a Catalyst 3750-E switch,
but the command does not take effect on the switch. This command only affects the system MTU
size on Fast Ethernet ports on Catalyst 3750 members in a mixed hardware switch stack. In this
stack, you can use the system mtu bytes global configuration command on a Catalyst 3750- E
member to configure the system MTU size on a Catalyst 3750 member.
•The system mtu, system mtu jumbo, and system mtu routing global configuration commands do
not take effect in these cases:
–
When you enter the system mtu command on a Catalyst 3750-E or 3560-E switch
–
In a mixed stack when you enter the system mtu jumbo command for the Fast Ethernet ports
on a Catalyst 3750 member
–
When you enter the system mtu routing command on a switch on which only Layer 2 ports are
configured
•When you use the system mtu bytes or system mtu jumbo bytes command to change the system
MTU or system 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
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 Cisco
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 swit ch 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.
In a switch stack, the MTU values applied to the members depend on the s tack configuration:
•A stack consisting of only Catalyst 3750-E switches, also referred to as a Catalyst 3750-E-only stack
•A stack consisting of Catalyst 3750-E switches and Catalyst 3750 switches, also referred to as a
mixed hardware stack
•A stack consisting of only Catalyst 3750 switches, also referred to as a Catalyst 3750-only stack
Table11-5 shows how the MTU values are applied depending on the configuration.