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Cisco ME 3400 Ethernet Access Switch Software Configuration Guide
78-17058-01
Chapter 9 Configuring Interface Characteristics
Configuring the System MTU
To remove an IP address from an interface, use the no ip address interface configuration command.
This example shows how to configure a port as a routed port and to assign it an IP address:
Switch# configure terminal
Enter configuration commands, one per line. End with CNTL/Z.
Switch(config)# interface gigabitethernet0/2
Switch(config-if)# no switchport
Switch(config-if)# ip address 192.20.135.21 255.255.255.0
Configuring the System MTU
The default maximum transmission unit (MTU) size for frames received and sent on all interfaces on the
switch is 1500 bytes. You can increase the MTU size for all interfaces operating at 10 or 100 Mbps by
using the system mtu global configuration command. You can increase the MTU size to support jumbo
frames on all Gigabit Ethernet interfaces by using the system mtu jumbo global configuration
command. Gigabit Ethernet ports are not affected by the system mtu command. Fast Ethernet ports are
not affected by the system jumbo mtu command because jumbo frames are not supported on 10/100
interfaces, including 100BASE-FX and 100BASE-BX SFP modules.
You cannot set the MTU size for an individual interface; you set it for all 10/100 or all Gigabit Ethernet
interfaces on the switch. When you change the MTU size, you must reset the switch before the new
configuration takes effect.
The size of frames that can be received by the switch CPU is limited to 1546 bytes, no matter what value
was entered with the system mtu or system mtu jumbo commands. On 10/100 interfaces, the maximum
size of frames is limited to 1546 bytes. 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.
Routed packets are subjected to MTU checks on the sending ports. The MTU value used for routed ports
is derived from the configured system mtu value (not the system mtu jumbo value). That is, the routed
MTU is never greater than the system MTU for any VLAN. The routing protocols use the system MTU
value when negotiating adjacencies and the MTU of the link. For example, the Open Shortest Path First
(OSPF) protocol uses this MTU value before setting up an adjacency with a peer router. To view the
MTU value for routed packets for a specific VLAN, use the show platform port-asic mvid privileged
EXEC command.
Step 3
no shutdown Enable the port, if necessary. By default, UNIs are
disabled, and NNIs are enabled.
Step 4
no switchport For physical ports only, enter Layer 3 mode.
Step 5
ip address ip_address subnet_mask Configure the IP address and IP subnet.
Step 6
no shutdown Enable the interface.
Step 7
end Return to privileged EXEC mode.
Step 8
show interfaces [interface-id]
show ip interface [interface-id]
show running-config interface [interface-id]
Verify the configuration.
Step 9
copy running-config startup-config (Optional) Save your entries in the configuration file.
Command Purpose