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Configuring the System MTU
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All Layer 3 interfaces require an IP address to route traffic. This procedure shows how to configure an
interface as a Layer 3 interface and how to assign an IP address to an interface.
If the physical port is in Layer 2 mode (the default), you must enter the no switchport interface
Note
configuration command to put the interface into Layer 3 mode. Entering a no switchport command
disables and then re-enables the interface, which might generate messages on the device to which the
interface is connected. Furthermore, when you put an interface that is in Layer 2 mode into Layer 3
mode, the previous configuration information related to the affected interface might be lost, and the
interface is returned to its default configuration
Beginning in privileged EXEC mode, follow these steps to configure a Layer 3 interface:
Command
Step 1
configure terminal
Step 2
interface {{fastethernet | gigabitethernet} interface-id}
| {vlan vlan-id} | {port-channel port-channel-number}
Step 3
no switchport
Step 4
ip address ip_address subnet_mask
Step 5
no shutdown
Step 6
end
Step 7
show interfaces [interface-id]
show ip interface [interface-id]
show running-config interface [interface-id]
Step 8
copy running-config startup-config
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.
Switch(config)# interface gigabitethernet1/0/2
Switch(config-if)# no switchport
Switch(config-if)# ip address 192.20.135.21 255.255.255.0
Switch(config-if)# no shutdown
Configuring the System MTU
The default maximum transmission unit (MTU) size for frames received and transmitted on all interfaces
on the switch stack 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
Catalyst 3750 Switch Software Configuration Guide
11-22
If the switch is notified by VLAN Trunking Protocol (VTP) of a new VLAN, it sends a message that
there are not enough hardware resources available and shuts down the VLAN. The output of the
show vlan user EXEC command shows the VLAN in a suspended state.
If the switch attempts to boot up with a configuration that has more VLANs and routed ports than
hardware can support, the VLANs are created, but the routed ports are shut down, and the switch
sends a message that this was due to insufficient hardware resources.
Chapter 11
Configuring Interface Characteristics
Purpose
Enter global configuration mode.
Enter interface configuration mode, and enter the
interface to be configured as a Layer 3 interface.
For physical ports only, enter Layer 3 mode.
Configure the IP address and IP subnet.
Enable the interface.
Return to privileged EXEC mode.
Verify the configuration.
(Optional) Save your entries in the configuration file.
End with CNTL/Z.
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