5-2. Layer 3 Ethernet Interfaces Layer 3 switching requires an interface on the switch that can forward packets based on Layer 3 addressing. Each Layer 3 interface defines a separate broadcast domain and therefore a separate network. After a Layer 3 interface has been configured with a protocol, it can act as a gateway for other devices in the same broadcast domain. On some switches, you can configure an Ethernet port (interface) as a Layer 3 interface.
Configuration A Layer 3 interface is a direct routed interface that is designed to provide Layer 3 processing of packets entering and exiting the interface. Not every physical interface on every switch is designed to be a Layer 3 interface; however, on some switches, each port is, or can be, configured to be a direct routed port. To configure these interfaces for Layer 3 processing, use the following steps. 1. | Select the physical Layer 3 interface:
IOS | (global) interface type mod/port
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Access global configuration mode and use this command to specify the interface and move to interface configuration mode in the device. You must specify the type of interfaces: ethernet, fastethernet, gigabitethernet, or tengigabitethernet. The mod/port specifies the module and port number of the interface. Fixed-configuration switches, such as the 2948G-L3, have no module option; and for the 3550 switches, the module (or slot) is always 0.
| 2. | Configure the interface for Layer 3 operation:
IOS | (interface)no switchport
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For multilayer IOS switches, such as the 4000 or 6000 running Supervisor IOS or such as the 3550, ports can be configured to act as Layer 2 (switchport) ports or Layer 3 (routed) ports. To configure a port to act as a Layer 3 port, use the no switchport command to disable Layer 2 operation and enable Layer 3 operation.
NOTE When the switchport or no switchport command is issued, the port will be disabled then reenabled. NOTE The default port operation for the 4000 and 6000 series switches running Supervisor IOS is routed mode; the 3550 defaults to switchport mode. | 3. | Configure protocol information.
- a. Assign an IP address:
IOS | (interface) ip address address netmask
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When an interface begins acting as a Layer 3 interface, you must configure it with information about the network connected to the broadcast domain. For IP networks, this means the interface must be given an IP address. This address becomes the gateway address used by clients in the broadcast domain to which the interface is connected.
NOTE Although all Layer 3 switch-routed interfaces support IP, not all of them support other protocols. For example, the 3550 and 4000 running Supervisor IOS support only the configuration of IP on a physical interface. Check your device notes for more details. - b. Assign the IPX network number:
IOS | (interface) ipx network networknumber
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To enable the IPX process on an interface, you must specify the network number with the networknumber option. Contact the NetWare administrator for the proper network number and encapsulation type.
- c. Assign an AppleTalk cable range and zone:
IOS | (interface) appletalk cable-range beginrange-endrange (interface) appletalk zone zonename
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To enable AppleTalk, you must specify the cable range using the beginrange-endrange option to specify the range numbers and configure at least one zone using the command appletalk zone.
NOTE The information presented here for configuring protocol information on a Layer 3 interface is the minimal requirements. You can find more detailed information concerning protocol configuration in Cisco Field Manual: Router Configuration, published by Cisco Press.
| 4. | Enable the interface:
IOS | (interface) no shutdown
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The default status of many Layer 3 interfaces is shutdown, which is a disabled state. To ensure that the interface is operational, enable the interface with the command no shutdown.
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Verifying the Configuration After you have configured a protocol on an interface, use the following command to verify the configuration: IOS | (privileged) show protocol interface type mod/port
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The protocol option is ip, ipx, appletalk, or other configured interface. Feature Example The following example shows the configuration of interface Gigabit Ethernet 1/1 on Distribution_Switch_A for Layer 3 processing. This interface acts as the gateway for all the clients connected to Access_Switch_A. Figure 5-1 shows the network topology for this example. Figure 5-1. Network Topology for Layer 3 Interface Configuration 
An example of the configuration for Distribution_Switch_A follows: Distribution_Switch_A(config)#interface gigabitethernet 1/1 Distribution_Switch_A (config-if)#no switchport Distribution_Switch_A (config-if)#ip address 192.168.10.1 255.255.255.0 Distribution_Switch_A (config-if)#no shut Distribution_Switch_A (config-if)#end Distribution_Switch_A #copy running-config startup-config
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