5-1. Layer 3 Switching Layer 3 switching is the movement of data between devices using tables or pathways containing Layer 3 network addressing. To perform Layer 3 switching, the device must have a Layer 3 switching processor that can be a separate module or card. A Layer 3 switching processor will use Layer 3 IOS to configure the Layer 3 switching components. To allow Layer 3 switching, the switch must have the routing function enabled for a given protocol. To provide connectivity between the different networks, the switch must have knowledge of available pathways for these networks.
Configuration To perform Layer 3 switching on a device, you must have a switching processor that makes decisions based on Layer 3 protocol addressing. This engine will run Cisco IOS and the configuration task will be performed in configuration mode of IOS only. This switching processor can be in the form of a route switch module (RSM), route switch feature card (RSFC), multilayer switch module (MSM), multilayer switch feature card (MSFC), Layer 3 services module, or integrated into the hardware in the switch like that of the 3550 or 2948G-L3. 1. | Access the Layer 3 switching processor.
If the switching processor is a card that runs as a subsystem in a COS device, you need to access this device to perform any configuration. To access the device, use the session command. This command is not required for a switch running Supervisor IOS, because you are communicating directly with the Layer 3 switch processor. If you are using a COS switch, you access the switch processor as follows:
COS | (privileged)session mod
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The mod number specifies the module number of the switch processor. Use the command show modules to locate this processor if you do not know where it is located in the switch.
NOTE For an MSFC or RSFC on a Supervisor in slot 1, the module number is always 15. For an MSFC or RSFC on a Supervisor in slot 2, the module number is always 16. | 2. | Enable the routing process.
In addition to having the processor, it must be configured to perform the switching process. This is accomplished by going to global configuration mode and enabling the routing process for the protocol in question:
IOS | (global) protocol routing
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This command enables the routing process. The protocol option specifies the protocol you want to enable. Some examples are appletalk, ip, and ipx. The default for all protocols except IP is that routing is disabled, so this command would not be required for IP packets to be switched by the Layer 3 engine.
If you have configured your switch to perform integrated routing and bridging (IRB), you will also have to enable the routing process for any bridge group that needs access outside of their broadcast domain. Use the following command to enable Layer 3 switching for the bridge groups:
IOS | (global) bridge groupnumber route protocol
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This command allows all the members of the bridge group, specified by the groupnumber option, to use a virtual interface to communicate through Layer 3 switching. The protocol option specifies which protocols will be Layer 3 switched.
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NOTE If you specify this command for a specific protocol on a bridge group, but do not assign the virtual interface a protocol address, the bridging function will be disabled for that protocol.
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