Configuring a Cisco Router as a Frame Relay Switch
Cisco routers can be configured as dedicated Frame Relay switches that act as DCE devices. On a Cisco router configured as a Frame Relay switch, frames from a Frame Relay PVC arriving on an incoming interface are switched to a Frame Relay PVC on outgoing interface. During this process, the incoming DLCI in the arriving frames is replaced by an outgoing DLCI. Frame Relay switching is performed completely in Layer 2, and the Frame Relay switch pays no attention to Layer 3 information contained within the frames. The paths taken by the switched frames are completely based on the Frame Relay route table constructed.
We use a simple example depicted in Figure 4-4 with two Frame Relay access routers and a Cisco router configured as a Frame Relay switch.
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4-4 Frame Relay Switching Using a Cisco Router
The simple Frame Relay network in Figure 4-4 shows a Cisco router configured as a Frame Relay switch with two serial interfaces set up as DCE interfaces. The Cisco router behaving as a Frame Relay switch switches Frame Relay frames received from interface serial4/3 on DLCI 403 to interface serial4/1 on the outgoing DLCI to 304. Example 4-25 shows the sample configurations for the router configured as a Frame Relay switch.
Example 4-25 Configurations for Frame Relay Switch
SW#show running-config Building configuration... <output omitted> ! hostname SW ! <output omitted> ! frame-relay switching ! interface Serial4/1 no ip address encapsulation frame-relay clockrate 64000 frame-relay intf-type dce frame-relay route 304 interface Serial4/3 403 ! interface Serial4/3 no ip address encapsulation frame-relay clockrate 64000 frame-relay intf-type dce frame-relay route 403 interface Serial4/1 304
To configure a Cisco router as a Frame Relay switch, follow the configuration steps listed below:
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Step 1 |
Enable Frame Relay switching on the router using the command frame-relay switching in the global configuration mode. |
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Step 2 |
Go to the interface configuration mode of the Frame Relay interface where you want to configure Frame Relay switching. Configure the interface as a DCE interface with the frame-relay intf-type dce interface configuration command. |
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Step 3 |
Configure the Frame Relay switching on the interface using the frame-relay route command, specifying the incoming DLCI, the outgoing interface, and the outgoing DLCI. Note that Frame Relay switching can be configured only on physical interfaces. |
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Step 4 |
The clockrate command is required on the serial interface of the Frame Relay switch (attached with the DCE end of the serial cable). It provides clocking signals to the connected Frame Relay routers, which are set up as DTE devices. |
The frame-relay route interface configuration command is used to route an incoming DLCI to an outgoing interface and corresponding outgoing DLCI.
To verify the contents of the Frame Relay route table, the show frame-relay route privileged EXEC mode command is used. Example 4-26 shows the contents of the table displayed by the show frame-relay route command.
Example 4-26 Contents of show frame-relay route Command
SW#show frame-relay route Input Intf Input Dlci Output Intf Output Dlci Status Serial4/1 304 Serial4/3 403 active Serial4/3 403 Serial4/1 304 active SW#
NOTE
Take note that the frame-relay route command must be configured in pairs on both the incoming interface and outgoing interface for the Frame Relay route to become active.
The Cisco Frame Relay switching feature can be used to support a Frame Relay network using only routers. Consider the example in Figure 4-5 in which two Cisco routers are configured as Frame Relay switches implementing a Network-to-Network Interface (NNI) between them. To enable communication between two Frame Relay switches, the NNI signaling protocol is used between them. Configuration examples involving two Frame Relay switches communicating in NNI mode are shown in Example 4-27.
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4-5 Two Frame Relay Switches Communicating with NNI Interface
Example 4-27 Configurations for the Frame Relay Switches
SW#show running-config Building configuration... <output omitted> ! hostname SW ! <output omitted> ! frame-relay switching ! interface Serial2/1 no ip address encapsulation frame-relay clockrate 64000 frame-relay intf-type dce frame-relay route 100 interface Serial2/3 300 ! interface Serial2/3 no ip address encapsulation frame-relay clockrate 64000 frame-relay intf-type nni frame-relay route 300 interface Serial2/1 100 SW2#show running-config Building configuration... <output omitted> ! hostname SW2 ! <output omitted> ! frame-relay switching ! interface Serial3/1 no ip address encapsulation frame-relay clockrate 64000 frame-relay intf-type dce frame-relay route 200 interface Serial3/3 300 ! interface Serial3/3 no ip address encapsulation frame-relay clockrate 64000 frame-relay intf-type nni frame-relay route 300 interface Serial3/1 200