The command used to enable Frame Relay switching on a Cisco router is as follows:
Router_A#config t
Router_A(config)#frame-relay switching
This command must come before any of the other Frame Relay switching–related commands
can be executed, or these commands won’t be allowed. When Frame Relay encapsulation is
enabled on an interface, it defaults to DTE, so you will need to change it to DCE for Frame
Relay switching. For a Frame Relay serial connection to function, you must have a DTE at one
end and a DCE at the other. You first configure the router with the following command:
Router_A(config)#interface serial0
Router_A(config-if)#frame-relay intf-type dce
The clocking on a serial link is provided by the DCE device, which is determined
by the type of cable connected to the serial interface. For a Frame Relay connection,
the DCE status is configured, whereas serial DCE status is cabled. For
Frame Relay, the DCE device is the one that provides LMI.
Because this interface is now functioning as the Frame Relay DCE device, you can change the
LMI type from the default of Cisco with the following command:
Router_A(config-if)#frame-relay lmi-type ?
cisco
ansi
q933a
Router_A(config-if)#frame-relay lmi-type
The next step in the configuration process is to create the proper DLCI forwarding rules.
These rules dictate that when a frame enters a particular interface on a certain DLCI, it will be
forwarded to another interface and DLCI. Let’s look at such an example on interface serial 1:
Router_A#config t
Router_A(config)#interface serial 1
Router_A(config-if)#frame-relay route 100 interface Serial2 101
This command states that any frame received on interface serial 1, with DLCI 100, shall be
forwarded to interface serial 2, with DLCI 101. You can view all the frame routing information
with the show frame-relay route command. The following router output shows the settings
of Router A:
Router_A#show frame-relay route
Input Intf Input Dlci Output Intf Output Dlci Status
Serial0 300 Serial1 200 active
Serial1 100 Serial2 101 active
Serial1 200 Serial0 300 active
Serial2 101 Serial1 100 active
Router_A#
The configuration of a router as a Frame Relay switch can be useful for a lab environment
or even as part of a production network.
Now, let’s look at the configuration of the Frame Relay switch:
Router_A#show running-config
Building configuration...
Current configuration:
!
version 11.2
!
hostname Router_A
!
frame-relay switching
!
interface Serial0
no ip address
encapsulation frame-relay
clockrate 56000
frame-relay intf-type dce
frame-relay route 300 interface Serial1 200
!
interface Serial1
no ip address
encapsulation frame-relay
clockrate 56000
frame-relay intf-type dce
frame-relay route 100 interface Serial2 101
frame-relay route 200 interface Serial0 300
!
interface Serial2
no ip address
encapsulation frame-relay
clockrate 56000
frame-relay intf-type dce
frame-relay route 101 interface Serial1 100
!
end
Router_A#
Notice the global command frame-relay switching is at the top of the configuration.
Also notice that both interfaces are configured with frame-relay intf-type dce commands.
On the serial interfaces, you’ll also see that the clock rate command is used to provide clocking
for the line because the router serving as the Frame Relay switch is a DCE device on the
physical interface.