Configuring a Cisco Router as a Frame Relay Switch

One of the most effective lab setups uses frame relay as its primary transport method. This is a configuration that many people use and praise for its ease of setup and maintenance. Topologies can be changed quickly and easily without touching a single cable. To accomplish this we configure a Cisco router as a dedicated Frame Relay switch that will act as a DCE device on the line. Frames from a Frame Relay PVC arriving on an incoming interface are switched to a Frame Relay PVC on the designated outgoing interface. The switching is performed completely in Layer 2, and the switch pays no attention to the Layer 3 information contained within the frames.

The path taken is completely based on the Frame Relay routing table we construct. Before we do this we need to think about ease of management. Choosing significant DLCI numbers can simplify this greatly, unfortunately the DLCI is limited to 1024, and only 16 – 991 are user definable. Using a method of concatenating Router X’s Number, the number 2 (to signify to the next router), and Router Y’s number. Using this method transport from router 4 to router 6 would use DLCI 426. This has the unfortunate side effect of limiting our lab to 9 routers.

Enough about theory, here is the configuration for a 3 router lab.

!
hostname FrameSwitch
!
frame-relay switching
!
interface Serial0/1
 description Router_1
 no ip address
 encapsulation frame-relay
 clockrate 64000
 frame-relay intf-type dce
 frame-relay route 122 interface Serial0/2 221
 frame-relay route 123 interface Serial0/3 321
!
interface Serial0/2
 description Router_2
 no ip address
 encapsulation frame-relay
 clockrate 64000
 frame-relay intf-type dce
 frame-relay route 221 interface Serial0/1 122
 frame-relay route 223 interface Serial0/1 322
!
interface Serial0/3
 description Router_3
 no ip address
 encapsulation frame-relay
 clockrate 64000
 frame-relay intf-type dce
 frame-relay route 321 interface Serial0/1 123
 frame-relay route 322 interface Serial0/1 223

You can check your configuration using the show frame-relay route command. Here is a sample output based on our configuration above.

FrameSwitch#show frame-relay route
Input Intf 	Input Dlci 	Output Intf 	Output Dlci 	Status
Serial0/1       122 		Serial0/2       221 		inactive
Serial0/1       123 		Serial0/3       321 		inactive
Serial0/2       221 		Serial0/1       122 		inactive
Serial0/2       223 		Serial0/1       322 		inactive
Serial0/3       321 		Serial0/1       123 		inactive
Serial0/3       322 		Serial0/1       223 		inactive

I hope this helps your lab setup and studies. If you have any questions about this or anything, feel free to leave a comment. I’d love to help.