Many CCNA certification candidates want to add a frame relay switch to their Cisco router lab, but aren't quite sure how to configure one. Other candidates aren't quite sure what a frame relay switch is, or what Cisco routers can serve as such a switch. This CCNA case study will examine how to add one of these pivotal devices to a Cisco lab.

A Cisco lab's frame relay switch is not a switch at all; it's a Cisco router. Almost any Cisco router can serve as your frame switch, but you will need multiple serial interfaces to make a router particularly effective in this role. I recommend you get a Cisco router with at least four serial interfaces. Cisco 2520s make excellent frame switches, and by doing a search on eBay for "frame relay switch", you'll quickly find several others that can as well.

The frame relay switch is going to play the role of the frame provider in your lab. In essence, you've got a one-switch frame relay cloud, which gives you a tremendous opportunity to practice frame relay scenarios.

In this example, I've got three Cisco routers that are going to be the production routers in my lab - R1, R2, and R3. I have a fourth router that will serve as the frame relay switch. The DLCI assignments I've come up with are as follows:

R1 is the hub and will use DLCI 122 to reach R2, DLCI 123 to reach R3.
R2 is a spoke router and will use DLCI 221 to reach both R1 and R3.
R3 is a spoke router and will use DLCI 321 to reach both R1 and R2.
The physical connections are as follows. All connections are using DTE/DCE cables with the DCE end of the cable connected to the frame relay switch.

R1 is connected to the frame switch's Serial1 port.
R2 is connected to the frame switch's Serial2 port.
R3 is connected to the frame switch's Serial3 port.
On the frame switch, the global command frame-relay switching is required to make the router act as a frame relay switch. Let's take a look at the commands we'll need on the frame switch's Serial1 port, which is connected to R1.

interface Serial1
no ip address
encapsulation frame-relay
logging event subif-link-status
logging event dlci-status-change
clockrate 56000
no frame-relay inverse-arp
frame-relay intf-type dce
frame-relay route 122 interface Serial2 221
frame-relay route 123 interface Serial3 321

Note that there is no IP address on the port, and frame relay encapsulation is enabled. The clockrate command is necessary on the DCE end of the connection, so you see it here. You also see that the interface is hard-coded as a DCE with the frame-relay intf-type dce command. Not all current IOS versions require this; just make sure you have the DCE end of the cable attached to the frame switch and verify that with show controller serial x.

Finally, we come to the frame-relay route command. The syntax seems a little tricky, but once you break it down it's pretty simple.

frame-relay route 122 = the incoming DLCI
interface serial2 = data coming in on DLCI 122 is sent out this port
221 = data sent out interface serial2 will use this DLCI
Getting those statements correct is the hardest part of configuring your frame relay switch. The good part is that once you have your frame switch configured and working properly, you can just leave the configuration there. Configuring a Cisco router as a frame switch is not a CCNA exam topic.

We'll take a look at how to verify your frame switch configuration in just a moment, but first here is the pertinent section of the frame switch configuration we've talked about here.

hostname FRAME_SWITCH
!
!
ip subnet-zero
no ip domain-lookup
frame-relay switching
!
!
!
interface Ethernet0
no ip address
no ip directed-broadcast
shutdown
!
interface Serial0
no ip address

interface Serial1
no ip address
no ip directed-broadcast
encapsulation frame-relay
logging event subif-link-status
logging event dlci-status-change
clockrate 56000
no frame-relay inverse-arp
frame-relay intf-type dce
frame-relay route 122 interface Serial2 221
frame-relay route 123 interface Serial3 321
!
interface Serial2
no ip address
no ip directed-broadcast
encapsulation frame-relay
logging event subif-link-status
logging event dlci-status-change
clockrate 56000
no frame-relay inverse-arp
frame-relay intf-type dce
frame-relay route 221 interface Serial1 122

interface Serial3
no ip address
no ip directed-broadcast
encapsulation frame-relay
logging event subif-link-status
logging event dlci-status-change
clockrate 56000
no frame-relay inverse-arp
frame-relay intf-type dce
frame-relay route 321 interface Serial1 123

To verify that your frame relay configuration is functioning correctly, run the global command show frame route on the frame relay switch. If you see active next to all frame routes as shown below, you're in good shape.

If you see anything else - say, the word "inactive" - then there is a problem.

Troubleshooting A Frame Relay Switch
The key to troubleshooting your frame switch is that you cannot concentrate on the frame switch's config. You can get that part perfect, but if you're using other DLCIs on your routers or there's a physical issue - perhaps you forgot to open some interfaces - you're not going to get the active frame routes you want. Just make sure you're got all the appropriate interfaces open, don't forget the clockrate and frame-relay route commands on the frame switch, and you'll successfully add this important device to your Cisco lab !

Chris Bryant,

CCIE #12933