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Frame Relay Switch Configuration
A Cisco Lab Case Study - Now Includes Two Video Tutorials!
By Chris Bryant, CCIE #12933
Many CCNA certification candidates want to add a frame relay
switch to theirCisco 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 bydoing 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 commandframe-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-changeclockrate 56000
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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 theframe-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 theframe-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-zerono ip domain-lookup
frame-relay switching!!!interface Ethernet0no ip addressno ip
directed-broadcast
shutdown!interface Serial0no ip address
interface Serial1no ip addressno ip
directed-broadcastencapsulation frame-relaylogging event
subif-link-status
logging event dlci-status-changeclockrate 56000no frame-relay
inverse-arpframe-relay intf-type dce
frame-relay route 122 interface Serial2 221frame-relay route 123
interface Serial3 321
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!
interface Serial2no ip addressno ip
directed-broadcastencapsulation frame-relaylogging event
subif-link-statuslogging event dlci-status-changeclockrate
56000
no frame-relay inverse-arpframe-relay intf-type dceframe-relay
route 221 interface Serial1 122
interface Serial3no ip addressno ip
directed-broadcastencapsulation frame-relaylogging event
subif-link-statuslogging event dlci-status-changeclockrate 56000no
frame-relay inverse-arp
frame-relay intf-type dceframe-relay route 321 interface Serial1
123
To verify that your frame relay configuration is functioning
correctly, run the globalcommand show frame route on the frame
relay switch. If you see active next to all frameroutes 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
cannotconcentrate on the frameswitch's config. You can get that
part perfect, but if you're using other DLCIs on your routersor
there's a physical issue - perhaps you forgot to open some
interfaces - you're not going toget the active frame routes you
want.
Just make sure you're got all the appropriate interfaces open,
don't forget the clockrate andframe-relay route commands on the
frame switch, and you'll successfully add thisimportant device to
your Cisco lab!
