Frame Relay Switch Setup

Read­ing Time: 5 min­utes

Edi­tor’s Note: Google Chrome seems to dis­like my site theme and is hyphen­at­ing absolute­ly every­thing.  Apolo­gies for that, and I’ll look into it just as soon as I get done with a few items on the “hon­ey-do” list.

If you are study­ing for the CCIE Rout­ing and Switch­ing exam, one of the tech­nolo­gies that is still heav­i­ly preva­lent is Frame Relay.  It is expect­ed that you know both the tech­nol­o­gy itself, and how to con­fig­ure it, but also how it inter­acts with and affects oth­er key tech­nolo­gies like OSPF and EIGRP.  Hav­ing the abil­i­ty to study Frame Relay, then, and get plen­ty of hands-on con­fig­u­ra­tion time becomes as impor­tant as with any­thing on the R&S 4.0 Blue­print.

While many net­work engi­neers are already famil­iar with Frame Relay from a con­sumer side–in oth­er words, from the per­spec­tive of an enti­ty which buys Frame Relay ser­vices from a provider–not many of us are famil­iar with the ser­vice provider por­tion of the equa­tion.  This makes set­ting up prac­tice labs dif­fi­cult if you are try­ing to study using your own equip­ment.  For­tu­nate­ly, you can set up your own Frame Relay switch fair­ly eas­i­ly, and that is what we’re going to walk through today.

A Frame Relay switch is the DCE device that sits inside a ser­vice provider’s net­work and moves the frames along from point A to point B.  There are many of these devices all work­ing togeth­er inside of your provider’s net­work to move your infor­ma­tion along, but for­tu­nate­ly for lab can­di­dates study­ing at home, you can eas­i­ly get by with just one.  Even more for­tu­nate is that you can use a fair­ly low pow­ered router to act as a Frame Relay switch, and not miss any­thing that you’ll need for pur­pos­es of the lab.

A quick note on the lab is in order here.  It used to be a part of the lab blue­print (dont’ ask me which one, or how far back in time) that you had to know how to set up a Frame Relay switch.  Cis­co has since tak­en that require­ment away, at least from the R&S lab, and so a lot of that knowl­edge isn’t com­mu­ni­cat­ed in teach­ing texts any longer.  What you’ll find in the lab itself is an already con­fig­ured Frame Relay switch that you’ll have no direct access to, but all of the infor­ma­tion you need to make your equip­ment talk to it.

It may seem coun­ter­in­tu­itive, but for a home lab the best device to use for a Frame Switch is actu­al­ly a router.  For instance, I’m using an old­er Cis­co 2621 mod­el for my Frame Switch, and it does every­thing I need it to do.  Ser­vice providers will typ­i­cal­ly use more spe­cial­ized gear, but all we’re going for in our stud­ies is a rea­son­able fac­sim­i­le.  If you want to spend a lot of mon­ey, fol­low the advice of so many oth­ers and spend it on your layer‑3 switch­es.

Anoth­er thing we want to briefly dis­cuss is inter­faces.  Gen­er­al­ly speak­ing, you can either fol­low the “run what’cha brung” phi­los­o­phy of just using what you have access to, or you can buy the inter­faces you want.  In my case I had a cou­ple of WIC-T1 cards that I’ve used, and then I bought a hand­ful of WIC-2T ser­i­al inter­face cards.  The key is to have a ser­i­al inter­face for each router you want to con­nect via the Frame switch.  So I have one T1 inter­face, and six ser­i­al inter­faces for a total of sev­en devices I can con­nect into the Frame “cloud”.  I find this to me more than ade­quate, though if you’re try­ing to dupli­cate a spe­cif­ic topol­o­gy you may need more or less.

The con­fig­u­ra­tion of a Frame switch is actu­al­ly very sim­ple, as you’ll see, though atten­tion to detail does mat­ter.  I’m assum­ing here, by the way, that you already know how to set up your router for basic access, clock, etc., so I won’t cov­er that here.  So, the first step in con­fig­ur­ing your router to be a Frame switch is to put it into Frame switch­ing mode using the com­mands:

ip cef

frame-relay switching

These com­mands turn on Cis­co Express For­ward­ing, put the router into a Frame switch­ing mode, and change quite a bit of the default behav­ior, so don’t expect to use this device as a router in any lab topol­o­gy you’re work­ing on.  This device will be just a Frame switch and noth­ing more.

The next step is to con­fig­ure the indi­vid­ual inter­faces you’ll con­nect your oth­er routers to, and you have a lot of choic­es here.  I don’t know exact­ly how the R&S lab devices are set up, so I’m just going to give you the con­fig­u­ra­tion I use.  I’ll post the con­fig­u­ra­tion below, and then go over the key comands:

interface Serial0/1

no ip address

encapsulation frame-relay

logging event subif-link-status

logging event dlci-status-change

clock rate 8000000

no frame-relay inverse-arp

frame-relay intf-type dce

frame-relay route 220 interface Serial0/2 120

frame-relay route 221 interface Serial0/0 320

The first few lines of the con­fig­u­ra­tion should be famil­iar to you already.  We’re set­ting our inter­face encap­su­la­tion to frame-relay, and then log­ging on a cou­ple of events.  The log­ging is com­plete­ly up to you, and not nec­es­sary one way or anoth­er.  I just find them help­ful.  Next we set the clock rate, and we tell the inter­face that we are the DCE end of the con­nec­tion.  Remem­ber, in a Frame Relay net­work the clock­ing (DCE end) comes from the line or provider side, so this is what you’ll want.  If I am work­ing with a T1 ser­i­al inter­face, I’ll also need a line for that:

service-module t1 clock source internal

This can change depend­ing on the type of card and how you have it con­fig­ured.

Now, the oth­er options we have here require a lit­tle more expla­na­tion.  The “no frame-relay inverse-arp” com­mand does just what it says, and you can argue for the Frame switch hav­ing this turned on, or off.  In most cas­es in the lab, you’ll be instruct­ed to not use inverse arp on the DTE devices, so I’ve just turned that func­tion­al­i­ty off on my Frame switch from the out­set.  It’s real­ly your call.

The next two lines begin­ning with frame-relay route are the ones that always seem to cause con­fu­sion.  You can read the first line as “If some traf­fic comes in from DLCI 220, with a des­ti­na­tion of DLCI 120, send it out inter­face Ser­i­al 0/2”.  Sub­sti­tute DLCI 221 and 320 on the next line, but oth­er­wise read it the same.  So if I now plug in a router to inter­face Ser­i­al 0/1, and assign DLCI 220 and 221 to two dif­fer­ent sub-inter­faces (for instance, dif­fer­ent options are pos­si­ble) the Frame switch will know what to do with that traf­fic.

So, if we have a dia­gram that looks like the fol­low­ing:

Then we have a con­fig­u­ra­tion for inter­faces that looks like so:

interface Serial0/0

no ip address

encapsulation frame-relay

logging event subif-link-status

logging event dlci-status-change

service-module t1 clock source internal

no frame-relay inverse-arp

frame-relay intf-type dce

frame-relay route 320 interface Serial0/1 221

frame-relay route 321 interface Serial0/2 121

!

interface Serial0/1

no ip address

encapsulation frame-relay

logging event subif-link-status

logging event dlci-status-change

clock rate 8000000

no frame-relay inverse-arp

frame-relay intf-type dce

frame-relay route 220 interface Serial0/2 120

frame-relay route 221 interface Serial0/0 320

!

interface Serial0/2

no ip address

encapsulation frame-relay

logging event subif-link-status

logging event dlci-status-change

clock rate 8000000

no frame-relay inverse-arp

frame-relay intf-type dce

frame-relay route 120 interface Serial0/1 220

frame-relay route 121 interface Serial0/0 321

I hope that helps out, and as always if you have any ques­tions or clar­i­fi­ca­tions please drop me a line here or on twit­ter where I’m known as @SomeClown.