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Frame Relay is still one of the most popular WAN services deployed over the past decade, and
there’s a good reason for this—cost. And it’s a rare network design or designer that has the
privilege to ignore that all-important cost factor!
By default, Frame Relay is classified as a non-broadcast multi-access (NBMA) network,
meaning it doesn’t send any broadcasts like RIP updates across the network.
Frame Relay has at its roots a technology called X.25, and it essentially incorporates the
components of X.25 that are still relevant to today’s reliable and relatively “clean” telecommunications
networks, while leaving out the no-longer-needed error-correction components.
It’s substantially more complex than the simple leased-line networks you learned about when
I discussed the HDLC and PPP protocols. The leased-line networks are easy to conceptualize—
but not as much when it comes to Frame Relay. It can be significantly more complex and versatile,
which is why it’s often represented as a “cloud” in networking graphics. I’ll get to that
in a minute—for right now, I’m going to introduce Frame Relay in concept and show you how
it differs from simpler leased-line technologies.
Frame Relay Implementation and Monitoring
As I’ve said, there are a ton of Frame Relay commands and configuration options, but I’m going
to zero in on the ones you really need to know when studying for the CCNA exam objectives.
I’m going to start with one of the simplest configuration options—two routers with a single PVC
between them. Next, I’ll show you a more complex configuration using subinterfaces, and demonstrate
some of the monitoring commands available to verify the configuration.
8.3 Configure and verify Frame Relay on Cisco routers 361
Single Interface
Let’s get started by looking at a simple example. Say that we just want to connect two routers
with a single PVC. Here’s how that configuration would look:
RouterA#config t
Enter configuration commands, one per line. End with CNTL/Z.
RouterA(config)#int s0/0
RouterA(config-if)#encapsulation frame-relay
RouterA(config-if)#ip address 172.16.20.1 255.255.255.0
RouterA(config-if)#frame-relay lmi-type ansi
RouterA(config-if)#frame-relay interface-dlci 101
RouterA(config-if)#^Z
RouterA#
The first step is to specify the encapsulation as Frame Relay. Notice that since I didn’t specify
a particular encapsulation type—either Cisco or IETF—the Cisco default type was used. If the
other router were non-Cisco, I would’ve specified IETF. Next, I assigned an IP address to the
interface, then I specified the LMI type of ANSI (the default being Cisco) based on information
provided by the telecommunications provider. Finally, I added the Data Link Connection
Identifier (DLCI) of 101, which indicates the PVC we want to use (again, given to me by my
ISP) and assumes that there’s only one PVC on this physical interface.
That’s all there is to it—if both sides are configured correctly, the circuit will come up.
Subinterfaces
You probably know by now that we can have multiple virtual circuits on a single serial interface
and yet treat each as a separate interface—I did mention this earlier. We can make this happen by
creating subinterfaces. Think of a subinterface as a logical interface defined by the IOS software.
Several subinterfaces will share a single hardware interface, yet for configuration purposes they
operate as if they were separate physical interfaces, something known as multiplexing.
To configure a router in a Frame Relay network so that it will avoid split horizon issues by
not permitting routing updates, just configure a separate subinterface for each PVC, with a
unique DLCI and subnet assigned to the subinterface.
You define subinterfaces using a command like int s0.subinterface number. First,
you have to set the encapsulation on the physical serial interface, and then you can define the
subinterfaces—generally one subinterface per PVC. Here’s an example:
RouterA(config)#int s0
RouterA(config-if)#encapsulation frame-relay
RouterA(config-if)#int s0.?
<0-4294967295> Serial interface number
RouterA(config-if)#int s0.16 ?
multipoint Treat as a multipoint link
point-to-point Treat as a point-to-point link
RouterA(config-if)#int s0.16 point-to-point
362 Chapter 8 Implement and verify WAN links
Make sure that you don’t have an IP address under the physical interface if
you have configured subinterfaces!
You can define a serious amount of subinterfaces on any given physical interface, but keep
in mind that there are only about a thousand available DLCIs. In the preceding example, I
chose to use subinterface 16 because that represents the DLCI number assigned to that PVC
by the carrier. There are two types of subinterfaces:
Point-to-point Used when a single virtual circuit connects one router to another. Each pointto-
point subinterface requires its own subnet.
A point-to-point subinterface maps a single IP subnet per DLCI and addresses
and resolves NBMA split horizon issues.
Multipoint This is when the router is the center of a star of virtual circuits that are using a
single subnet for all routers’ serial interfaces connected to the frame switch. You’ll usually find
this implemented with the hub router in this mode and the spoke routers in physical interface
(always point-to-point) or point-to-point subinterface mode.
Monitoring Frame Relay
Several commands are used frequently to check the status of your interfaces and PVCs once
you have Frame Relay encapsulation set up and running. To list them, use the show frame ?
command, as shown here:
RouterA>sho frame ?
end-to-end Frame-relay end-to-end VC information
fragment show frame relay fragmentation information
ip show frame relay IP statistics
lapf show frame relay lapf status/statistics
lmi show frame relay lmi statistics
map Frame-Relay map table
pvc show frame relay pvc statistics
qos-autosense show frame relay qos-autosense information
route show frame relay route
svc show frame relay SVC stuff
traffic Frame-Relay protocol statistics
vofr Show frame-relay VoFR statistics
The most common parameters that you view with the show frame-relay command are
lmi, pvc, and map.
Now, let’s take a look at the most frequently used commands and the information they provide.
8.3 Configure and verify Frame Relay on Cisco routers 363
The show frame-relay lmi Command
The show frame-relay lmi command will give you the LMI traffic statistics exchanged
between the local router and the Frame Relay switch. Here’s an example:
Router#sh frame lmi
LMI Statistics for interface Serial0 (Frame Relay DTE)
LMI TYPE = CISCO
Invalid Unnumbered info 0 Invalid Prot Disc 0
Invalid dummy Call Ref 0 Invalid Msg Type 0
Invalid Status Message 0 Invalid Lock Shift 0
Invalid Information ID 0 Invalid Report IE Len 0
Invalid Report Request 0 Invalid Keep IE Len 0
Num Status Enq. Sent 0 Num Status msgs Rcvd 0
Num Update Status Rcvd 0 Num Status Timeouts 0
Router#
The router output from the show frame-relay lmi command shows you any LMI errors,
plus the LMI type.
The show frame pvc Command
The show frame pvc command will present you with a list of all configured PVCs and DLCI
numbers. It provides the status of each PVC connection and traffic statistics too. It will also
give you the number of BECN and FECN (BECN and FECN and discussed in detail in the
CCNA Study Guide) packets received on the router per PVC.
Here is an example:
RouterA#sho frame pvc
PVC Statistics for interface Serial0 (Frame Relay DTE)
DLCI = 16,DLCI USAGE = LOCAL,PVC STATUS =ACTIVE,
INTERFACE = Serial0.1
input pkts 50977876 output pkts 41822892
in bytes 3137403144
out bytes 3408047602 dropped pkts 5
in FECN pkts 0
in BECN pkts 0 out FECN pkts 0 out BECN pkts 0
in DE pkts 9393 out DE pkts 0
pvc create time 7w3d, last time pvc status changed 7w3d
DLCI = 18,DLCI USAGE =LOCAL,PVC STATUS =ACTIVE,
364 Chapter 8 Implement and verify WAN links
INTERFACE = Serial0.3
input pkts 30572401 output pkts 31139837
in bytes 1797291100
out bytes 3227181474 dropped pkts 5
in FECN pkts 0
in BECN pkts 0 out FECN pkts 0 out BECN pkts 0
in DE pkts 28 out DE pkts 0
pvc create time 7w3d, last time pvc status changed 7w3d
If you only want to see information about PVC 16, you can type the command show
frame-relay pvc 16.
The show interface Command
You can use the show interface command to check for LMI traffic. The show interface
command displays information about the encapsulation, as well as layer 2 and layer 3 information.
It also displays line, protocol, DLCI, and LMI information. Check it out:
RouterA#sho int s0
Serial0 is up, line protocol is up
Hardware is HD64570
MTU 1500 bytes, BW 1544 Kbit, DLY 20000 usec, rely
255/255, load 2/255
Encapsulation FRAME-RELAY, loopback not set, keepalive
set (10 sec)
LMI enq sent 451751,LMI stat recvd 451750,LMI upd recvd
164,DTE LMI up
LMI enq recvd 0, LMI stat sent 0, LMI upd sent 0
LMI DLCI 1023 LMI type is CISCO frame relay DTE
Broadcast queue 0/64, broadcasts sent/dropped 0/0,
interface broadcasts 839294
The LMI DLCI above is used to define the type of LMI being used. If it happens to be 1023,
it’s the default LMI type of Cisco. If LMI DLCI is zero, then it’s the ANSI LMI type (Q.933A
uses 0 as well). If LMI DLCI is anything other than 0 or 1023, it’s a 911—call your provider;
they’ve got major issues!
The show frame map Command
The show frame map command displays the Network layer–to–DLCI mappings. Here’s how
that looks:
RouterB#show frame map
Serial0 (up): ipx 20.0007.7842.3575 dlci 16(0x10,0x400),
dynamic, broadcast,, status defined, active
8.3 Configure and verify Frame Relay on Cisco routers 365
Serial0 (up): ip 172.16.20.1 dlci 16(0x10,0x400),
dynamic, broadcast,, status defined, active
Serial1 (up): ipx 40.0007.7842.153a dlci 17(0x11,0x410),
dynamic, broadcast,, status defined, active
Serial1 (up): ip 172.16.40.2 dlci 17(0x11,0x410),
dynamic, broadcast,, status defined, active
Notice that the serial interfaces have two mappings—one for IP and one for IPX. Also important
is that the Network layer addresses were resolved with the dynamic protocol Inverse ARP
(IARP). After the DLCI number is listed, you can see some numbers in parentheses. The first one
is 0x10, which is the hex equivalent for the DLCI number 16, used on serial 0. And the 0x11 is
the hex for DLCI 17 used on serial 1. The second numbers, 0x400 and 0x410, are the DLCI
numbers configured in the Frame Relay frame. They’re different because of the way the bits are
spread out in the frame.
The debug frame lmi Command
The debug frame lmi command will show output on the router consoles by default (as with any
debug command). The information this command gives you will enable you to verify and troubleshoot
the Frame Relay connection by helping you determine whether the router and switch are
exchanging the correct LMI information. Here’s an example:
Router#debug frame-relay lmi
Serial3/1(in): Status, myseq 214
RT IE 1, length 1, type 0
KA IE 3, length 2, yourseq 214, myseq 214
PVC IE 0x7 , length 0x6 , dlci 130, status 0x2 , bw 0
Serial3/1(out): StEnq, myseq 215, yourseen 214, DTE up
datagramstart = 0x1959DF4, datagramsize = 13
FR encap = 0xFCF10309
00 75 01 01 01 03 02 D7 D6
Serial3/1(in): Status, myseq 215
RT IE 1, length 1, type 1
KA IE 3, length 2, yourseq 215, myseq 215
Serial3/1(out): StEnq, myseq 216, yourseen 215, DTE up
datagramstart = 0x1959DF4, datagramsize = 13
FR encap = 0xFCF10309
00 75 01 01 01 03 02 D8 D7
Exam Objectives
Understand what the LMI is in Frame Relay. The LMI is a signaling standard between a
CPE device (router) and a frame switch. The LMI is responsible for managing and maintaining
366 Chapter 8 Implement and verify WAN links
the status between these devices. This also provides transmission keepalives to ensure that the
PVC does not shut down because of inactivity.
Understand the different Frame Relay encapsulations. Cisco uses two different Frame
Relay encapsulation methods on their routers. Cisco is the default, and means that the router
is connected to a Cisco Frame Relay switch; Internet Engineering Task Force (IETF) means
that your router is connecting to anything except a Cisco Frame Relay switch.
Remember what the CIR is in Frame Relay. The CIR is the rate, in bits per second, at which
the Frame Relay switch agrees to transfer data.
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