The command used to enable Frame Relay switching on a Cisco router is as follows:
Router_A#config t
Router_A(config)#frame-relay switching
This command must come before any of the other Frame Relay switching–related commands
can be executed, or these commands won’t be allowed. When Frame Relay encapsulation is
enabled on an interface, it defaults to DTE, so you will need to change it to DCE for Frame
Relay switching. For a Frame Relay serial connection to function, you must have a DTE at one
end and a DCE at the other. You first configure the router with the following command:
Router_A(config)#interface serial0
Router_A(config-if)#frame-relay intf-type dce
The clocking on a serial link is provided by the DCE device, which is determined
by the type of cable connected to the serial interface. For a Frame Relay connection,
the DCE status is configured, whereas serial DCE status is cabled. For
Frame Relay, the DCE device is the one that provides LMI.
Because this interface is now functioning as the Frame Relay DCE device, you can change the
LMI type from the default of Cisco with the following command:
Router_A(config-if)#frame-relay lmi-type ?
cisco
ansi
q933a
Router_A(config-if)#frame-relay lmi-type
The next step in the configuration process is to create the proper DLCI forwarding rules.
These rules dictate that when a frame enters a particular interface on a certain DLCI, it will be
forwarded to another interface and DLCI. Let’s look at such an example on interface serial 1:
Router_A#config t
Router_A(config)#interface serial 1
Router_A(config-if)#frame-relay route 100 interface Serial2 101
This command states that any frame received on interface serial 1, with DLCI 100, shall be
forwarded to interface serial 2, with DLCI 101. You can view all the frame routing information
with the show frame-relay route command. The following router output shows the settings
of Router A:
Router_A#show frame-relay route
Input Intf Input Dlci Output Intf Output Dlci Status
Serial0 300 Serial1 200 active
Serial1 100 Serial2 101 active
Serial1 200 Serial0 300 active
Serial2 101 Serial1 100 active
Router_A#
The configuration of a router as a Frame Relay switch can be useful for a lab environment
or even as part of a production network.
Now, let’s look at the configuration of the Frame Relay switch:
Router_A#show running-config
Building configuration...
Current configuration:
!
version 11.2
!
hostname Router_A
!
frame-relay switching
!
interface Serial0
no ip address
encapsulation frame-relay
clockrate 56000
frame-relay intf-type dce
frame-relay route 300 interface Serial1 200
!
interface Serial1
no ip address
encapsulation frame-relay
clockrate 56000
frame-relay intf-type dce
frame-relay route 100 interface Serial2 101
frame-relay route 200 interface Serial0 300
!
interface Serial2
no ip address
encapsulation frame-relay
clockrate 56000
frame-relay intf-type dce
frame-relay route 101 interface Serial1 100
!
end
Router_A#
Notice the global command frame-relay switching is at the top of the configuration.
Also notice that both interfaces are configured with frame-relay intf-type dce commands.
On the serial interfaces, you’ll also see that the clock rate command is used to provide clocking
for the line because the router serving as the Frame Relay switch is a DCE device on the
physical interface.
IT Certification CCIE,CCNP,CCIP,CCNA,CCSP,Cisco Network Optimization and Security Tips
Frame Relay Switching Network
Routers are typically edge devices that connect your LANs to the Frame Relay network. However,
you can use a router as part of the Frame Relay cloud or you can use it to create your own Frame
Relay network. Frame Relay switching is the forwarding of Frame Relay frames based upon their
DLCI assignments. You have seen how to configure a Frame Relay DTE device; now, let’s look
at how to configure a Frame Relay DCE switch. Routers are DTE devices by default; however, by
changing the Frame Relay interface type to a DCE, you can provide switching of frames.
Compare Figure 29.8 to Figure 29.9. Both of these diagrams represent the same network. In
Figure 29.8, you see the Frame Relay cloud without any detail. Each router on the right side will
send traffic to the router on the left by using DLCI 100. Keep in mind that the DLCI is an identifier
and that the DLCIs in this diagram could be the same or different and still communicate.
The router on the left will use DLCIs 101 and 300 to reach each router on the right. This is the
normal way that you should think about the frame cloud. It is typically not your concern what
happens within the Frame Relay network. Figure 29.9 shows that this particular Frame Relay
cloud is a single router configured as a switch.
FIGURE 2 9 . 8 Logical Frame Relay network
DLCI 300
DLCI 100 DLCI 101
DLCI 100
Frame Relay cloud
Physical Frame Relay network
DLCI 101
DLCI 100
Frame Relay cloud
Serial 1
Serial 0
Serial 2
you can use a router as part of the Frame Relay cloud or you can use it to create your own Frame
Relay network. Frame Relay switching is the forwarding of Frame Relay frames based upon their
DLCI assignments. You have seen how to configure a Frame Relay DTE device; now, let’s look
at how to configure a Frame Relay DCE switch. Routers are DTE devices by default; however, by
changing the Frame Relay interface type to a DCE, you can provide switching of frames.
Compare Figure 29.8 to Figure 29.9. Both of these diagrams represent the same network. In
Figure 29.8, you see the Frame Relay cloud without any detail. Each router on the right side will
send traffic to the router on the left by using DLCI 100. Keep in mind that the DLCI is an identifier
and that the DLCIs in this diagram could be the same or different and still communicate.
The router on the left will use DLCIs 101 and 300 to reach each router on the right. This is the
normal way that you should think about the frame cloud. It is typically not your concern what
happens within the Frame Relay network. Figure 29.9 shows that this particular Frame Relay
cloud is a single router configured as a switch.
FIGURE 2 9 . 8 Logical Frame Relay network
DLCI 300
DLCI 100 DLCI 101
DLCI 100
Frame Relay cloud
Physical Frame Relay network
DLCI 101
DLCI 100
Frame Relay cloud
Serial 1
Serial 0
Serial 2
The debug frame-relay lmi Command
The debug frame-relay lmi command is used to help you troubleshoot and verify Frame
Relay connections. As you’ll see in the router output shown next, the out parameter is an LMI
status inquiry sent out from the router, and the (in) parameter is a reply from the Frame
Relay switch:
Router#debug frame-relay lmi
Serial0(in): Status, myseq 128
RT IE 1, length 1, type 0
KA IE 3, length 2, yourseq 128, myseq 128
PVC IE 0x7, length 0x6, dlci 16, status 0x2, bw 0
Serial0(out): StEnq, myseq 128, yourseen 214, DTE up
datagramstart = 0x1959DF4, datagramsize = 13
FR encap = 0xFCF10309
00 75 01 01 01 03 02 C6 E5
Serial0(in): Status, myseq 129
RT IE 1, length 1, type 1
KA IE 3, length 2, yourseq 129, myseq 129
Serial0(out): StEnq, myseq 130, yourseen 129, DTE up
datagramstart = 0x1959DF4, datagramsize = 13
FR encap = 0xFCF10309
00 74 01 01 01 03 02 C9 E3
The type 1 is an LMI keepalive from the router to the Frame Relay switch every 10 seconds.
This tells the router that the switch is still active and vice versa. The type 0 is an IARP exchanged
between routers every 60 seconds. Now notice that when the type is 0, you have a full status message
on your hands. When the type is 1, it is the standard status message, which is the keepalive
mentioned earlier (sometimes called the heartbeat).
Remember that the DLCI is known. There is no IP address in this output. Therefore, this
would be a poor excuse for an InARP reply (notice the in designation, corresponding to a reply).
Furthermore, Status and StEnq messages go between the DTE and DCE only. These messages
do not traverse the cloud, meaning they couldn’t possibly have anything to do with InARP. Full
status messages from the switch include all PVCs known by the switch. In this case, there’s only
one—DLCI = 16. The type 1 message will always have three lines, whereas the type 0 message
will have four or more, assuming PVCs exist. You would see InARP messages coming in the
exact same way you would see any other non-LMI frames coming in (such as ICMP pings), by
using the debug frame-relay packet command. Status indicators are pretty straightforward
and adhere to the following rules:
0x0 (no bits turned on) means “inactive.”
0x2 (second bit turned on) means “active.”
0x3 means that the active DLCI cannot accept any more traffic without drops occurring—
sort of a flow-control code. (Active bit [2nd] is on, but the first bit is also on as sort of a
Receive Not Ready (RNR) bit because Cisco uses this bit even though the ITU-T indicates
the first bit is reserved.)
0x4 (third bit turned on) means “deleted.”
Looking at our status, the DLCI 16, status 0x2 means the DLCI is active.
Relay connections. As you’ll see in the router output shown next, the out parameter is an LMI
status inquiry sent out from the router, and the (in) parameter is a reply from the Frame
Relay switch:
Router#debug frame-relay lmi
Serial0(in): Status, myseq 128
RT IE 1, length 1, type 0
KA IE 3, length 2, yourseq 128, myseq 128
PVC IE 0x7, length 0x6, dlci 16, status 0x2, bw 0
Serial0(out): StEnq, myseq 128, yourseen 214, DTE up
datagramstart = 0x1959DF4, datagramsize = 13
FR encap = 0xFCF10309
00 75 01 01 01 03 02 C6 E5
Serial0(in): Status, myseq 129
RT IE 1, length 1, type 1
KA IE 3, length 2, yourseq 129, myseq 129
Serial0(out): StEnq, myseq 130, yourseen 129, DTE up
datagramstart = 0x1959DF4, datagramsize = 13
FR encap = 0xFCF10309
00 74 01 01 01 03 02 C9 E3
The type 1 is an LMI keepalive from the router to the Frame Relay switch every 10 seconds.
This tells the router that the switch is still active and vice versa. The type 0 is an IARP exchanged
between routers every 60 seconds. Now notice that when the type is 0, you have a full status message
on your hands. When the type is 1, it is the standard status message, which is the keepalive
mentioned earlier (sometimes called the heartbeat).
Remember that the DLCI is known. There is no IP address in this output. Therefore, this
would be a poor excuse for an InARP reply (notice the in designation, corresponding to a reply).
Furthermore, Status and StEnq messages go between the DTE and DCE only. These messages
do not traverse the cloud, meaning they couldn’t possibly have anything to do with InARP. Full
status messages from the switch include all PVCs known by the switch. In this case, there’s only
one—DLCI = 16. The type 1 message will always have three lines, whereas the type 0 message
will have four or more, assuming PVCs exist. You would see InARP messages coming in the
exact same way you would see any other non-LMI frames coming in (such as ICMP pings), by
using the debug frame-relay packet command. Status indicators are pretty straightforward
and adhere to the following rules:
0x0 (no bits turned on) means “inactive.”
0x2 (second bit turned on) means “active.”
0x3 means that the active DLCI cannot accept any more traffic without drops occurring—
sort of a flow-control code. (Active bit [2nd] is on, but the first bit is also on as sort of a
Receive Not Ready (RNR) bit because Cisco uses this bit even though the ITU-T indicates
the first bit is reserved.)
0x4 (third bit turned on) means “deleted.”
Looking at our status, the DLCI 16, status 0x2 means the DLCI is active.
The show frame-relay lmi Command
The show frame-relay lmi command shows you the LMI statistics for an interface, as in the
following example:
Router#show 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 109087 Num Status msgs Rcvd 109087
Num Update Status Rcvd 0 Num Status Timeouts 0
Router#
The important statistics to notice are the number of inquiries sent and received. This
indicates the number of LMI status messages sent by the DTE device. The DCE sends a
status message in return. Noticing this statistic enables you to see whether data is passing
between the two devices.
following example:
Router#show 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 109087 Num Status msgs Rcvd 109087
Num Update Status Rcvd 0 Num Status Timeouts 0
Router#
The important statistics to notice are the number of inquiries sent and received. This
indicates the number of LMI status messages sent by the DTE device. The DCE sends a
status message in return. Noticing this statistic enables you to see whether data is passing
between the two devices.
The show frame-relay map Command
You can see the current map entries and information about the connections by using the show
frame-relay map command. This command shows the Network-layer-to-DLCI mappings
table in the router. An example output of this command is given here:
Router_A#show frame-relay map
Serial0 (up): ip 172.16.1.2 dlci 500(0x64,0x1840), dynamic,
broadcast, status defined, active
Router_A#
LMI used IARP to determine the address of the remote router and created this dynamic mapping.
If you want to clear the Network layer-to-DLCI mappings on a router, you can
use the command clear frame-relay-inarp, which clears dynamically created
maps on the router.
frame-relay map command. This command shows the Network-layer-to-DLCI mappings
table in the router. An example output of this command is given here:
Router_A#show frame-relay map
Serial0 (up): ip 172.16.1.2 dlci 500(0x64,0x1840), dynamic,
broadcast, status defined, active
Router_A#
LMI used IARP to determine the address of the remote router and created this dynamic mapping.
If you want to clear the Network layer-to-DLCI mappings on a router, you can
use the command clear frame-relay-inarp, which clears dynamically created
maps on the router.
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