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
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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.
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