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Cisco CCENT / CCNA 640-802 Certification Exam Training :

Detecting And Troubleshooting Frame Relay LMI Mismatches

By Chris Bryant, CCIE #12933

In my last CCNA 640-802 / CCENT certification exam training tutorial, you learned all about Frame Relay and LMI Autosense. Today, we'll create an LMI mismatch, practice our debugging and troubleshooting tools, and prepare you for exam success!

Right now, our Cisco router is sending the LMI type Cisco. We expect that, since Cisco is the default LMI type and we haven't changed it. show frame lmi verifies this.

R1#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 50               Num Status msgs Rcvd 50
Num Update Status Rcvd 0              Num Status Timeouts 0

The top line of the output indicates both the interface and the LMI type set on that interface. Here, it's the default of Cisco, since we haven't changed the LMI type -- yet.

The three fields that we're most interested in here are the two bolded fields and the "Num Status Timeouts" column. As the LMIs continue to be exchanged, the "Enq Sent" and "Msgs Rcvd" should continue to increment and the Timeouts value should remain where it is. Let's take another look at this output just a few minutes later.

R1#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 64               Num Status msgs Rcvd 64
Num Update Status Rcvd 0              Num Status Timeouts 0

show interface serial 0 verifies that the interface is physically up and the line protocol (the logical state of the interface) is up as well. The keepalive for Frame Relay is set to 10 seconds - that's how often LMI messages are going out.

Now that we know how things look when the LMI matches, let's set the LMI type on the router to ansi and see what happens.

LMI Mismatch

R1(config)#int serial0
R1(config-if)#frame lmi-type ansi

About 30 seconds later, the line protocol comes down.

R1(config)#int serial0
R1(config-if)#frame lmi-type ansi
R1(config-if)#
3d04h: %LINEPROTO-5-UPDOWN: Line protocol on Interface Serial0, changed state to down

R1#show int s0
Serial0 is up, line protocol is down

You and I know why the line protocol is down, since we did it deliberately. But what if you had just walked into a client site and they're complaining that their Frame Relay WAN link is down? The first step is to run show interface serial, which we just did. We see that the line protocol is down and that the interface is running Frame Relay.

The "Serial0 is up" part of the show int s0 output tells us that everything is fine physically, but there is a logical problem. 99% of the time with Frame Relay, that's an LMI issue. Let's run show frame lmi twice, a few minutes apart, and see what we can see.

R1#show frame lmi

LMI Statistics for interface Serial0 (Frame Relay DTE) LMI TYPE = ANSI
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 121          Num Status msgs Rcvd 94
Num Update Status Rcvd 0              Num Status Timeouts 26

R1#show frame lmi

LMI Statistics for interface Serial0 (Frame Relay DTE) LMI TYPE = ANSI
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 134          Num Status msgs Rcvd 94
Num Update Status Rcvd 0              Num Status Timeouts 39

LMI message are still going out, so that's good. The bad part is that the timeout counter is incrementing while the msgs rcvd counter is not. Let's dig a little deeper and run debug frame lmi.

R1#debug frame lmi
Frame Relay LMI debugging is on
Displaying all Frame Relay LMI data
R1#
3d04h: Serial0(out): StEnq, myseq 49, yourseen 94, DTE down
3d04h: datagramstart = 0xE329E4, datagramsize = 14
3d04h: FR encap = 0x00010308
3d04h: 00 75 95 01 01 00 03 02 31 5E
3d04h:
R1#
3d04h: Serial0(out): StEnq, myseq 50, yourseen 94, DTE down
3d04h: datagramstart = 0xE24444, datagramsize = 14
3d04h: FR encap = 0x00010308
3d04h: 00 75 95 01 01 00 03 02 32 5E
3d04h:
R1#
3d04h: Serial0(out): StEnq, myseq 51, yourseen 94, DTE down
3d04h: datagramstart = 0xE24574, datagramsize = 14
3d04h: FR encap = 0x00010308
3d04h: 00 75 95 01 01 00 03 02 33 5E
3d04h:
R1#undebug all
All possible debugging has been turned off

When myseq continues to increment but yourseen is not, that's another indicator of an LMI mismatch. I'll turn the debug back on, change the LMI type back to Cisco, and we'll see the result.

R1#debug frame lmi
Frame Relay LMI debugging is on
Displaying all Frame Relay LMI data

R1#conf t
Enter configuration commands, one per line. End with CNTL/Z.
R1(config)#int s0
R1(config-if)#frame lmi-type cisco
R1(config-if)#
3d04h: Serial0(out): StEnq, myseq 63, yourseen 94, DTE down
3d04h: datagramstart = 0xE0183C, datagramsize = 14
3d04h: FR encap = 0x00010308
3d04h: 00 75 95 01 01 00 03 02 3F 5E
3d04h:
R1(config-if)#
3d04h: Serial0(out): StEnq, myseq 64, yourseen 94, DTE down
3d04h: datagramstart = 0xE01A9C, datagramsize = 13
3d04h: FR encap = 0xFCF10309
3d04h: 00 75 01 01 00 03 02 40 5E
3d04h:
3d04h: Serial0(in): Status, myseq 64
3d04h: RT IE 1, length 1, type 0
3d04h: KA IE 3, length 2, yourseq 1 , myseq 64
3d04h: PVC IE 0x7 , length 0x6 , dlci 122, status 0x0 , bw 0
3d04h: PVC IE 0x7 , length 0x6 , dlci 123, status 0x0 , bw 0
R1(config-if)#
3d04h: Serial0(out): StEnq, myseq 65, yourseen 1, DTE down
3d04h: datagramstart = 0xE01CFC, datagramsize = 13
3d04h: FR encap = 0xFCF10309
3d04h: 00 75 01 01 01 03 02 41 01
3d04h:
3d04h: Serial0(in): Status, myseq 65
3d04h: RT IE 1, length 1, type 1
3d04h: KA IE 3, length 2, yourseq 2 , myseq 65
R1(config-if)#
3d04h: Serial0(out): StEnq, myseq 66, yourseen 2, DTE up
3d04h: datagramstart = 0xE23BD0, datagramsize = 13
3d04h: FR encap = 0xFCF10309
3d04h: 00 75 01 01 01 03 02 42 02
3d04h:
3d04h: Serial0(in): Status, myseq 66
3d04h: RT IE 1, length 1, type 0
3d04h: KA IE 3, length 2, yourseq 3 , myseq 66
3d04h: PVC IE 0x7 , length 0x6 , dlci 122, status 0x0 , bw 0
3d04h: PVC IE 0x7 , length 0x6 , dlci 123, status 0x0 , bw 0
3d04h: %LINEPROTO-5-UPDOWN: Line protocol on Interface Serial0, changed state t
up
R1(config-if)#^Z
R1#
3d04h: Serial0(out): StEnq, myseq 67, yourseen 3, DTE up
3d04h: datagramstart = 0xE23D00, datagramsize = 13
3d04h: FR encap = 0xFCF10309
3d04h: 00 75 01 01 01 03 02 43 03
3d04h:
3d04h: Serial0(in): Status, myseq 67
3d04h: RT IE 1, length 1, type 1
3d04h: KA IE 3, length 2, yourseq 4 , myseq 67

R1#undebug all
All possible debugging has been turned off

As yourseq and yourseen begin to increment, the line protocol comes back up. Once you see that, you should be fine, but always stick around for a minute or so and make sure the line protocol stays up. Once you're done, always do two things....

Verify the line protocol is up with show interface serial. Note that you can see other information relating to the LMI in this output.

R1#show int s0
Serial0 is up, line protocol is up
Hardware is HD64570
Internet address is 172.12.123.1/24
MTU 1500 bytes, BW 1544 Kbit, DLY 20000 usec,
reliability 255/255, txload 1/255, rxload 1/255
Encapsulation FRAME-RELAY, loopback not set
Keepalive set (10 sec)
LMI enq sent 180, LMI stat recvd 114, LMI upd recvd 0, 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

And before you leave the client site... turn off your debugs, either individually or with the undebug all command.

R1#undebug all
All possible debugging has been turned off

The LMI isn't the only required match in Frame Relay - the encapsulation type must match as well. We'll take a detailed look at that requirement in our next CCNA 802-640 / CCENT Exam tutorial!

Five Minutes From Now, You Can Be Studying For CCNA Exam Success Just As Thousands Of Other CCNA Candidates Around The World Have - With Chris Bryant, CCIE #12933 With You Every Step Of The Way - With The Ultimate CCNA Exam Study Package!