Troubleshooting Frame Relay

Using Frame Relay is a common solution for connecting one or more LANs through a WAN. Frame Relay uses PVCs to establish a data link from one location to another (Point-to-Point) or from one location to multiple locations (Multi-point). These virtual circuits are built by using a Data-Link Connection Identifier (DLCI), which is used to identify the virtual circuits in a Frame Relay cloud.

DLCI numbers are only significant locally. The packets sent through the Frame Relay cloud are actually encapsulated with an identifier to enable each packet to traverse the local service provider's switched network. This encapsulation is added by the CSU/DSU and is stripped off by the remote end's CSU/DSU when the packet reaches its destination.

The frame does not make a straight connection to its destination. In fact, many other companies will use the same link that your packet is traveling on.

The local service provider shares the bandwidth with multiple customers and each connection is identified by the DLCI number. Each customer is given a guarantee of bandwidth called the committed information rate (CIR).

Frame Relay Troubleshooting Steps

The following sections list a number of Frame Relay troubleshooting problems. Each section also offers a series of steps you can take to troubleshoot the associated Frame Relay problem.

Frame Relay Link Is Down

To troubleshoot this problem, use the following steps:

1. Check the cabling.

2. Check for faulty hardware.

3. Check with the local service provider for problems.

4. Check for an LMI type mismatch.

5. Check whether keepalives are being sent.

6. Check the encapsulation type to see if it matches the external device's configuration.

7. Check for a DLCI mismatch.

Can't Ping a Remote Host Across a Frame Relay Network

To troubleshoot this problem, use the following steps:

1. Check that the DLCI is not assigned to the wrong subinterface.

2. Check the encapsulation types on both ends.

3. Check access list entries to make sure the proper data traffic is permitted. For more information see the "Access List Issues" section later in this chapter.

4. Check the Serial interface configuration for configuration problems.

Faulty Cabling

To troubleshoot this problem, use the following steps:

1. Check the cabling for physical breaks and to make sure they are secure.

2. Use a breakout box or other cable tester to test the cabling leads.

3. Replace faulty cables.

Faulty Hardware

To troubleshoot this problem, use the following steps:

1. Use loopback tests to isolate the faulty hardware components .

2. Move the cable to a different interface on the router and configure that interface to see whether the link comes up; if it does, you know that you have a configuration or hardware issue on the other interface. If it doesn't come up there is most likely another hardware or software issue you need to look for.

3. Replace the hardware component that has failed.

Local Service Provider Issue

One of the best indications that a local service provider issue exists is if you perform a loopback test and the LMI state changes to "up." To troubleshoot this problem, use the following steps:

1. Check for a DLCI mismatch or an encapsulation mismatch. If this is not the issue, go to Step 2.

2. Contact the local service provider to help resolve the issue.

LMI Type Mismatch

To troubleshoot this problem, use the following steps:

1. Check that the LMI type on the router matches the LMI used by the CO switches on the DCE end of the circuit.

2. If you do not know the LMI type the local service provider uses, you need to contact them, or set the LMI type to dynamically detect the LMI type (if you are using IOS version 11.2 or later).

Keepalive Issues

To troubleshoot a keepalive issue, follow these steps:

1. Use the show interface command to verify that keepalives are not disabled or misconfigured.

2. Verify that the keepalive interval is correct on the interface.

Encapsulation Type Issues

To troubleshoot this issue, follow these steps:

1. Check that the same encapsulation types are used on both sides of the router.

2. Use the show frame-relay map command if you are using non-Cisco equipment. Also verify that the encapsulation types on both interface in the PVC are set to IETF. Use the encapsulation frame-relay ietf command to change the encapsulation type from Cisco to IETF.

DLCI Mismatch

To troubleshoot this issue, follow these steps:

1. Use the show running-config command to display the DLCI number, to verify the DLCI number is assigned to the proper interface. The show frame-relay pvc command can also display the DLCI assigned to the interface.

2. If the correct DLCI number is configured on the proper interface, contact the local carrier to verify that it has the same DLCI configured on the Frame Relay switch.

Access List Issues

Access lists are very complex and if you don't have a handle on how they work, you may be in trouble. You need to have a good understanding of how protocols and their addressing work with the router because many problems can occur as a result of misconfiguring an access list. Problems can be caused by a blocked IP address, port, or any number of protocols. Not only that, many administrators forget that there is an invisible "deny all" at the end of every access list. So if you don't permit the traffic you want in the access list, it will be blocked.

To troubleshoot this issue, follow these steps:

1. Use the show ip interface command to display all the access lists applied to the routers interfaces.

2. Check each access list, keeping in mind that an invisible, implied "deny all" is at the end of the list.

3. Remove the access list to see whether the problem is resolved.

4. Make modifications to the access list and then reapply the access list. For more information on access lists see Appendix C.

Frame Relay Troubleshooting Commands

You can use the following show and debug commands, discussed in the following sections, to troubleshoot Frame Relay:

• debug frame-relay events

• debug frame-relay lmi

• show frame-relay lmi

• show frame-relay map

• show frame-relay pvc

• show interfaces

The following sections discuss the use of these commands in troubleshooting frame relay problems.

The debug frame-relay events Command

The debug frame-relay events command enables you to analyze packets and events occurring on a Frame Relay network. Data provided by this command is useful because it gives details about protocols and applications using the DLCI. This includes the interface of arrival, the datagram size, and the type of frame received.

 Sean2514#  debug frame-relay events  Frame Relay events debugging is on Sean2514# 07:05:20: Serial0: FR ARP input 07:05:20: datagramstart = 0x628970, datagramsize = 30 07:05:20: FR encap = 0x18E10300 07:05:20: 80 00 00 00 08 06 00 0F 08 00 02 04 00 09 00 00 07:05:20: 3F 4E 26 AE 18 E1 3F 4E 27 AE 07:05:20: Sean2514# 

The preceding output shows that the Serial0 interface received an ARP reply, and also displays the datagram size. The numbers 08 06 mean 0x0806, which indicates an Ethernet type code. The packet type also indicates the types of applications on the circuit. Use this command to troubleshoot connectivity problems during the installation of a new Frame Relay network.

The debug frame-relay lmi Command

The debug frame-relay lmi command enables you to obtain information with the router and the local service provider's switched network. The following is sample output:

 Sean2514#  debug frame-relay lmi  Frame Relay LMI debugging is on Displaying all Frame Relay LMI data 06:53:30: Serial0(out): StEnq, myseq 174, yourseen 173, DTE up 06:53:30: datagramstart = 0x622EA4, datagramsize = 13 06:53:30: FR encap = 0xFCF10309 06:53:30: 00 75 01 01 01 03 02 AE AD 06:53:30: 06:53:30: Serial0(in): Status, myseq 174 06:53:30: RT IE 1, length 1, type 1 06:53:30: KA IE 3, length 2, yourseq 174, myseq 174 

Notice above that the Frame Relay process is displayed step by step in the output above. For the exam you might want to brush up on your CCNA knowledge of Frame Relay connection steps.

The show frame-relay lmi Command

Using the show frame-relay lmi command, you can obtain LMI statistical information. The LMI provides communication and synchronization between the network and the local demarcation point devices. The following is an example of the output produced by using the show frame-relay lmi command:

 Sean2514#  show frame-relay 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 288              Num Status msgs Rcvd 288   Num Update Status Rcvd 17             Num Status Timeouts 0 Sean2514# 

The highlighted line indicates the interface and its role in the network. In this case, it acts as the DTE side of the interface. If the number of sent messages does not match the number received, a problem may exist with the sending and receiving keepalive messages. This type of problem can indicate a potential problem with the network equipment.

The show frame-relay map Command

Using the show frame-relay map command, you can obtain information about the DLCI numbers, encapsulation type, and status of all the Frame Relay interfaces. The following is an example of the output produced by using the show frame-relay map command:

 Serial0 (up): ip 207.212.78.174 dlci 120(0x78,0x1C80), static,               broadcast,               IETF, status deleted Serial0 (up): ip 207.212.78.175 dlci 102(0x66,0x1860), dynamic,               broadcast,               IETF, status defined, active Serial0 (up): ip 207.212.78.174 dlci 120(0x78,0x1C80), static,               broadcast,               IETF, status deleted Serial0.2 (down): point-to-point dlci, dlci 202(0xCA,0x30A0), broadcast                   status deleted 

Notice in the output above that whether the interface is active or not is indicated with the up or down state. This command also indicates whether this is a static or dynamic interface and whether the interface type is point-to-point or multipoint.

The show frame-relay pvc Command

The show frame-relay pvc command provides statistics about the PVCs and the LMI status of every DLCI on the router.

You should know for the exam that two types of DLCI numbers exist: those used on the local DTE and those used on the CO switched network.

An example of the output produced by this command follows :

 Sean2514#  show frame-relay pvc  PVC Statistics for interface Serial0 (Frame Relay DTE) DLCI = 120, DLCI USAGE = LOCAL, PVC STATUS = DELETED, INTERFACE = Serial0   input pkts 0             output pkts 0            in bytes 0   out bytes 0              dropped pkts 0           in FECN pkts 0   in BECN pkts 0           out FECN pkts 0          out BECN pkts 0   in DE pkts 0             out DE pkts 0   out bcast pkts 0         out bcast bytes 0   pvc create time 00:33:13, last time pvc status changed 00:24:49 DLCI = 202, DLCI USAGE = LOCAL, PVC STATUS = DELETED, INTERFACE = Serial0.2   input pkts 0             output pkts 0            in bytes 0   out bytes 0              dropped pkts 0           in FECN pkts 0   in BECN pkts 0           out FECN pkts 0          out BECN pkts 0   in DE pkts 0             out DE pkts 0   out bcast pkts 0         out bcast bytes 0   pvc create time 00:37:35, last time pvc status changed 00:24:20 Sean2514# 

You should monitor the number of Forward Explicit Congestion Notifications (FECNs) and Backward Explicit Congestion Notifications (BECNs), which are packets created when the transmitted rate is above the CIR. Each packet sent is given a discard eligible bit, which means that if the CO switches get congested , they will drop those packets with the discard eligible bit. When the packets are discarded, the FECN packets are sent to the receiving DTE participating devices to notify them to implement flow control. BECN messages notify the sending station that congestion was experienced and to reduce the transmission rate.

The show interfaces Command

The show interfaces command, previously discussed with regard to troubleshooting serial links, can also be used to troubleshoot Frame Relay problems. Line-by-line detail has already been given for a normal serial interface earlier in the chapter. The following is an example of the command's output:

 Sean2514#  show interface serial 0  Serial0 is up, line protocol is up   Hardware is HD64570   Internet address is 207.212.77.174/24   MTU 1500 bytes, BW 64 Kbit, DLY 20000 usec, rely 255/255, load 1/255  Encapsulation FRAME-RELAY IETF  , loopback not set, keepalive set (10 sec)  LMI enq sent  270, LMI stat recvd 127, LMI upd recvd 0, DTE LMI up   LMI enq recvd 119, 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 22/0, interface broadcasts 0   Last input 00:00:06, output 00:00:06, output hang never   Last clearing of "show interface" counters 02:51:08   Input queue: 0/75/0 (size/max/drops); Total output drops: 0   Queueing strategy: weighted fair   Output queue: 0/1000/64/0 (size/max total/threshold/drops)      Conversations  0/1/256 (active/max active/max total)      Reserved Conversations 0/0 (allocated/max allocated)   5 minute input rate 0 bits/sec, 0 packets/sec   5 minute output rate 0 bits/sec, 0 packets/sec      1238 packets input, 66402 bytes, 0 no buffer      Received 888 broadcasts, 0 runts, 0 giants, 0 throttles      0 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored, 0 abort      1245 packets output, 57524 bytes, 0 underruns      0 output errors, 0 collisions, 51 interface resets      0 output buffer failures, 0 output buffers swapped out      124 carrier transitions      DCD=up  DSR=up  DTR=up  RTS=up  CTS=up 

Table 10.3 below lists and describes the troubleshooting fields that are presented in bold text in the preceding output.

Table 10.3. Troubleshooting Fields Found in the show interface Command

Field	Description
Encapsulation	One of two encapsulation methods supported by Cisco switches for Frame Relay, which are Cisco and IETF
LMI enq sent	The number of LMI enquiries sent
LMI stat recvd	The number of LMI status packets received
LMI upd recvd	The number of LMI updates received
DTE LMI	The DTE LMI status
LMI enq recvd	The number of LMI enquiries received
LMI stat sent	The number of LMI status updates sent
LMI upd sent	The number of LMI updates sent
LMI DLCI	The DLCI number used for LMI
LMI type	The LMI type used by the interface

When the router is working properly, the Cisco LMI type should state 1023 for the LMI DLCI, and the ANSI type should state 0. The three types are Cisco, ANSI, and ITU-T. The default is Cisco. The LMI type configured on the router must be the same LMI type used by the CO switches.