Home > Articles > Cisco Network Technology > General Networking > Cisco Frame Relay Configurations

Cisco Frame Relay Configurations

Chapter Description

Jonathan Chin dicussess basic Frame Relay operations on Cisco routers in a router-based Frame Relay network. He also explains how to configure a basic frame relay network involving Cisco equipment and how to perform basic monitoring and troubleshooting using relevant Cisco IOS show and debug commands.

Configuring the LMI Type on a Frame Relay Interface

Cisco supports three different Local Management Interface (LMI) types for Frame Relay: Cisco, ANSI Annex D, and Q933-A Annex A. Beginning with Cisco IOS Software Release 11.2, the LMI autosense feature allows a Frame Relay interface to autodetect the LMI type supported by the directly connected Frame Relay switch. Based on the LMI status messages it receives from the Frame Relay switch, the router automatically configures its Frame Relay interface with the supported LMI type acknowledged by the Frame Relay switch.

No extra configuration command is required on a Cisco router to activate the LMI autosense feature. With Cisco IOS Release 11.2 or later, LMI autosense is activated by default when an LMI type is not explicitly configured on the interface. After the no shutdown interface configuration command is used to activate the Frame Relay interface, the interface starts polling the Frame Relay switch for the supported LMI type by sending out LMI status requests for all three supported LMI types—ANSI, Q933-A, and Cisco—in quick succession.

In the debug output shown in Example 4-7, the debug frame-relay lmi command is used on a Cisco router to display the LMI exchanges between the router and the connected Frame Relay switch. The router sends out LMI status enquiries to the Frame Relay switch in an attempt to determine an LMI type supported by the switch. This is indicated by the observation of StEnq in the debugs. A status enquiry message is sent out for each LMI type in the following sequence: ANSI, Q933-A, and Cisco. The status reply message returned by the switch carries information on the supported LMI type, as well as the status of active permanent virtual circuits (PVCs). A successful exchange of LMI status messages with the Frame Relay switch increments the LMI sequence counter on the router.

After the router learns the LMI type supported by the Frame Relay switch, it installs the supported LMI type on its Frame Relay interface.

Example 4-7 Frame Relay Interface on Router R1 Sends Out LMI Status Requests to the Switch When Activated

R1#debug frame-relay lmi
02:51:41: %LINK-3-UPDOWN: Interface Serial4/2, changed state to up
*Jul 5 00:20:53.535: Serial4/2(out): StEnq, myseq 1, yourseen 0, DTE up
*Jul 5 00:20:53.535: datagramstart = 0x7000214, datagramsize = 14
*Jul 5 00:20:53.535: FR encap = 0x00010308
*Jul 5 00:20:53.535: 00 75 95 01 01 00 03 02 01 00 
*Jul 5 00:20:53.535: 
*Jul 5 00:20:53.535: Serial4/2(out): StEnq, myseq 1, yourseen 0, DTE up
*Jul 5 00:20:53.535: datagramstart = 0x70000D4, datagramsize = 13
*Jul 5 00:20:53.535: FR encap = 0x00010308
*Jul 5 00:20:53.535: 00 75 51 01 00 53 02 01 00 
*Jul 5 00:20:53.535: 
*Jul 5 00:20:53.535: Serial4/2(out): StEnq, myseq 1, yourseen 0, DTE up
*Jul 5 00:20:53.535: datagramstart = 0x7000214, datagramsize = 13
*Jul 5 00:20:53.535: FR encap = 0xFCF10309
*Jul 5 00:20:53.535: 00 75 01 01 00 03 02 01 00 
*Jul 5 00:20:53.535: 
*Jul 5 00:20:53.547: Serial4/2(in): Status, myseq 1
*Jul 5 00:20:53.547: RT IE 1, length 1, type 0
*Jul 5 00:20:53.547: KA IE 3, length 2, yourseq 1 , myseq 1 
*Jul 5 00:20:53.547: PVC IE 0x7 , length 0x6 , dlci 100, status 0x0 , bw 0 
*Jul 5 00:21:03.535: Serial4/2(out): StEnq, myseq 2, yourseen 1, DTE up
*Jul 5 00:21:03.535: datagramstart = 0x7000214, datagramsize = 13
*Jul 5 00:21:03.535: FR encap = 0xFCF10309
*Jul 5 00:21:03.535: 00 75 01 01 01 03 02 02 01 
*Jul 5 00:21:03.535: 
*Jul 5 00:21:03.539: Serial4/2(in): Status, myseq 2
*Jul 5 00:21:03.539: RT IE 1, length 1, type 0
*Jul 5 00:21:03.539: KA IE 3, length 2, yourseq 2 , myseq 2 
*Jul 5 00:21:03.539: PVC IE 0x7 , length 0x6 , dlci 100, status 0x2 , bw 0
*Jul 5 00:21:03.543: Serial4/2(o): dlci 100(0x1841), pkt encaps 0x0300 0x8000 0x0000 0x806 (ARP), datagramsize 34
*Jul 5 00:21:03.543: FR: Sending INARP Request on interface Serial4/2 dlci 100 for link 7(IP)

After the router has determined the supported LMI type to use via the LMI autosense feature, the show frame-relay lmi privileged EXEC mode command can be used to verify the LMI type used. Example 4-8 shows an output of the show frame-relay lmi command.

Example 4-8 Sample Output of show frame-relay lmi Command

R1#show frame-relay lmi

LMI Statistics for interface Serial4/2 (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 144       Num Status msgs Rcvd 145
 Num Update Status Rcvd 0       Num Status Timeouts 0

LMI autosense on the interface can be turned off by explicitly configuring an LMI type with the frame-relay lmi-type lmi-type interface configuration command. Disabling LMI autosense permits the user to specifically configure either the ANSI Annex D, the ITU Q933-A, or the Cisco LMI type to be used on an interface.

NOTE

Unlike Frame Relay encapsulation, LMI type cannot be configured on a per-DLCI basis. It has to be configured at the interface level.

When manually configuring the LMI type on the Frame Relay interface, the selected LMI type on the router must match the LMI type supported by the connected Frame Relay switch. If there is a mismatch of the LMI type running on the Cisco router and its connected Frame Relay switch, the router will not be able to discover any assigned Frame Relay PVCs or maintain LMI status with the switch.

Furthermore, individual Frame Relay PVCs configured under the same physical interface or subinterfaces cannot be set up to use a different LMI type. LMI type is configured only at the interface level. However, it is possible to allow individual Frame Relay PVCs under the same physical interface to use different Frame Relay encapsulations. The frame-relay map command allows the selected DLCI to use either Cisco or IETF Frame Relay encapsulation. The Frame Relay encapsulation type configured on the near-end Frame Relay device must match the Frame Relay encapsulation type configured on the far-end Frame Relay device. Table 4-1 summarizes the key points on the consistency of LMI and Frame Relay encapsulation types.

Table 4-1 Matching Frame Relay Encapsulation and LMI Type

Must Match Between

Configurable on Per-Interface Basis?

Configurable on Per-DLCI Basis?

Frame Relay Encapsulation Type

End-to-end Frame Relay devices

Yes

Yes

Frame Relay LMI Type

Frame Relay device and connected Frame Relay switch

Yes

No


Example 4-9 shows a configuration example of the frame-relay lmi-type interface configuration command, which is used to explicitly configure an LMI type on a Frame Relay interface. Three LMI options are available: ansi, Cisco, and q933a. They represent the ANSI Annex D, Cisco, and ITU Q933-A (Annex A) LMI types, respectively. The no form of the frame-relay lmi-type command removes the explicit LMI type configured on the interface. Take note that after the explicit LMI type configuration is removed from an interface, the LMI autosense feature is used again for LMI type discovery on that interface. In an all-Cisco environment, the recommended LMI type to use is Cisco LMI.

Example 4-9 Configuring the LMI Type on the Interface

R1(config)#interface serial4/2
R1(config-if)#frame-relay lmi-type ?
 cisco 
 ansi  
 q933a 

R1(config-if)#frame-relay lmi-type q933a
R1(config-if)#no shutdown

After router R1 is set up to use Q933-A LMI type on its serial interface 4/2 in Example 4-9, the next example, in Example 4-10, shows that R1 no longer sends out all three LMI status enquiry messages to poll for a supported LMI type on that interface. Instead, R1 starts exchanging status enquiry messages directly with the Frame Relay switch using the selected Q933-A LMI. In the debug output in Example 4-10, R1 sends out a lone status enquiry message to the Frame Relay switch as noted by the single StEng message. The Frame Relay switch acknowledges the enquiry with a status update message.

Example 4-10 Router Begins LMI Status Exchanges Directly with the Explicitly Configured LMI Type

R1#debug frame-relay lmi
*Jul 5 01:08:28.279: Serial4/2(out): StEnq, myseq 1, yourseen 0, DTE up
*Jul 5 01:08:28.279: datagramstart = 0x70000D4, datagramsize = 13
*Jul 5 01:08:28.279: FR encap = 0x00010308
*Jul 5 01:08:28.279: 00 75 51 01 00 53 02 01 00 
*Jul 5 01:08:28.279: 
*Jul 5 01:08:28.283: Serial4/2(in): Status, myseq 1
*Jul 5 01:08:28.283: RT IE 51, length 1, type 0
*Jul 5 01:08:28.283: KA IE 53, length 2, yourseq 1 , myseq 1 
*Jul 5 01:08:28.283: PVC IE 0x57, length 0x3 , dlci 102, status 0x2 
*Jul 5 01:08:38.279: Serial4/2(out): StEnq, myseq 2, yourseen 1, DTE up
*Jul 5 01:08:38.279: datagramstart = 0x70000D4, datagramsize = 13
*Jul 5 01:08:38.279: FR encap = 0x00010308
*Jul 5 01:08:38.279: 00 75 51 01 01 53 02 02 01 
*Jul 5 01:08:38.279: 
*Jul 5 01:08:38.283: Serial4/2(in): Status, myseq 2
*Jul 5 01:08:38.283: RT IE 51, length 1, type 1
*Jul 5 01:08:38.283: KA IE 53, length 2, yourseq 2 , myseq 2

When manually setting up the LMI type, it is necessary to configure the keepalive interval on the Frame Relay interface to prevent LMI status exchanges between the router and the Frame Relay switch from timing out. The LMI status exchange messages are used for the purpose of communication between the router and the switch to determine the status of the PVC connection. For example, a large mismatch in the keepalive interval on the router and the switch can cause the switch to declare the router dead.

By default, the keepalive time interval is 10 seconds on Cisco serial interfaces. The keepalive interval can be changed with the keepalive interface configuration command. Refer to Example 4-11 for an example of configuring the keepalive on the interface.

Example 4-11 Configuring the Keepalive on the Interface

Enter configuration commands, one per line. End with CNTL/Z.
R1(config)#interface serial4/2
R1(config-if)#keepalive 30

To keep the LMI status exchanges between the router and the switch in synchronization, the keepalive interval configured at the router has to be equal to or lower than the corresponding keepalive interval configured on the switch interface. Failure to do so can result in a mismatch of sequence numbers in the status exchange messages, interface flapping, or even dropped connections. Then, when connecting to a public Frame Relay network and the LMI autosense feature is not supported, explicit configuration of the LMI type on the router interface is required.

Consider the following example to illustrate this issue. After the Frame Relay PVC connection becomes active, the keepalive interval of the Frame Relay router R1's interface is readjusted to a value three times higher than the default 10-second interval used by the Frame Relay switch's interfaces. Hence, keepalive 30 is used to increase the keepalive of router R1's interface to 30 seconds, while the keepalive value on the switch interface remains at the default 10 seconds. As a result of the configuration change, the Frame Relay switch interface continues to expect LMI status messages from router R1 at 10-second intervals but it hears an LMI status message from R1 only after every 30 seconds. This keepalive mismatch causes a timeout on the switch, and the Frame Relay switch declares the PVC connection to the router R1 as inactive. This can be observed in the output of the show frame-relay route command depicted in Example 4-12. On the router, it is not possible to reach its remote destination on the inactive Frame Relay PVC.

Example 4-12 Frame Relay Connection Status on the Frame Relay Switch with Mismatch Keepalive Intervals Between the Switch and R1

SW#show frame-relay route
Input Intf   Input Dlci   Output Intf   Output Dlci   Status
Serial1/0    102       Serial1/1    201       active
Serial1/1    201       Serial1/0    102       inactive

On the Frame Relay switch, the hardware interface connected to R1 transitions to the line protocol is down state because of the keepalive mismatch. The output of the show interface command executed on the Frame Relay switch in Example 4-13 reflects this.

Example 4-13 Mismatch Keepalive on R1 Causes an Interface Flap on the Frame Relay Switch

SW#show interface serial1/0
Serial1/0 is up, line protocol is down 
 Hardware is cxBus Serial
 MTU 1500 bytes, BW 1544 Kbit, DLY 20000 usec, 
   reliability 255/255, txload 1/255, rxload 1/255
 Encapsulation FRAME-RELAY, crc 16, loopback not set
 Keepalive set (10 sec)
 Restart-Delay is 0 secs
 LMI enq sent 0, LMI stat recvd 0, LMI upd recvd 0
 LMI enq recvd 65, LMI stat sent 65, LMI upd sent 0, DCE LMI down
 LMI DLCI 1023 LMI type is CISCO frame relay DCE
 Broadcast queue 0/64, broadcasts sent/dropped 0/0, interface broadcasts 0
 Last input 00:00:00, output 00:00:00, output hang never
 Last clearing of "show interface" counters 00:17:56
 Input queue: 0/75/0/0 (size/max/drops/flushes); 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)
   Available Bandwidth 1158 kilobits/sec
 5 minute input rate 0 bits/sec, 0 packets/sec
 5 minute output rate 0 bits/sec, 0 packets/sec
   73 packets input, 1426 bytes, 0 no buffer
   Received 0 broadcasts, 0 runts, 0 giants, 0 throttles
   0 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored, 0 abort
   71 packets output, 1503 bytes, 0 underruns
   0 output errors, 0 collisions, 1 interface resets
   0 output buffer failures, 0 output buffers swapped out
   1 carrier transitions
   RTS up, CTS up, DTR up, DCD up, DSR up

In Example 4-14, a standard ping is performed at Router R1, targeted to the destination address 172.16.1.2 at Router R2. As expected, with the Frame Relay PVC in the inactive state, all the packets sent out on DLCI 100 are dropped. The cause of this problem is the mismatch of the keepalive intervals for exchanging LMI updates between the router and the Frame Relay switch interface. This problem can be identified with the use of show and debug commands for Frame Relay verifying LMI, which will be introduced and explained subsequently in this chapter.

Example 4-14 Frame Relay Router's Connectivity to Remote Destination Is Lost After Keepalive Mismatch

R1#show ip route
Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP
    D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area 
    N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
    E1 - OSPF external type 1, E2 - OSPF external type 2, E - EGP
    i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, ia - IS-IS inter area
    * - candidate default, U - per-user static route, o - ODR
    P - periodic downloaded static route

Gateway of last resort is not set

   172.16.0.0/24 is subnetted, 1 subnets
C    172.16.1.0 is directly connected, Serial4/2
R1#ping 172.16.1.2

Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 172.16.1.2, timeout is 2 seconds:
.....
Success rate is 0 percent (0/5)

After the keepalive interval at Router R1's serial interface 4/2 is restored to 10 seconds to match the Frame Relay switch's settings, the LMI status messages are exchanged properly between R1 and the Frame Relay switch. The LMI status on R1 goes to the UP state and the PVC connection is in the active state again on the switch. Router R1 is able to ping router R2's address at 172.16.1.2, as shown in Example 4-15.

Example 4-15 Connectivity Is Restored After Correcting the Keepalive Interval Mismatch

R1#show interface serial 4/2
Serial4/2 is up, line protocol is up 
 Hardware is M4T
 Internet address is 172.16.1.1/24
 MTU 1500 bytes, BW 2048 Kbit, DLY 20000 usec, 
   reliability 255/255, txload 1/255, rxload 1/255
 Encapsulation FRAME-RELAY, crc 16, loopback not set
 Keepalive set (10 sec)
 LMI enq sent 94, LMI stat recvd 91, LMI upd recvd 0, DTE LMI up
 LMI enq recvd 0, LMI stat sent 0, LMI upd sent 0
 LMI DLCI 0 LMI type is CCITT frame relay DTE
 FR SVC disabled, LAPF state down
 Broadcast queue 0/64, broadcasts sent/dropped 1/0, interface broadcasts 0
 Last input 00:00:06, output 00:00:06, output hang never
 Last clearing of "show interface" counters 00:27:06
 Input queue: 0/75/0/0 (size/max/drops/flushes); 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)
   Available Bandwidth 1536 kilobits/sec
 5 minute input rate 0 bits/sec, 0 packets/sec
 5 minute output rate 0 bits/sec, 0 packets/sec
   108 packets input, 2901 bytes, 0 no buffer
   Received 0 broadcasts, 0 runts, 0 giants, 0 throttles
   0 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored, 0 abort
   128 packets output, 4276 bytes, 0 underruns
   0 output errors, 0 collisions, 1 interface resets
   0 output buffer failures, 0 output buffers swapped out
1 carrier transitions   DCD=up DSR=up DTR=up RTS=up CTS=up
R1#ping 172.16.1.2

Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 172.16.1.2, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 56/57/60 ms

The discussion in this section demonstrates that a successful LMI status exchange between a Frame Relay DTE device (router) and a Frame Relay DCE device (Frame Relay switch) is required for communication and maintenance of the Frame Relay PVC status. A router cannot communicate with a Frame Relay network via a Frame Relay PVC in the inactive state. However, in order for the router to really send information to a remote destination network address, it needs to know which DLCI to use. This is accomplished by mapping a remote destination network address to a local DLCI address and is explained in the next section.

4. Configuring Static and Dynamic DLCI to Network Layer Address Mapping | Next Section Previous Section