Loopbacks are an important part of troubleshooting; they are used to isolate the fault on and end-to-end circuit (especially when the circuit is down). This document examines the two types of loopbacks on Cisco router ATM interfaces:
loopback diagnostic - this helps you determine if your interface is working properly.
loopback line - this helps determine if there is a problem with the service provider.
Refer to Cisco Technical Tips Conventions for more information on document conventions.
There are no specific prerequisites for this document.
The information in this document is based on the software and hardware versions below.
The examples in this document do not require any particular software versions. The AIP supports loopback diagnostic while PA-A1, PA-A2 and PA-A3 support both modes. The LANE modules support loopback line and other Cisco routers support both modes.
This applies only to the main interface, not the subinterface.
The information presented in this document was created from devices in a specific lab environment. All of the devices used in this document started with a cleared (default) configuration. If you are working in a live network, ensure that you understand the potential impact of any command before using it.
We will use the following network to illustrate the loopback modes.
The loopback diagnostic command under an ATM interface of a Cisco router causes traffic going out of the interface to come back to the router. Figure 1 and Example 1 illustrate how that process. To see loopback information for non-ATM interfaces, please click here and scroll down to the section on Special Serial Line Tests.
Note: Tx=transmit interface and Rx=Receive interface.
Note: To ensure proper clocking, set the router as the clock source. To do this, specify the atm clock internal setting in the main interface configuration.
Before loopback diagnostic is set on the interface
LT-7507b |
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lt7507b# show running-configg Building configuration... Current configuration: ! version 12.0 ! hostname lt-7507b ! ip subnet-zero no ip domain-lookup ! interface ATM1/0 no ip address no ip directed-broadcast no ip mroute-cache no atm ilmi-keepalive ! interface ATM1/0.1 multipoint ip address 10.1.1.2 255.255.255.0 no ip directed-broadcast pvc 0/85 protocol ip 10.1.1.1 encapsulation aal5snap ! ip classless no ip http server ! line con 0 transport input none line aux 0 line vty 0 4 login ! end |
lt-7507b# show interface atm 1/0 ATM1/0 is up, line protocol is up Hardware is cxBus ATM MTU 4470 bytes, sub MTU 4470, BW 155520 Kbit, DLY 80 usec, reliability 255/255, txload 1/255, rxload 1/255 Encapsulation ATM, loopback not set Keepalive not supported Encapsulation(s): AAL5, PVC mode 256 TX buffers, 256 RX buffers, 2048 maximum active VCs, 1024 VCs per VP, 1 current VCCs VC idle disconnect time: 300 seconds Last input 00:01:55, output 00:01:55, output hang never Last clearing of "show interface" counters never Queueing strategy: fifo Output queue 0/40, 0 drops; input queue 0/75, 0 drops 5 minute input rate 0 bits/sec, 0 packets/sec 5 minute output rate 0 bits/sec, 0 packets/sec 163861 packets input, 3164940 bytes, 0 no buffer Received 0 broadcasts, 0 runts, 0 giants, 0 throttles 33 input errors, 33 CRC, 0 frame, 0 overrun, 0 ignored, 0 abort 144191 packets output, 2138298 bytes, 0 underruns 0 output errors, 0 collisions, 0 interface resets 0 output buffer failures, 0 output buffers swapped out
Note: To illustrate the loopback mechanism, we will show that we can send ATM cells on the PVC. If loopback is not set, those cells will not be loopbacked to us. Let's shut down the ATM interface at the other router lt-7200b.
lt-7507b#debug atm packet interface a1/0.1 vc 0/85 ATM packets debugging is on Displaying packets on interface ATM1/0 VPI 0, VCI 85 only lt-7507b#ping Protocol [ip]: Target IP address: 10.1.1.1 Repeat count [5]: 2 Datagram size [100]: Timeout in seconds [2]: Extended commands [n]: Sweep range of sizes [n]: Type escape sequence to abort. Sending 2, 100-byte ICMP Echos to 10.1.1.1, timeout is 2 seconds:
The ping fails, and ATM debug show traffic is going out and not coming back.
1w1d: ATM1/0.1(O): VCD:0x5 VPI:0x0 VCI:0x55 DM:0x100 SAP:AAAA CTL:03 OUI:000000 TYPE:0800 Length:0x70 1w1d: 4500 0064 001C 0000 FF01 B778 0A01 0102 0A01 0101 0800 9994 0E58 068B 0000 1w1d: 0000 2D42 A290 ABCD ABCD ABCD ABCD ABCD ABCD ABCD ABCD ABCD ABCD ABCD ABCD 1w1d: ABCD ABCD ABCD ABCD ABCD ABCD ABCD ABCD ABCD ABCD ABCD ABCD ABCD ABCD ABCD 1w1d: ABCD ABCD ABCD ABCD ABCD 1w1d: . 1w1d: ATM1/0.1(O): VCD:0x5 VPI:0x0 VCI:0x55 DM:0x100 SAP:AAAA CTL:03 OUI:000000 TYPE:0800 Length:0x70 1w1d: 4500 0064 001D 0000 FF01 B777 0A01 0102 0A01 0101 0800 91C3 0E59 068B 0000 1w1d: 0000 2D42 AA60 ABCD ABCD ABCD ABCD ABCD ABCD ABCD ABCD ABCD ABCD ABCD ABCD 1w1d: ABCD ABCD ABCD ABCD ABCD ABCD ABCD ABCD ABCD ABCD ABCD ABCD ABCD ABCD ABCD 1w1d: ABCD ABCD ABCD ABCD ABCD 1w1d: . Success rate is 0 percent (0/2)
Now add loopback diagnostic, and show interface will show that loopback is set.
lt-7507b#configure terminal Enter configuration commands, one per line. End with CNTL/Z. lt-7507b(config)#int a1/0 lt-7507b(config-if)#loopback diagnostic lt-7507b(config-if)#atm clock internal
Note: The loopback command must be configured on a main interface, not on a subinterface.
lt-7507b# show interface a1/0 ATM1/0 is up, line protocol is up Hardware is cxBus ATM MTU 4470 bytes, sub MTU 4470, BW 155520 Kbit, DLY 80 usec, reliability 255/255, txload 1/255, rxload 1/255 Encapsulation ATM, loopback set Keepalive not supported Encapsulation(s): AAL5, PVC mode 256 TX buffers, 256 RX buffers, 2048 maximum active VCs, 1024 VCs per VP, 1 current VCCs VC idle disconnect time: 300 seconds Last input 00:03:16, output 00:03:16, output hang never Last clearing of "show interface" counters never Queueing strategy: fifo Output queue 0/40, 0 drops; input queue 0/75, 0 drops 5 minute input rate 0 bits/sec, 0 packets/sec 5 minute output rate 0 bits/sec, 0 packets/sec 163861 packets input, 3164940 bytes, 0 no buffer Received 0 broadcasts, 0 runts, 0 giants, 0 throttles 33 input errors, 33 CRC, 0 frame, 0 overrun, 0 ignored, 0 abort 144191 packets output, 2138298 bytes, 0 underruns 0 output errors, 0 collisions, 1 interface resets 0 output buffer failures, 0 output buffers swapped out
The ping will fail but the ATM debug will show traffic going out (O) and coming back in (I).
lt-7507b#ping Protocol [ip]: Target IP address: 10.1.1.1 Repeat count [5]: 2 Datagram size [100]: Timeout in seconds [2]: Extended commands [n]: Sweep range of sizes [n]: Type escape sequence to abort. Sending 2, 100-byte ICMP Echos to 10.1.1.1, timeout is 2 seconds: 1w1d: ATM1/0.1(O): VCD:0x5 VPI:0x0 VCI:0x55 DM:0x100 SAP:AAAA CTL:03 OUI:000000 TYPE:0800 Length:0x70 1w1d: 4500 0064 001A 0000 FF01 B77A 0A01 0102 0A01 0101 0800 119A 13A2 07C5 0000 1w1d: 0000 2D41 2408 ABCD ABCD ABCD ABCD ABCD ABCD ABCD ABCD ABCD ABCD ABCD ABCD 1w1d: ABCD ABCD ABCD ABCD ABCD ABCD ABCD ABCD ABCD ABCD ABCD ABCD ABCD ABCD ABCD 1w1d: ABCD ABCD ABCD ABCD ABCD 1w1d: 1w1d: ATM1/0.1(I): VCD:0x5 VPI:0x0 VCI:0x55 Type:0x0 SAP:AAAA CTL:03 OUI:000000 TYPE:0800 Length:0x70 1w1d: 4500 0064 001A 0000 0101 B57B 0A01 0102 0A01 0101 0800 119A 13A2 07C5 0000 1w1d: 0000 2D41 2408 ABCD ABCD ABCD ABCD ABCD ABCD ABCD ABCD ABCD ABCD ABCD ABCD 1w1d: ABCD ABCD ABCD ABCD ABCD ABCD ABCD ABCD ABCD ABCD ABCD ABCD ABCD ABCD ABCD 1w1d: ABCD ABCD ABCD ABCD ABCD 1w1d: . 1w1d: ATM1/0.1(O): VCD:0x5 VPI:0x0 VCI:0x55 DM:0x100 SAP:AAAA CTL:03 OUI:000000 TYPE:0800 Length:0x70 1w1d: 4500 0064 001B 0000 FF01 B779 0A01 0102 0A01 0101 0800 09C9 13A3 07C5 0000 1w1d: 0000 2D41 2BD8 ABCD ABCD ABCD ABCD ABCD ABCD ABCD ABCD ABCD ABCD ABCD ABCD 1w1d: ABCD ABCD ABCD ABCD ABCD ABCD ABCD ABCD ABCD ABCD ABCD ABCD ABCD ABCD ABCD 1w1d: ABCD ABCD ABCD ABCD ABCD 1w1d: 1w1d: ATM1/0.1(I): VCD:0x5 VPI:0x0 VCI:0x55 Type:0x0 SAP:AAAA CTL:03 OUI:000000 TYPE:0800 Length:0x70 1w1d: 4500 0064 001B 0000 0101 B57A 0A01 0102 0A01 0101 0800 09C9 13A3 07C5 0000 1w1d: 0000 2D41 2BD8 ABCD ABCD ABCD ABCD ABCD ABCD ABCD ABCD ABCD ABCD ABCD ABCD 1w1d: ABCD ABCD ABCD ABCD ABCD ABCD ABCD ABCD ABCD ABCD ABCD ABCD ABCD ABCD ABCD 1w1d: ABCD ABCD ABCD ABCD ABCD 1w1d: . Success rate is 0 percent (0/2)
The loopback line command under an ATM interface of a Cisco router causes incoming traffic to be sent back to the network. See Figure 2 and Example 2 below.
Configuration of router lt-7200b
LT-7200b |
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lt-7200b# show running-configg Building configuration... Current configuration: ! version 12.0 service timestamps debug uptime service timestamps log uptime no service password-encryption ! hostname lt-7200b ! interface ATM2/0 no ip address no ip directed-broadcast no atm ilmi-keepalive ! interface ATM2/0.1 multipoint ip address 10.1.1.1 255.255.255.0 no ip directed-broadcast pvc 0/85 protocol ip 10.1.1.2 encapsulation aal5snap ! ! ip classless no ip http server ! line con 0 transport input none line aux 0 line vty 0 4 login ! end |
Before loopback line is set on the interface
lt-7200b# show interface atm 2/0 ATM2/0 is up, line protocol is up Hardware is TI1570 ATM MTU 4470 bytes, sub MTU 4470, BW 155520 Kbit, DLY 80 usec, reliability 255/255, txload 1/255, rxload 1/255 Encapsulation ATM, loopback not set Keepalive not supported Encapsulation(s): AAL5, PVC mode 2048 maximum active VCs, 1024 VCs per VP, 1 current VCCs VC idle disconnect time: 300 seconds Last input 00:00:43, output 00:00:43, output hang never Last clearing of "show interface" counters never Queueing strategy: fifo Output queue 0/40, 0 drops; input queue 0/75, 0 drops 5 minute input rate 0 bits/sec, 0 packets/sec 5 minute output rate 0 bits/sec, 0 packets/sec 94912 packets input, 1637823 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 102893 packets output, 2042225 bytes, 0 underruns 0 output errors, 0 collisions, 3 interface resets 0 output buffer failures, 0 output buffers swapped out
Note: ATM packets debugging is on and ping from router lt-7507b to router lt-7200b. You can see that the tests are done again from the 7500 (where we removed the initial loopback). This is normal since the Loopback Line allows us to see if the Telco's network is working properly.
lt-7507b#debug atm packet ATM packets debugging is on Displaying all ATM packets lt-7507b#debug atm packet sh debug lt-7507b#ping 10.1.1.1 Type escape sequence to abort. Sending 1, 100-byte ICMP Echos to 10.1.1.1, timeout is 2 seconds: ! Success rate is 100 percent (1/1), round-trip min/avg/max = 1/2/4 ms
The ping is successful, and ATM debug show traffic is going out and coming back in.
00:45:14: ATM1/0.1(O): VCD:0x1 VPI:0x0 VCI:0x55 DM:0x100 SAP:AAAA CTL:03 OUI:000000 TYPE:0800 Length:0x70 00:45:14: 4500 0064 0005 0000 FF01 B78F 0A01 0102 0A01 0101 0800 028C 02D4 0C51 0000 00:45:14: 0000 0029 6C70 ABCD ABCD ABCD ABCD ABCD ABCD ABCD ABCD ABCD ABCD ABCD ABCD 00:45:14: ABCD ABCD ABCD ABCD ABCD ABCD ABCD ABCD ABCD ABCD ABCD ABCD ABCD ABCD ABCD 00:45:14: ABCD ABCD ABCD ABCD ABCD 00:45:14: 00:45:14: ATM1/0.1(I): VCD:0x1 VPI:0x0 VCI:0x55 Type:0x0 SAP:AAAA CTL:03 OUI:000000 TYPE:0800 Length:0x70 00:45:14: 4500 0064 0005 0000 FE01 B88F 0A01 0101 0A01 0102 0000 0A8C 02D4 0C51 0000 00:45:14: 0000 0029 6C70 ABCD ABCD ABCD ABCD ABCD ABCD ABCD ABCD ABCD ABCD ABCD ABCD 00:45:14: ABCD ABCD ABCD ABCD ABCD ABCD ABCD ABCD ABCD ABCD ABCD ABCD ABCD ABCD ABCD 00:45:14: ABCD ABCD ABCD ABCD ABCD Success rate is 100 percent (1/1)
Configure loopback line on the ATM interface of router lt-7200b
lt-7200b#configure terminal Enter configuration commands, one per line. End with CNTL/Z. lt-7200b(config)#int a2/0 lt-7200b(config-if)#loopback line lt-7200b(config-if)#^Z lt-7200b#config show interface a2/0 ATM2/0 is up, line protocol is up Hardware is TI1570 ATM MTU 4470 bytes, sub MTU 4470, BW 155520 Kbit, DLY 80 usec, reliability 255/255, txload 1/255, rxload 1/255 Encapsulation ATM,loopback set Keepalive not supported Encapsulation(s): AAL5, PVC mode 2048 maximum active VCs, 1024 VCs per VP, 1 current VCCs VC idle disconnect time: 300 seconds Last input 00:02:45, output 00:02:45, output hang never Last clearing of "show interface" counters never Queueing strategy: fifo Output queue 0/40, 0 drops; input queue 0/75, 0 drops 5 minute input rate 0 bits/sec, 0 packets/sec 5 minute output rate 0 bits/sec, 0 packets/sec 94917 packets input, 1638383 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 102898 packets output, 2042785 bytes, 0 underruns 0 output errors, 0 collisions, 5 interface resets 0 output buffer failures, 0 output buffers swapped out
Note: Ping from the router lt-7507b. The ping will fail as expected, but the traffic is looped back towards router lt-7507b from the network, and the ATM debug shows the traffic going out (O) and then coming back in (I).
lt-7507b#ping 10.1.1.1 Type escape sequence to abort. Sending 2, 100-byte ICMP Echos to 10.1.1.1, timeout is 2 seconds: .. Success rate is 0 percent (0/2) 00:52:00: ATM1/0.1(O): VCD:0x1 VPI:0x0 VCI:0x55 DM:0x100 SAP:AAAA CTL:03 OUI:000000 TYPE:0800 Length:0x70 00:52:00: 4500 0064 000F 0000 FF01 B785 0A01 0102 0A01 0101 0800 CE44 1 21D 0009 0000 00:52:00: 0000 002F 9DB0 ABCD ABCD ABCD ABCD ABCD ABCD ABCD ABCD ABCD ABCD ABCD ABCD 00:52:00: ABCD ABCD ABCD ABCD ABCD ABCD ABCD ABCD ABCD ABCD ABCD ABCD ABCD ABCD ABCD 00:52:00: ABCD ABCD ABCD ABCD ABCD 00:52:00: 00:52:00: ATM1/0.1(I): VCD:0x1 VPI:0x0 VCI:0x55 Type:0x0 SAP:AAAA CTL:03 OUI:000000 TYPE:0800 Length:0x70 00:52:00: 4500 0064 000F 0000 0101 B586 0A01 0102 0A01 0101 0800 CE44 121D 0009 0000 00:52:00: 0000 002F 9DB0 ABCD ABCD ABCD ABCD ABCD ABCD ABCD ABCD ABCD ABCD ABCD ABCD 00:52:00: ABCD ABCD ABCD ABCD ABCD ABCD ABCD ABCD ABCD ABCD ABCD ABCD ABCD ABCD ABCD 00:52:00: ABCD ABCD ABCD ABCD ABCD 00:52:00: . 00:52:02: ATM1/0.1(O): VCD:0x1 VPI:0x0 VCI:0x55 DM:0x100 SAP:AAAA CTL:03 OUI:000000 TYPE:0800 Length:0x70 00:52:02: 4500 0064 0010 0000 FF01 B784 0A01 0102 0A01 0101 0800 C673 121E 0009 0000 00:52:02: 0000 002F A580 ABCD ABCD ABCD ABCD ABCD ABCD ABCD ABCD ABCD ABCD ABCD ABCD 00:52:02: ABCD ABCD ABCD ABCD ABCD ABCD ABCD ABCD ABCD ABCD ABCD ABCD ABCD ABCD ABCD 00:52:02: ABCD ABCD ABCD ABCD ABCD 00:52:02: 00:52:02: ATM1/0.1(I): VCD:0x1 VPI:0x0 VCI:0x55 Type:0x0 SAP:AAAA CTL:03 OUI:000000 TYPE:0800 Length:0x70 00:52:02: 4500 0064 0010 0000 0101 B585 0A01 0102 0A01 0101 0800 C673 121E 0009 0000 00:52:02: 0000 002F A580 ABCD ABCD ABCD ABCD ABCD ABCD ABCD ABCD ABCD ABCD ABCD ABCD 00:52:02: ABCD ABCD ABCD ABCD ABCD ABCD ABCD ABCD ABCD ABCD ABCD ABCD ABCD ABCD ABCD 00:52:02: ABCD ABCD ABCD ABCD ABCD Success rate is 0 percent (0/2)
Loopbacks are a useful tool to use when you are troubleshooting failed circuits. They can:
help you determine physical interface problems.
help you troubleshoot ATM service providers.
We ran the tests in this document in the usual way: pinging the other side. If you use the right mapping, or if you use a point-to-point sub-interface, the user could ping his own IP address. Therefore, the ICMP echo would definitely be sent on the interface and would be answered by the router. In other words, in a live environment, you can run the same tests without any debugging enabled. You can simply check if you can ping your own IP address and see if the test succeeds.