Sometimes ATM T3 interfaces, like PA-T3 on the Cisco 7x00 or NM-1A-T3 on Cisco 3600 series routers, have connectivity problems. Some of the symptoms caused by connectivity problems include:
Line code violations (LCVs)
Continuous interface flaps
CRC errors in the ATM interface
This document will provide a description of these symptoms and will study when those correspond to a sensitive receiver or attenuation problem and provides with a workaround.
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There are no specific prerequisites for this document.
This document is not restricted to specific software and hardware versions.
The information in this document is based on the software and hardware versions below.
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.
A sensitive receiver or attenuation problem can be detected on an atm interface by the symptoms listed in the introduction. The following sections explain each of those symptoms.
The output of the show controller command may report incrementing line code violations (LCVs) and other facility statistics on an ATM DS-3 port adapter or network module. Normally, LCV error counters increment when the interface experiences a mismatch in the line coding type. However, in rare cases, this problem may also occur when the router is connected to the next device using a short cable (50 foot or less). The root cause of the problem is the line interface unit (LIU) used on these PAs is overly sensitive to some far-end equipment's transmitted signal when short cables are employed. With short cables, the LIU receiver may be saturated, thus resulting in LCV's.
The following are some examples of what the CLI outputs will look like (depending on which PA/NM you are using, the actual output may vary slightly)
Router#show controllers atm 1/0/0
ATM1/0/0: Port adaptor specific information
Hardware is DS3 (45Mbps) port adaptor
Framer is PMC PM7345 S/UNI-PDH, SAR is LSI ATMIZER II
Framing mode: DS3 C-bit ADM
No alarm detected
Facility statistics: current interval elapsed 796 seconds
lcv fbe ezd pe ppe febe hcse
lcv: Line Code Violation
be: Framing Bit Error
ezd: Summed Excessive Zeros
PE: Parity Error
ppe: Path Parity Error
febe: Far-end Block Error
hcse: Rx Cell HCS Error
Router#show controller atm 3/0
Interface ATM3/0 is down<
Hardware is RS8234 ATM DS3
Framer Chip Type PM7345
Framer Chip ID 0x20
Framer State RUNNING
Defect FRMR OOF
Defect ADM OOCD
Loopback Mode NONE
Clock Source INTERNAL
DS3 Scrambling ON
Framing DS3 C-bit direct mapping
TX cells 0
Last output time 00:00:00
RX cells 1
RX bytes 53
Last input time 1w6d
Line Code Violations (LCV) 25558650
DS3: F/M-bit errors 401016
DS3: parity errors 2744053
DS3: path parity errors 1879710
DS3/E3: G.832 FEBE errors 3099127
T3/E3: excessive zeros 25689720
uncorrectable HEC errors 554
idle/unassigned cells dropped 0
LCV errored secs 392
DS3: F/M-bit errored secs 392
DS3: parity errored secs 389
DS3: path parity errored secs 389
T3/E3: excessive zeros errored secs 392
DS3/E3: G.832 FEBE errored secs 380
uncorrectable HEC errored secs 67
LCV error-free secs 0
DS3: F/M-bit error-free secs 0
DS3: parity error-free secs 3
DS3: path parity error-free secs 3
T3/E3: excessive zeros error-free secs 0
DS3/E3: G.832 FEBE error-free secs 12
uncorrectable HEC error-free secs 325
LCV indicates the number of bipolar violation (BPV) or excessive zeros (EXZ) errors. The conditions under which these errors increment will vary with the line coding.
Alternate Mark Inversion (AMI) - Receiving two successive pulses of the same polarity.
Bipolar Three Zero Substitution (B3ZS) or High-Density Bipolar Three (HDB3) - Receiving two successive pulses of the same polarity, but these pulses are not part of zero substitution.
Refer to the following document for more information about LCV errors:
Troubleshooting Line Problems and Errors on DS-3 and E3 ATM Interfaces
Execute the show log command. Does it show a series of link up messages on the console without a corresponding link down? Cisco bug ID CSCdm84527 resolves this problem. Normally, you should see the following log messages when the interface flaps:
Aug 11 02:54:46.243 UTC: %LINK-3-UPDOWN: Interface ATM2/0, changed state to down
Aug 11 02:54:47.243 UTC: %LINEPROTO-5-UPDOWN: Line protocol on Interface ATM2/0,
changed state to down
Aug 11 02:54:57.003 UTC: %LINK-3-UPDOWN: Interface ATM2/0, changed state to up
Aug 11 09:59:14.544 UTC: %LINEPROTO-5-UPDOWN: Line protocol on Interface ATM2/0,
changed state to up
Messages similar to the following appear when enabling debug atm errors:
Aug 11 10:01:27.940 UTC: pmon_change 0x3E, cppm_change 0x53
pmon_change 0x3E - Performance monitoring (pmon) reports line code violations,
parity errors, path parity problems, and related errors.
cppm_change 0x53 - Cell and PLCP performance monitoring (cppm) reports bit
interleaved parity (BIP) errors and framing errors.
The following is an example of the output for the show interfaces command:
Router#show interfaces atm 4/0
ATM4/0 is up, line protocol is up
Last clearing of "show interface" counters never
Output queue 0/40, 0 drops; input queue 0/75, 0 drops
Five minute input rate 0 bits/sec, 0 packets/sec
Five minute output rate 0 bits/sec, 0 packets/sec
144 packets input, 31480 bytes, 0 no buffer
Received 0 broadcasts, 0 runts, 0 giants
13 input errors, 12 CRC, 0 frame, 0 overrun, 1 ignored, 0 abort
154 packets output, 4228 bytes, 0 underruns
0 output errors, 0 collisions, 1 interface resets, 0 restarts
Refer to the following document for more information on CRC errors and how to troubleshoot them:
CRC Troubleshooting Guide for ATM Interfaces
The problem may be that the attenuator is causing a sensitive receiver. Follow the steps in the troubleshooting section first, then perform the following steps to resolve the sensitive receiver issue.
Reduce the transmit level of the device attached to the T3 network module. Many devices have a Line Build Out (LBO) configuration setting for this purpose.
Increase the cable length to reduce the signal strength and edge rates. The exact length required cannot be calculated but a total length of at least 100' is recommended.
Use an 75-ohm in-line coaxial attenuator. This should reduce or eliminate LCV errors. Cisco offers an attenuator kit (ATTEN-KIT-PA=) that contains a 3-dB, 6-dB, 10-dB, 15-dB and 20-dB attenuators with standard BNC connectors. This kit is not available through RMA. Instead, the customer needs to order the kit through their account representative or Customer Service.
Start with the 3-dB attenuator first, then move to the next higher value attenuator if LCV errors persist. Note that attenuators are a normal requirement, depending on line conditions and far-end equipment involved.
If the interface in the router is configured correctly for all physical-layer parameters and still reports numerous LCV and/or CRC errors, then your ATM interface may have a sensitive receiver.
Before deciding whether a sensitive receiver is the problem, please do the following:
Check that the PA (or NM) and the far end equipment are properly configured for short cable use and that the clocking is set correctly. If the network provides the clock reference, then both ends should be set to "line" clock. Otherwise, one end must be set to internal (or local) clock and the other end set to line clock.
Check that the CRC errors are not caused by traffic shaping misconfiguration or dropping of the ATM cells on the ATM switch.
The following are recommended troubleshooting steps:
Check all physical-layer parameter configurations such as framing and scrambling.
Check the clocking configuration on both ends of the ATM connection.
Enable debug atm error and compare collected output with above example
Use the loopback diagnostic command and check CRC errors counter on ATM interface.
If you have LCV and other physical layers errors, CRC errors and debug atm error shows above output the most probable root cause is sensitive receiver.