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.
For more information on document conventions, see the Cisco Technical Tips Conventions.
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 [output omitted] 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.
AMI - Receiving more than 15 contiguous zeros.
B3ZS - Receiving more than seven contiguous zeros.
Refer to the following document for more information about LCV errors:
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 [output omitted] 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:
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.
The Cisco Support Community is a forum for you to ask and answer questions, share suggestions, and collaborate with your peers.
Refer to Cisco Technical Tips Conventions for information on conventions used in this document.