CRC Errors on an ATM Trunk

How does one localise the errors on the ATM trunk to a specific VC?

Assume for a second that the following interface ATM1/0 is terminating multiple VCs (Virtual Circuits), and when you issue the following command you see CRC errors. How would you know which one of VCs are the problem child?

#show interfaces atm 1/0
ATM1/0 is up, line protocol is up
  Hardware is ENHANCED ATM PA Plus
  Description: bob's ATM
  MTU 4470 bytes, sub MTU 4470, BW 149760 Kbit, DLY 80 usec,
     reliability 255/255, txload 7/255, rxload 5/255
  Encapsulation ATM, loopback not set
  Encapsulation(s): AAL5
  8191 maximum active VCs, 16 current VCCs
  VC Auto Creation Disabled.
  VC idle disconnect time: 300 seconds
  Signalling vc = 1, vpi = 0, vci = 5
         UNI Version = 4.0, Link Side = user
  0 carrier transitions
  Last input 00:00:01, output 00:00:00, output hang never
  Last clearing of "show interface" counters 00:23:50
  Input queue: 0/75/0/0 (size/max/drops/flushes); Total output drops: 1115
  Queueing strategy: Per VC Queueing
  30 second input rate 1966000 bits/sec, 1032 packets/sec
  30 second output rate 3226000 bits/sec, 1025 packets/sec
     885563 packets input, 129820445 bytes, 0 no buffer
     Received 0 broadcasts, 0 runts, 0 giants, 0 throttles
     350 input errors, 350 CRC, 0 frame, 0 overrun, 0 ignored, 0 abort            '<----Not cool'
     1373823 packets output, 456299872 bytes, 0 underruns
     0 output errors, 0 collisions, 0 interface resets
     0 output buffer failures, 0 output buffers swapped out

Issue the following command to see which VC (Vicrutal Circuit) is causing the problem

#show atm vc detail | include ^ATM|Crc
ATM1/0.489: VCD: 7, VPI: 1, VCI: 489
 CrcErrors: 0, SarTimeOuts: 0, OverSizedSDUs: 0, LengthViolation: 0, CPIErrors: 0
ATM1/0.490: VCD: 8, VPI: 1, VCI: 490
 CrcErrors: 0, SarTimeOuts: 0, OverSizedSDUs: 0, LengthViolation: 0, CPIErrors: 0
ATM1/0.201: VCD: 9, VPI: 1, VCI: 201
 CrcErrors: 0, SarTimeOuts: 0, OverSizedSDUs: 0, LengthViolation: 0, CPIErrors: 0
ATM1/0.300: VCD: 14, VPI: 10, VCI: 300
 CrcErrors: 350, SarTimeOuts: 0, OverSizedSDUs: 0, LengthViolation: 0, CPIErrors: 0
  ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^

Confirm that the amount of CRC errors match up, to confirm only one VC is faulty.

.

The following configuration mistakes are potential reasons for ATM CRC errors::

  • Having the link set at different rates between the router and the ATM node.
    • Example, when the ATM node is set to 10000 and the router is set to 10240.
  • Using a different bitrate between the router and the ATM node.
    • Example, when the ATM node is set to VBR-NRT and the router is set to CBR.

The following physical problems are some potential reasons for ATM CRC errors:

  • Dropped cells due to traffic policing within the ATM cloud on one or more VCs attached to that ATM interface.
  • Noise, gain hits, or other transmission problems on the data-link equipment.
  • A faulty or failing ATM interface or card.

Now you check with the vendor to make sure the configuration is correct and matches up.
If it does, I would suggest asking your provider to test the link, else try a new ATM interface card.

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