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CRC Errors on ATM IMA

I have 4 T1's configured in an IMA bundle in a 7200 with an IMA PA card. I'm getting CRC errors on the IMA interface. The carrier tested each T1 individually and saw no errors. I also shut each T1 down individually and while each one was down, the errors continued to increment on the IMA interface. This tells me that no single T1 is causing the errors. What else could be causing these errors? Here are some outputs:

ROUTER>sh int atm4/ima0

ATM4/ima0 is up, line protocol is up

Hardware is IMA PA

MTU 4470 bytes, sub MTU 4470, BW 6093 Kbit, DLY 20000 usec,

reliability 255/255, txload 3/255, rxload 11/255

Encapsulation ATM, loopback not set

Encapsulation(s): AAL5

2047 maximum active VCs, 4 current VCCs

VC Auto Creation Disabled.

VC idle disconnect time: 300 seconds

0 carrier transitions

Last input 00:00:00, output 00:00:00, output hang never

Last clearing of "show interface" counters 00:31:50

Input queue: 0/75/0/0 (size/max/drops/flushes); Total output drops: 0

Queueing strategy: Per VC Queueing

5 minute input rate 279000 bits/sec, 58 packets/sec

5 minute output rate 87000 bits/sec, 58 packets/sec

142955 packets input, 68773978 bytes, 0 no buffer

Received 0 broadcasts, 0 runts, 2 giants, 0 throttles

754 input errors, 752 CRC, 0 frame, 0 overrun, 0 ignored, 0 abort

144675 packets output, 25655149 bytes, 0 underruns

0 output errors, 0 collisions, 0 interface resets

0 output buffer failures, 0 output buffers swapped out


ROUTER>sh ver

Cisco IOS Software, 7200 Software (C7200-IK9S-M), Version 12.4(5a), RELEASE SOFTWARE (fc3)

Technical Support:

Copyright (c) 1986-2006 by Cisco Systems, Inc.

Compiled Sat 14-Jan-06 00:25 by alnguyen

ROM: System Bootstrap, Version 12.3(4r)T3, RELEASE SOFTWARE (fc1)

BOOTLDR: 7200 Software (C7200-KBOOT-M), Version 12.3(9), RELEASE SOFTWARE (fc2)

LexKY1-8 uptime is 11 weeks, 2 days, 8 hours, 23 minutes

System returned to ROM by power-on

System restarted at 10:42:09 edt Tue Aug 8 2006

System image file is "disk2:c7200-ik9s-mz.124-5a.bin"

Cisco 7206VXR (NPE-G1) processor (revision B) with 491520K/32768K bytes of memory.

Processor board ID 34188349

SB-1 CPU at 700MHz, Implementation 1025, Rev 0.2, 512KB L2 Cache

6 slot VXR midplane, Version 2.9

Last reset from power-on

PCI bus mb1 (Slots 1, 3 and 5) has a capacity of 600 bandwidth points.

Current configuration on bus mb1 has a total of 600 bandwidth points.

This configuration is within the PCI bus capacity and is supported.

PCI bus mb2 (Slots 2, 4 and 6) has a capacity of 600 bandwidth points.

Current configuration on bus mb2 has a total of 0 bandwidth points.

This configuration is within the PCI bus capacity and is supported.

3 Gigabit Ethernet interfaces

9 ATM interfaces

1 Virtual Private Network (VPN) Module

509K bytes of NVRAM.

62592K bytes of ATA PCMCIA card at slot 2 (Sector size 512 bytes).

16384K bytes of Flash internal SIMM (Sector size 256K).

Configuration register is 0x2102


Re: CRC Errors on ATM IMA

Since ATM divides packets into cells to transmit them across the network, all cells need to be present at the other end in order to reassemble the IP packet. If even 1 cell is dropped by the carrier network, you'll see CRC errors on your interface.

This can be caused by other sites not conforming to the CIR the provider gave them, or the provider having misconfigured policing, or even a link problem somewhere in the provider network.

Testing the T1's alone wont help here, since that is only the connection between you and the ATM edge. More times than not CRC errors are from cells being dropped within the ATM cloud.

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