Output drops increasing on ATM

Unanswered Question
Jul 31st, 2007


I have a weird issue . . . On a router connected to a passport via an ATM card, I was seeing the link flapping, and I was able to see these flaps occuring while in the router and in the logs. After checking the interface, I was seeing increasing output drops even though the link was not over utilized nor was there any QoS setup causing the drops. To be on the safe side, The card has been replaced on the router. I am no longer seeing the link flapping on the router (using term mon or in the log) yet the output drops are still increasing, and our monitoring tool is still reporting link flaps. At this point I am trying to find out what could be causing these Output drops when there is no QoS setup and I am no seeing the "no Buffer" counter incrementing:

lalaland#sho int atm1/0

ATM1/0 is up, line protocol is up

Hardware is RS8234 ATM DS3

MTU 4470 bytes, sub MTU 4470, BW 45000 Kbit, DLY 190 usec,

reliability 255/255, txload 1/255, rxload 1/255<<<<<<<<<<<<<<<<<<<<<<<Low load

Encapsulation ATM, loopback not set

Encapsulation(s): AAL5

1023 maximum active VCs, 1 current VCCs

VC Auto Creation Disabled.

VC idle disconnect time: 300 seconds

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

Last clearing of "show interface" counters 05:01:04

Input queue: 0/75/0/0 (size/max/drops/flushes); Total output drops: 237<<<<<<<<<<<<<<<Increasing

Queueing strategy: Per VC Queueing

5 minute input rate 136000 bits/sec, 80 packets/sec

5 minute output rate 95000 bits/sec, 80 packets/sec

1310894 packets input, 374723235 bytes, 0 no buffer<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<Not incrementing

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

0 input errors, 5 CRC, 0 frame, 0 overrun, 0 ignored, 0 abort

1190427 packets output, 221624609 bytes, 0 underruns

0 output errors, 0 collisions, 0 interface resets

0 output buffer failures, 0 output buffers swapped out


lalaland#sho int atm1/0.213

ATM1/0.213 is up, line protocol is up

Hardware is RS8234 ATM DS3

Description: ----- Area 31 Default PVC to blablabla -----

Internet address is

MTU 4470 bytes, BW 6000 Kbit, DLY 190 usec,

reliability 255/255, txload 1/255, rxload 1/255

Encapsulation ATM

1312105 packets input, 374883790 bytes

1191509 packets output, 221776787 bytes

0 OAM cells input, 0 OAM cells output

AAL5 CRC errors : 5

AAL5 SAR Timeouts : 0

AAL5 Oversized SDUs : 0

AAL5 length violation : 5

AAL5 CPI Error : 0

Last clearing of "show interface" counters never


Any hints or suggestions is welcomed . . .


I have this problem too.
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Paolo Bevilacqua Tue, 07/31/2007 - 00:51


Is your PVC SBR 6 mbps ? You have not included "show atm pvc" for it.

Anyway, it could be traffic spikes causing the drops. Note the drops are a very small number compared to total sent.

You will not see any buffer failure for a drop cause by filling the output queue.

Hope this helps, please rate post if it does!

samyverizon Tue, 07/31/2007 - 01:02

Thanks for your prompt reply . . . here is the output:

lalaland#sho atm vc

VCD / Peak Avg/Min Burst

Interface Name VPI VCI Type Encaps SC Kbps Kbps Cells Sts

1/0.213 213 13 213 PVC SNAP UBR 40000 UP


Paolo Bevilacqua Tue, 07/31/2007 - 05:05

Do show atm pvc 13/213 and let's see if the drops happened on that particular PVC.

Alos you should include the full service policy applied on that PVC.


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