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Community Member

Output drops on ATM SubInt.

One of our ATM links is reporting output drops:

show interfaces atm 1/0

ATM1/0 is up, line protocol is up


Description: ATM TRUNK

MTU 4470 bytes, sub MTU 4470, BW 149760 Kbit, DLY 80 usec,

reliability 255/255, txload 5/255, rxload 2/255

Encapsulation ATM, loopback not set

Encapsulation(s): AAL5

4095 maximum active VCs, 3 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 1d15h

Input queue: 0/75/0/0 (size/max/drops/flushes); Total output drops: 152699

Queueing strategy: Per VC Queueing

30 second input rate 1723000 bits/sec, 716 packets/sec

30 second output rate 3118000 bits/sec, 672 packets/sec

66722977 packets input, 568696940 bytes, 0 no buffer

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

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

62762717 packets output, 1367514775 bytes, 0 underruns

0 output errors, 0 collisions, 0 interface resets

0 output buffer failures, 0 output buffers swapped out

Appears to be caused by one subint in particular:

#sh atm pvc 10/101

Description: N/A

ATM1/0.101: VCD: 2, VPI: 10, VCI: 101, Connection Name: QLD_C1

VBR-NRT, PeakRate: 14000 (33019 cps), Average Rate: 7000 (16510 cps)

Burst Cells: 94

AAL5-LLC/SNAP, etype:0x0, Flags: 0x20, VCmode: 0x0, Encapsize: 12

OAM frequency: 10 second(s), OAM retry frequency: 1 second(s)

OAM up retry count: 3, OAM down retry count: 5

OAM Loopback status: OAM Received

OAM VC Status: Verified

ILMI VC status: Not Managed

VC is managed by OAM.

VC TxRingLimit: 3 particles

VC Rx Limit: 165 particles

InARP frequency: 15 minutes(s)

Transmit priority 4

InPkts: 59112577, OutPkts: 62488267, InBytes: 906189222, OutBytes: 1355211759

InCells: 584410574, OutCells: 775588246

InPRoc: 57684, OutPRoc: 57595

InFast: 59054893, OutFast: 61150799, InAS: 0, OutAS: 0

InPktDrops: 0, OutPktDrops: 152699/0/152699 (holdq/outputq/total)

InCellDrops: 0, OutCellDrops: 0

InByteDrops: 0, OutByteDrops: 0

CrcErrors: 0, SarTimeOuts: 0, OverSizedSDUs: 0, LengthViolation: 0, CPIErrors: 0

Out CLP=1 Pkts: 0, Cells: 0

OAM cells received: 14272

F5 InEndloop: 14272, F5 InSegloop: 0, F5 InAIS: 0, F5 InRDI: 0

F4 InEndloop: 0, F4 InSegloop: 0, F4 InAIS: 0, F4 InRDI: 0

OAM cells sent: 14272

F5 OutEndloop: 14272, F5 OutSegloop: 0, F5 OutRDI: 0

F4 OutEndloop: 0, F4 OutSegloop: 0, F4 OutRDI: 0

OAM cell drops: 0

Status: UP

From Cisco Site:

"Note: Any interface that is overdriven (that is, when the offered rate is greater than the line rate) presents output drops.

ATM interfaces typically use ATM-layer traffic shaping to limit the maximum amount of bandwidth used by a virtual connection. If you present more traffic to the virtual circuit (VC) than it is configured to transmit, the ATM interface tries to store the packet until it can be scheduled for transmission. However, the interface may need to drop some packets. This can particularly happen if you burst above the traffic-shaping parameters for a period of time longer than the virtual circuit is configured to handle. Traffic shaping is often implemented as part of a traffic contract with the circuit provider."

It is a 7Mb PVC (Burstable to 14Mb), and I've never seen the txload or rxload get over 9/255 - And our SolarWinds monitoring has never reported anything above 6Mb/sec? Just wondering what could be causing this?

7204 VXR with NPE400 (12.3-15)



Community Member

Re: Output drops on ATM SubInt.

It seems that you have too short TXRing and too short hold queue. Your TXRing for the PVC can fit just 1 Ethernet packet ( 3 particles = 3 * 512 = 1536 Bytes). So after one packet is transferred to TXRing and waiting to be sent all other packets for the PVC goes to hold queue whos lenght is also limited. If packets are coming fast enough hold queue overflows and you get all these discards.

Check this one:

You can also check

MVH/Best regards,

Mikhail Galiulin

Siemens Business Services AB

ORS Server & Communications

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