Total output Drops Incrementing

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Hi Experts,

We have a P2P link between India and UK. What we have noticed is that we are seeing incrementing Output Drops on our serial interface in UK.

Pls find screenshot

Serial0/0/1 is up, line protocol is up

Hardware is GT96K Serial

Internet address is 192.168.20.129/30

MTU 1500 bytes, BW 1544 Kbit, DLY 20000 usec,

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

Encapsulation HDLC, loopback not set

Keepalive set (10 sec)

Last input 00:00:01, output 00:00:01, output hang never

Last clearing of "show interface" counters 2d13h

Input queue: 0/75/0/0 (size/max/drops/flushes); Total output drops: 69891

Queueing strategy: fifo

Output queue: 0/40 (size/max)

5 minute input rate 0 bits/sec, 1 packets/sec

5 minute output rate 0 bits/sec, 1 packets/sec

122629643 packets input, 889580392 bytes, 0 no buffer

Received 25992 broadcasts, 0 runts, 0 giants, 0 throttles

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

98870494 packets output, 2723332531 bytes, 0 underruns

0 output errors, 0 collisions, 0 interface resets

0 output buffer failures, 0 output buffers swapped out

0 carrier transitions

DCD=up DSR=up DTR=up RTS=up CTS=up

Please find below the config of the interface-

interface Serial0/0/1

ip address x.x.x.x y.y.y.y

no ip redirects

no ip unreachables

no ip proxy-arp

ip tcp header-compression iphc-format

ip tcp compression-connections 256

no ip mroute-cache

no fair-queue

no cdp enable

ip rtp header-compression iphc-format

ip rtp compression-connections 1000

!

We are running VoIP trafic over this link. Please can someone suggest/advise as to why we are seeing these output drops increments as we are facing some voice quality issues in this setup.

Many thanks in advance.

I have this problem too.
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Paolo Bevilacqua Mon, 03/26/2007 - 11:19

Your link is getting congested, and you have not configured QoS. Not surpisingly you have voice quality issues.

is simple as that. Probably you don't need that many pre-allocated cRTP sessions.

Hope this helps, please reate all useful posts!

Hi There,

Much appreciate the response.

Just an update, we have now separated the voice and data traffic on two different serial interfaces.

The utilization on the Voice link is around 40% max, however we are still observing these output drops.

Any suggestions.Config of interface is attached-

interface Serial0/0/1

description ****link Connected to Chennai****

ip address x.x.x.x y.y.y.y

no ip redirects

no ip unreachables

no ip proxy-arp

ip tcp header-compression iphc-format

ip tcp compression-connections 256

no ip mroute-cache

no fair-queue

no cdp enable

ip rtp header-compression iphc-format

ip rtp compression-connections 1000

Also if it helps, this is a cisco 2821 router with Version 12.3(8)T11 IOS.

Paolo Bevilacqua Wed, 03/28/2007 - 05:45

Is cef enabled on the router ?

You can increate the output queue a little bit, and monitor it together with router cpu, buffer utilization, etc.

If you have a link that is frequently in a congestive state then neither a QoS policy nor CEF is going to help. In fact, although you use a QoS policy to insure a certain level of service for specific traffic/applications that would only make the problem worse for other traffic flows. QoS is not a solution for a congested link; it?s more like a safety net for what should be rare occurrences of congestion. Also, increasing the output queue size can be dangerous. If packets sit in the queue too long its as good as if they had been dropped altogether.

Paolo Bevilacqua Wed, 03/28/2007 - 09:21

griffijo, all what you said is correct, however in certain cases the router can exhibit drops not directly related to congestion, one typical case is buffer exhaustion. This is why the TAC has published literature on-line on how to diagnose and address 'strange' output drops cases.

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