How to check what traffic causes output drops?

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Jul 17th, 2008
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Hi all


I have a GigE trunk interface connecting two 6513. On one of the 6513 the "Total output drops" is constantly incremented by 7000 an hour. Total packets outbound is 118000000 an hour.

I know outbound drops might be caused by burst traffic but is there is a way for me to check exactly what traffic is causing this?


Regards

Fredrik

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cpembleton Thu, 07/17/2008 - 08:14
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To see what traffic is being sent you need to capture the traffic with a SPAN or hardware tape. Then use something like Wireshark to analyze the traffic.


The number of packets dropped because the output queue is full. A common cause of this might be traffic from a high bandwidth link being switched to a lower bandwidth link or traffic from multiple inbound links being switched to a single outbound link. For example, if a large amount of bursty traffic comes in on a gigabit interface and is switched out to a 100Mbps interface, this might cause output drops to increment on the 100Mbps interface. This is because the output queue on that interface is overwhelmed by the excess traffic due to the speed mismatch between the incoming and outgoing bandwidths.




http://www.cisco.com/en/US/docs/switches/lan/catalyst6500/ios/12.2SXF/native/configuration/guide/swcg.html



Hope this helps.

Chad

Giuseppe Larosa Fri, 07/18/2008 - 11:39
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Hello Frederick,

I would prefer to look for the reasons these drops happen.


Have you enabled any form of broadcast storm-control or multicast storm-control on the port ?


Packets dropped by broadcast storm-control will appear as output drops


This regular dropping rate could be a multicast stream that is near the threshold.


What kind of flow control is on the GE link ?

You can see from sh int gigabit x/y

In this case do you see a counter of pause frames incrementing ?


What type of linecard is the GE port on ?

Some types of linecards are not really full rate for all the GE ports they host and groups of ports share some resources on the card including access to the switching matrix /backplane


Hope to help

Giuseppe


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