Total Output Drops increasing on WAN facing FE port

Unanswered Question
Jan 7th, 2009

Would you help me to understand the reason why Total Output Drops increasing on WAN interface fa0/0( facing to VPLS/L2 service link) on my 1841 router?

To my understanding there are two reasons that could cause this output drop counter increasing as follows.

1. speed/duplex setting that is not matching to the ether port that is directly facing to the equipment.

2. There is short burst of traffic coming in from LAN port that is exceeding 10M (in this case as the speed/duplex is hard-coded to 10M/Full)

Speed/duplex setting was varified that is matching, thus it should be the reason number 2 above, however, the LAN end is 10M and also the WAN interface is at the same speed,10M. How could a router would drop packets from the output queue when the LAN interface speed = WAN interface speed?

I have this problem too.
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glen.grant Wed, 01/07/2009 - 05:00

As it gets close to the max 10 meg the queues fill up and it can cannot process the packets fast enough and starts dropping the packets. We have seen this as it gets around 95% of capacity of the link. I would look at the utilization of that link and see if you are getting close to the max at times.

takimoto2008 Thu, 01/08/2009 - 23:08

Thanks a lot for your suggestion, we are currently setting up a lab to replicate the condition. will post when result comes up.

Phillip Hichens Wed, 01/07/2009 - 05:13

Remember output on the router WAN interface facing the VPLS/L2 link is towards the “WAN”, understanding the direction of this traffic flow is very important.

Thus these drops are traffic originating from the “LAN?” going towards the VPLS/L2 WAN link.

I suspect the router is getting more packets destined for the interface Fa0/0 than what it can put on the line clocking at 10M, have a look at the output rate and consider implementing/revising congestion management and congestion avoidance policies.

takimoto2008 Thu, 01/08/2009 - 23:11

Thanks a lot, understand that congestion management queueing policy need to be implemented to avoid the output drop, and setting up the lab to see what actually happens when there are more traffic than 10M generated from LAN to WAN. will post when the result comes up.

Joseph W. Doherty Fri, 01/09/2009 - 05:05

Any chance the 1841 is sourcing traffic that would also leave via the WAN facing interface, e.g. SNMP? If it were, then traffic in combined with locally sourced traffic could exceed the bandwidth out.


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