total output drops on switch port on Cisco 6509E

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May 23rd, 2012
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What is the cause of having a huge number ( 875349) of total output drops on one of my gigabit utp port gi 2/12 which is connected to Cisco 1841 fa0/0 router by mean of cat5e cable.I did change the cable from cat5e to cat 6 and tried to increase hold queue to 4096 and to tweak wrr queue bandwidth and still I do have the same problem

Any ideas and solutions.

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nkarpysh Mon, 05/28/2012 - 17:59
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Usually that is sign of oversubscription which is caused of multiple ports sending out of single one. It can cause random burst which are spiking and cause interface to buffer and eventually drop. Interesting thing is that average rate can still be low.

Way to check it is to configure and etherchannel adding new port and see if drops going down.

if you need some other info- let me know.


@EngZiad@ Tue, 05/29/2012 - 03:39
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Hello Nik

You mean L3  etherchannel from the Switch to the router , Is this doeable for the C1841 i know it can be done for high end routers like 7200 series .?

nkarpysh Tue, 05/29/2012 - 06:52
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Right 1841 does not support etherchannel.

Generally problem is due to much traffic over short period of time. There is no specific cure for that other than changing flow or adding new links.

What is your current connection between switch and router - is it L2 trunk or L3 link? IN case of L3 - you can just add one more and load-balance using routing. Though output drops are a sign of a need for upgrade in HW.


Joseph W. Doherty Tue, 05/29/2012 - 10:11
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If you're running the link to your 1841 at 100 Mbps, there may be little point to directing more bandwidth to it as the 1841 is only rated at 75 Kpps.

If your traffic is mainly TCP, it might be possible to use WRED to decrease the overall number of drops.  As Nik has described, you're probably oversubscribing your egress bandwidth, so you're going to have some drops, but WRED might slow a flow, or flows, and avoid really excessive dropping.  (NB: it can be difficult to "tune" WRED for optimal performance.)

Also BTW, if your clients are ECN capable, WRED might also be used to slow flows without any dropping.  (NB: I don't recall whether 6500 line cards support ECN WRED.)


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