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Output Drops on 3750X interface

customer has a pair of 3750X switches to the 2 port 10G ports.

They are "worried" about when they do a Show Interface TE1/1/1 as part of the output they see

"Input queue: 0/75/0/0 (size/max/drops/flushes); Total output drops: 2727051"

I am not seeing where the output drop buffer might be and what might be done to modify so it does not drop as many packets.

It is connected to a Nexus 7702 10Gig port that is not showing any received packet issues.

 

Thanks

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Hall of Fame Expert

Re: Output Drops on 3750X interface

Hello Richard,

to understand the impact of the output drops compare the output drops to output packets

calculate the following ratio

output drops / (output drops + output packets)

 

taken from show interface tengiga1/1/1

 

if this ratio is equal or lower to 10^-4 you should be fine, TCP traffic should not suffer excessive retransmissions.

 

Hope to help

Giuseppe

 

4 REPLIES 4
Hall of Fame Expert

Re: Output Drops on 3750X interface

Hello Richard,

to understand the impact of the output drops compare the output drops to output packets

calculate the following ratio

output drops / (output drops + output packets)

 

taken from show interface tengiga1/1/1

 

if this ratio is equal or lower to 10^-4 you should be fine, TCP traffic should not suffer excessive retransmissions.

 

Hope to help

Giuseppe

 

Contributor

Re: Output Drops on 3750X interface

good information.  quick calculation came to 0.00000926  well below your .00001

Thank you

VIP Expert

Re: Output Drops on 3750X interface

Giuseppe is correct, that a low drop percentages is often not a major concern, but as a part of such an analysis you should determine if it happens at fairly constant rate or whether it happens in "clusters". If the latter, it might be adverse to some of your traffic, occasionally.

The 3560/3750 series of switches are a bit "infamous" for interface drops as they only have a couple of MB of RAM per 24 copper or uplink ports. Additionally their default QoS "reserves" buffers for what can be unused egress queues.

Often if QoS is enabled, and using default settings, and if it's not needed, disabling QoS will reduce interface drops counts.

If QoS is needed, buffer tuning might dramatically mitigate interface drops. (For example, a couple years a go I had a 3750 with a couple of SAN devices dropping multiple packets per second, but after buffer tuning, I reduced the drops to just a few per day.
VIP Mentor

Re: Output Drops on 3750X interface

Hello,

 

on a side note, which IOS are you running ? There is a cosmetic bug in IOS 15.0(2)SE1:

 

Output drops occur on Tengigabit Ethernet at low traffic.
This problem does not occur on Gigabit Ethernet.

Conditions:
3750X Swich running 15.0(2)SE1

Workaround:
None

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