Output drops on Cisco 3750

[Robert Milner]

Hello everyone,

I could really do with some help with my Cisco 3750X; basically I am using the 3750 as a dedicated iSCSI Switch which is connected to 3 Hyper-V Servers via 1GB (each Hyper-V Server has 4x1GB connections), and there is also a Hybrid Storage Device connected to the Switch using 10GB SFP+.

On the 1GB interfaces I am seeing quite a few output drops, but on the 10GB interfaces I am not seeing any output drops.  I really need to find what is causing the output drops, and how I can possible resolve this issue.

This is what I have configured on the Switch:

• Jumbo Frames set to an MTU of 9000
• Flow control receive desired is on all interfaces
• QoS has been disabled (seen issues before with having QoS enabled)
• Spanning-Tree Portfast enabled on all interfaces

I have also uploaded the output of some show commands which should hopefully help out (let me know if there are any other commands that you would like me to run).

Thanks in advance to any help anyone can give.

[Joseph W. Doherty · Accepted solution]

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Posting

You've probably are bumping into the buffer limitations of the 3750X series.

You might upgrade your IOS to the SE8 release, doubt that will help in this case.

About the only thing that might mitigate your drops is active flow control, although that can cause latency.

PS:

Disabling jumbo Ethernet might decrease drops too, if it slows the effective data transfer rate.

[Robert Milner]

Hi,

Thanks for the information; I was considering turning off Jumbo Frames as in a past environment I have had nothing but problems using Jumbo Frames with iSCSI.

I have since disabled Jumbo Frames on my Hyper-V hosts and the iSCSI SAN, and I have also disabled all TCP offloading functionality on the Intel NICs that are partaking in iSCSI traffic, and since then I haven't had a single packet output drop in over 20 minutes which is looking much better.

If anyone wants to know how to configure your NIC settings for a Microsoft Hyper-V 2008 R2 environment, there is a good blog post that can be found here: http://gallery.technet.microsoft.com/scriptcenter/Configure-iSCSI-Network-81232a5e (I would have thought it would also apply to 2012 and 2012R2).

Thanks again for your help.

[Joseph W. Doherty]

Thanks for the update - especially about the impact of removing TCP offloading.