Re: [CentOS] centos 6 and super jumbo frames

[Steven Tardy]

> On Nov 9, 2015, at 11:34 AM, Steve Clark  wrote:
> 
> IP 10.79.4.53.64327 > 10.79.2.53.24294: Flags [.], seq 16060:29200, ack 1, 
> win 32767, length 13140


Do you have RSS enabled? With RSS the software/tcpdump sees larger "packets" 
but the physical NIC chunks down to the wire MTU. What does a capture on the 
destination show?

https://en.m.wikipedia.org/wiki/Receive-side_scaling#RSS
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Re: [CentOS] centos 6 and super jumbo frames

[Steve Clark]

On 11/09/2015 12:36 PM, Gordon Messmer wrote:

On 11/09/2015 08:34 AM, Steve Clark wrote:

Has anyone using CentOS 6 been able to successfully set an mtu larger
than 9710
on an interface.

Maximum frame size varies from implementation to implementation:
http://pages.uoregon.edu/joe/jumbo-clean-gear.html

It's also worth noting that that due to offloading features in modern
NICs, there's often very little benefit to large frames. Since you
*really* need all of the devices on any network segment to use the same
MTU, the best option might be to eliminate jumbo frames (and test the
impact of doing so in terms of throughput and CPU utilization).


Hi Gordon,

Thanks for the response.

The real issue relates to doing pcap on an interface that is
hooked to a span port. The super jumbo frames cause rx_long_length_errors: 
2701813
which show up in our monitoring software and the customer thinks there are 
error on his
network.

So I wanted to increase the mtu on the interface so these errors would not be 
reported.


--
Stephen Clark
*NetWolves Managed Services, LLC.*
Director of Technology
Phone: 813-579-3200
Fax: 813-882-0209
Email: steve.cl...@netwolves.com
http://www.netwolves.com
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Re: [CentOS] centos 6 and super jumbo frames

[Gordon Messmer]

On 11/09/2015 08:34 AM, Steve Clark wrote:


Has anyone using CentOS 6 been able to successfully set an mtu larger 
than 9710
on an interface. 


Maximum frame size varies from implementation to implementation:
http://pages.uoregon.edu/joe/jumbo-clean-gear.html

It's also worth noting that that due to offloading features in modern 
NICs, there's often very little benefit to large frames. Since you 
*really* need all of the devices on any network segment to use the same 
MTU, the best option might be to eliminate jumbo frames (and test the 
impact of doing so in terms of throughput and CPU utilization).

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[CentOS] centos 6 and super jumbo frames

[Steve Clark]

Hi,

Has anyone using CentOS 6 been able to successfully set an mtu larger than 9710
on an interface.

I am seeing super jumbo frames with length > 1.
...
IP 10.79.4.53.64327 > 10.79.2.53.24294: Flags [.], seq 16060:29200, ack 1, win 
32767, length 13140
...

CentOS release 6.7 (Final)

Thanks,

--
Stephen Clark
*NetWolves Managed Services, LLC.*
Director of Technology
Phone: 813-579-3200
Fax: 813-882-0209
Email: steve.cl...@netwolves.com
http://www.netwolves.com
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Re: [CentOS] centos 6 and super jumbo frames

[Gordon Messmer]

On 11/09/2015 09:57 AM, Steve Clark wrote:
So I wanted to increase the mtu on the interface so these errors would 
not be reported. 


Well, ifconfig can be used to set the MTU.  The maximum size may depend 
on the hardware and driver that you're using.

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Re: [CentOS] centos 6 and super jumbo frames

[John R Pierce]

On 11/9/2015 9:57 AM, Steve Clark wrote:

The real issue relates to doing pcap on an interface that is
hooked to a span port. The super jumbo frames cause 
rx_long_length_errors: 2701813
which show up in our monitoring software and the customer thinks there 
are error on his

network.

So I wanted to increase the mtu on the interface so these errors would 
not be reported.


9K or so is the upper limit for MTU on most NIC cards, and many are less 
than that.   this is a hardware thing, has nothign to do with software.




--
john r pierce, recycling bits in santa cruz

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