measure GRE tunnel bandwidth using iperf

[Yuefeng Wang]

Hi, 

I have a question about GRE tunnel. I built GRE tunnels over two aggregates (GPO and NYU, WISC and NYU, GPO and Illinois, GPO and WISC), and I found sometimes I cannot measure the bandwidth of the GRE tunnel using iperf TCP, basically when I open iperf server on one end of the GRE tunnel, and iperf client on the other end, it will not return any measurement after iperf client is connected to iperf server using TCP.  

On the other hand, when I was using iperf UDP, and it can always return the measurement of the bandwidth. So I would like to know whether the GRE tunnel has some constraints on TCP connections. 

Thanks, 

Yuefeng 

[Nicholas Bastin]

On Wed, Jun 11, 2014 at 4:59 PM, Yuefeng Wang <w...@bu.edu> wrote:

I have a question about GRE tunnel. I built GRE tunnels over two aggregates (GPO and NYU, WISC and NYU, GPO and Illinois, GPO and WISC), and I found sometimes I cannot measure the bandwidth of the GRE tunnel using iperf TCP, basically when I open iperf server on one end of the GRE tunnel, and iperf client on the other end, it will not return any measurement after iperf client is connected to iperf server using TCP.  

This happens when your MTU is too large and you lose an IP fragment early in the connection setup.  If you adjust your MTU down to 1400 on each interface this should work.

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Nick

[Yuefeng Wang]

Hi Nick, 

Thanks a lot for your reply. 

I am using VMs, so I should change the MTU for the interfaces of the VM ? Can I do that when I am reserving the VMs?

Regards,

Yuefeng 

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[Nicholas Bastin]

On Wed, Jun 11, 2014 at 8:03 PM, Yuefeng Wang <w...@bu.edu> wrote:

I am using VMs, so I should change the MTU for the interfaces of the VM ? Can I do that when I am reserving the VMs?

You'll have to change the interface MTUs yourself (ifconfig ethX mtu 1400, for example).  You are free to change the MTU for VM interfaces to anything that is lower than the host interface MTU (which is probably 1500).  Obviously if you try to make it larger you will have more problems.. :-)

You may have to push your interface MTU as low as 1360 depending on the GRE implementation and other issues in your path.

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Nick

[Yuefeng Wang]

Hi Nick, 

I tried what you suggested, and it works!

Thanks a lot. 

Regards,

Yuefeng

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