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Beginner

OTV over 1500 MTU carrier DCI

We have just configured OTV on an ISP link between two datacenters. After everything was set up we found out about the 42bytes of overhead and the DF bit being set. Naturally that isn't working out on a 1500 MTU network.

Our ISP won't raise the MTU to an acceptable level.  So, my question is. Has anyone else ever faced this problem? Are there any tips or tricks to get around the DF bit or is shopping for a new ISP my only option?

Thanks!

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Cisco Employee

OTV over 1500 MTU carrier DCI

You can lower your servers MTU to allow the additional OTV header. If you choose this route, 1400 is a good number to test with.

HTH,

jerry

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Beginner

OTV over 1500 MTU carrier DCI

Hi Jerry, thanks for your response.

Sure lowering the MTU on all the devices would work, but that is Thousands of hosts.

Its not a viable option to lower the MTU for every workstation on our network.  Is there any way to lower the MTU only on the join interfaces and fragment the frames before they are put on the overlay?

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Cisco Employee

OTV over 1500 MTU carrier DCI

Packets traverse the system as the following:

Internal (physical, L2) -> Overlay (virtual) -> Joint (physical, L3).

The packet already included the additional OTV header and DF bit at the Overlay. Changing the Joint is not going to do anything, and it will actually break ISIS.

Your other option is shop for another ISP like you said.

Regards,

jerry

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Cisco Employee

Re: OTV over 1500 MTU carrier DCI

are you using ASR1K to perform OTV, in that case you have the option to fragment the packet. This feature however AFAIK is not available in case of Nexus.

Command to be used: "otv fragmentation join-interface "

Just wanted to add one more information. This command has to be enabled on all your ED's [for most this is known, but just wanted to be specific]

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Frequent Contributor

OTV over 1500 MTU carrier DCI

Why is TCP-adjust-MSS not supported in OTV? It would solve 90% of the problems.

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