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How do you check MTU Size??

unknown

We are currently using an SCG200, and we are using the Default Tunnel Profile, upon checking the manual it is configured as MTU AUTO or we can manually configure it between 850-1500. Our client is asking if we have a way to check the MTU size?

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Ankush_Chandan

SCG allows maximum 1500 MTU and that basically sets the MTU on AP as 1440. You can check this from AP CLI with command “get mtu gre”.

unknown

Hi Ankush, we are only getting 1392 when we issued the get mtu gre, what does this mean? Image_ images_messages_5f91c44e135b77e247a161bb_8b0c6c42433c54b98cfb9b7fe3703cfe_RackMultipart2019031879864ebvd-235631b4-b1df-46b9-9149-d7171e987a7c-1454958047.jpeg1552918769

Image_ images_messages_5f91c44e135b77e247a161bb_8b0c6c42433c54b98cfb9b7fe3703cfe_RackMultipart2019031879864ebvd-235631b4-b1df-46b9-9149-d7171e987a7c-1454958047.jpeg1552918769

Ankush_Chandan

Hi Clifford,

The MTU is negotiated between the two end points and it depends upon the path MTU. AP sends ICMP packet to the vDP every 10 minutes to discover the PMTU and if any fragmentation needed for the ICMP packet, AP reduces the gre MTU from 1440 to 1392.

unknown

Hi Ankush,

Thanks. What is the composition of the 60 bytes that was subtracted, 20 bytes IP + 16 bytes Ruckus GRE(this are just assumption on my end). We want to know so we could explain to customer the 1440.

Ankush_Chandan

Hi Clifford,

The calculation will be Eth + IP + UDP + Ruckus GRE : 14+20+8+16 =58 . This negotiates MTU to no more than 1440.

Regards,
-Ankush

unknown

Got it. Thank you very much.

Greg_WiGuy

@ankush_ankush , Is it possible to set the tunnel to not use UDP encapsulation for networks that dont need NAT traversal?