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I did some research for my question beforehand such as this one:Cisco routers THROUGHPUT - MTU and packet size

however, it seemed not precise to explain the doubts in my head.

My primary goal is to solve the tunnel encapsulation problem for my VMware NSX-T environment. I'm currently hanging at tunnel status down problem and would like to ensure that the physical links are all correct.

in my infrastructure, the servers nodes are connected to Cisco 2960X switch and the inter-vlan routing has been enabled. all the MTU size(system, jumbo, routing...etc) were configured to 1700(for the GENEVE encapsulation).

The encapsulation tunnel between ESXi server and edge VM(which is located on ESXi) had unknown problem which prevent them from connecting to each other via tunnel encapsulation. the Pings are unreachable.(but the pings between ESXi hosts' tunnel are normal.)

I would like to doubt that there's issue on physical links before troubleshoot the ESXi virtual switch setting(its MTU is 1700 too).

here's the question:

  1. what kind of MTU setting/system setting will the switch disrupt the encapsulation? should i set routing MTU to 1500? or maybe the routing MTU size does nothing to this GENEVE encapsulation? (in my infrastructure, I must to utilize two different VLANs to establish the encapsulation tunnels therefore the inter-vlan routing is mandatory.)
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    Changing the MTU has to be done EVERYWHERE. I would bet the vmnic isn't set to 1700. (the machine that houses most of my brain isn't on, or I'd share the iSCSI networking scripts from my setup.) – Ricky Nov 20 at 4:26
  • @Ricky thanks for the suggestion! but I dont see MTU setting on any esxi vmnic. all my esxi hosts' NICs were managed by a VDS and this VDS had MTU 1700. – user53815 Nov 20 at 5:06
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You cannot increase the routing MTU because of the hardware limitation (the switch cannot fragment in hardware). The system mtu routing will only affect the router ports but at the same time would not affect the SVIs.

In brief, you cannot have routed ports and SVIs on the same box with different MTU sizes.

The only thing you can do on that hardware is:

  • increase the system mtu 1700 (This command would only affect 10/100 interfaces.)
  • increase the system mtu jumbo 9000 (This command only affects Gigabit Ethernet interfaces.)

Have you tested the MTU size with the ping and df-bit set?

Best

| improve this answer | |
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    As a note, anything above 1500 is technically "jumbo". – Jesse P. Nov 21 at 0:08

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