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What's the difference between MTU and IP MTU on Tunnel Interfaces?

When i configure a GRE Tunnel the "int tu0" shows MTU 1514. By Default the IP MTU of this Tunnel is 24 Byte less than the physical interface IP MTU (whitch means the "next hop" Serial i.e.(?), so the GRE interface MTU is 1476. When i want to set the Tunnel MTU to 1500 (to avoid fragmentation) i only set Serial interface MTU to 1524 an GRE Tunnel IP MTU to 1500. The "int tu0" still shows MTU 1514. Do i have to increase also the MTU of this Tunnel interfaces?

  • Other Network Infrastructure Subjects
3 REPLIES

Re: What's the difference between MTU and IP MTU on Tunnel Inter

When your tunnel is corssing any public network you will have to stick to 1500 bytes for the GRE-frame. Otherwise you run the risk of yor frames being dropped as giants.

Having a smaller mtu is one of the drawbacks of GRE-tunneling.

New Member

Re: What's the difference between MTU and IP MTU on Tunnel Inter

I'm not crossing a public network, just a Serial-to-Serial WAN where i have increased the MTU to 1524 Byte. My problem is to find the difference between the 'MTU' an the 'IP MTU' of an Tunnel interface?

Re: What's the difference between MTU and IP MTU on Tunnel Inter

Ok, I understand your question.

MTU is the size of the payload of an ethernet frame. Over GRE you are only sending layer 3 and up, encapsulated in IP & GRE. Therefore IP MTU.

Layer 2 has only local significance, thus there is no need to send it along the tunnel. On the receiving side the mac-layer is rebuilt from ARP.

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