Can anyone tell me the overhead due to GRE tunnel?
Acc to my calculations, it will be:
Normal Packet = 66 bytes (including 6 bytes of MLPPP - L2 Header)
GRE tunnel overhead = 24 bytes + 6 (L2 ethernet header)
Total GRE tunnel = 24 + 6 + 66 = 96 bytes
i.e. 30 bytes extra w.r.t normal packet.
Is it correct?
It would be an additional 24 bytes overhead.
In the case of the GRE tunnel interface, the IP maximum transmission unit (MTU) is 24 bytes less than the IP MTU of the real outgoing interface. For an Ethernet outgoing interface that means the IP MTU on the tunnel interface would be 1500 minus 24, or 1476 bytes.
Hence most of the times it becomes necesarry to adjust the MSS or MTU while using GRE
Have a look at this link
HTH, rate if it does