Mar 28th, 2007

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)

Total GRE tunnel = 24 + 6 + 66 = 96 bytes

i.e. 30 bytes extra w.r.t normal packet.

Is it correct?

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

http://www.netcraftsmen.net/welcher/seminars/intro-ipsec.pdf

HTH, rate if it does

Narayan

Overall Rating: 5 (2 ratings)

## Replies

royalblues Wed, 03/28/2007 - 04:53
• Green, 3000 points or more

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

http://www.netcraftsmen.net/welcher/seminars/intro-ipsec.pdf

HTH, rate if it does

Narayan