IPSec and Router MTU values

[JohnTylerPearce]

Hey guys,

So, let's say I have an ISR G1 router, and a Lan-to-Lan IPSec tunnel built. On the outside interface(WAN) I have an MTU of 1260, with the 'ip mtu 1260' command. Now if I put this command in, the max IPSec (ESP) packet will be 58 bytes, and with tunnel mode you add an additional 20 bytes as well, for the new IP header, since the original or encapsulation within ESP, encrypted and authenticated.

So this would come to 1338 with my math. Now if I ping from the source of my WAN to the remote IP (Extended Ping) with  1338 as the size, and DF-bit set I don't get any successful pings. If I do an Extended Ping with the byte size of 1178 which is (1100+50+20) I get successfully pings. So would it be wise to get ride of 'ip mtu 1260' and put 'ip mtu 1100' ?

[milan.kulik]

Hi,

I believe you should subtract, not add, whithin your calculations.

So with  'ip mtu 1260' command on your WAN interface the maximum Ping packet size you are able to transfer while DF bit set is 1260 - 58 - 20 = 1182 Bytes.

(In fact there are additional 4 Bytes lost somewhere, I'd need to search my archive to find the exact fromula.)

So if you want to get a 1338-Byte Ping through with DF bit set, you need to use  'ip mtu 1420' command instead of 'ip mtu 1100'.

Best regards,

Milan