MTU issue with IPsec Tunnel?

[TRACY HARTMANN]

I have a IPSEC tunnel setup with a vendor, not GRE and we can FTP files if they are smaller than 1394 but anything larger fails.  I thought it may be a MTUissue but I am not sure how to set an MTU on an IPSEC tunnel only.  I understand how to with a gre tunnel.   If the answer is to place it on the Ethernet what other issues might this cause since this router is used for outbound browsing.

[Richard Burts]

Tracy

Since IPSec adds additional header information to the packet it is common to have MTU issues when you do IPSec tunnels (and even more of an issue with the tunnel is IPSec with GRE). Make the adjustment on the Ethernet interface. The effect will be that other traffic going through the Ethernet interface will use the smaller MTU. But should not have much impact. And you need it to fix the IPSec traffic.

HTH

Rick

[Kishore Chennupati]

HI Tracy,

In addition to just what Rick has mentioned, if you like to know more about calculating and fixing the MTU for IPSec, GRE's then below is a good reference which explains it clearly.

http://www.cisco.com/en/US/tech/tk827/tk369/technologies_white_paper09186a00800d6979.shtml

HTH

Regards