Clear the df bit in packets comming from the LAN

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May 14th, 2009
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Hi there.

I'm wondering, is there any available method or command which is provided to clear the DF bit from the packets which are coming from the LAN interface? I know that I can manually set up a route-map for this, but I'm searching for a different options. I have the following (standard) scenario:

LAN which is connected to a router who creates a GRE (with tunnel protection feature) tunnel to the tail end customer's HQ. The (ip) MTU of the Tunnel is setup to 1400 where the physical ("outside"/"inside") interfaces has a normal ip mtu of 1500. The real problem actually is that when a sender (who is located on lan interface fa0/0) trying to send an IP packet bigger than 1400 bytes with DF bit set (and application that has set this df bit doesn't understand the ICMP notification about fragmentation) the router is dropping the packet because it's too large to enter in Tunnel interface (because of ip mtu 1400 configured on it).

Any piece of advice?

Kind regards,

Danail Petrov

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danail-petrov Thu, 05/14/2009 - 23:18
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I just unintentionally did a double posting. Sorry about that ..


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