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Help with mtu issue on an ipsec gre tunnel

Dear expert I am having a problem with mtu. spoke is setup with an ipsec gre tunnel using ospf. User are not able to access the internet or internal application. user are able to ping to the internet, ping to internal ip's. I all had configured ip nut 1300 under the tunnel interface and ip tcp adjust-mss 1300 under lan interface ofice had work for afew hrs but went back down. Any idea what to try will help
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lanboyo

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Make sure there are no PMTU issues...

do a series of ping commands with the do not fragment (DF) bit set.

  ping -f -l PACKET_SIZE  SERVER

Where PACKET_SIZE is the data size of the ping and SERVER is the IP of the server with the shares.

You should look for a gap in packet sizes between when you get ping replies and when you get the message :

Packet needs to be fragmented but DF set.

This error message is not a problem. The problem is no response at all.

If you have a gap then you have a bit of a blackhole where certain packet sizes can not traverse the network AND icmp can not fragment messages do not return. If you have no problems from the client to server do the same thing server to client.


Also, tcp mss command only works for TCP. For UDP I have had to delete the DF bit with a route map. Note to microsoft: Honestly, just send the packets at 1500 bytes and let the network handle fragments.