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VPN client session through two routers


For a customer I am providing a Ipsec VPN client connection with a Cisco 851 router. The Wan side of the router is connected through a wireless network to the LAN side of a SnapGear router from another vendor. There is two more SnapGear routers connected over the same wireless network to the first SnapGear router as well. MTU is set to 1380 on the LAN interface of the first router, and also to 1380 on the WAN side of the other two routers. VPN sessions through theese routers works fine. I have tried to set "ip mtu 1380" on the WAN interface of the Cisco router. I am then able to connect, and receive a ip-address on the LAN. As soon as I generate trafic (icmp, http or else) the session is disconnected.

Everything works fine if I connect through the Cisco router only. The problem occurs when establishing a sesion through two routers. I have attached a network diagram.

What should I do ?

regards Kjetil