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1720 Router Config Issue

I am no router guru. I used cofigmaker (yes everyone can now grumble) to set up a 1720 router to provide NAT on the ethernet side to my one public IP number. It is working perfectly for internet access. I have this hooked to a T-1 using HDLC at my house for the specific reason of being able to VPN into work. It does not allow the VPN connection. Logs in, checks user name and password and then times out. XP Pro gives an error 610 (?) that NAT may not be configured correctly... Once again using config maker i try to set up the firewall (which is currnetly not active)and it tells me that due to using NAT i can not allow WAN connections to the LAN when i try to tell it that internet can access all on the ethernet. The router has IOS 12.1. So there is no firewalling being done by the router, has NAT inside and out, is only configured for TCP traffic and it works for everything except VPN into a Snap Gear firewall.

Any help and a point in the right direction would be greatly appreciated. My ISP said "everything looked ok" in my config and my IT guy agreed.


VIP Purple

Re: 1720 Router Config Issue

Hello Todd,

I found this in the SnapGear Knowledge Base:

This is usually due to fragmentation problems of routers in between the IPSec tunnel endpoints. You will need to reduce the mtu of the Internet interface. For a "Direct Connection" to the Internet edit the /etc/config/start file and add the line:

ifconfig eth1 mtu 1400

to the end of the file.

You can also reduce the mtu of the ipsec interface. Guidelines on how to do this can be found at



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