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PPPOE DSL issues with double NAT and 1720 router and MTU

I am having an issue with a DSL link running PPPOE over Ethernet and NAT on a 1720 router. The 1720 router has (1) T! interface and (2) Ethenret interfaces. My internal clients use a spoofed IP address. When they go to the internet they go through a firewall that NAT's them to a public IP address belonging on Ethernet 0. This public IP address is provided by T1 ISP (T1 link) . I am trying to use PPPOE DSL for a backup link if the T1 fails. So in the instance that the T1 fails, I would enable the Ethenret0 interface (pppoe) and the dialer interface. I would configure NAT on the 1720 router to go from the public IP provided by the T1 ISP to the public IP provided by the DSL ISP.

In this scenario, the DSL link is up and working fine. IP traffic to the internet goes thorugh the DSL link. The issue that I am having is that certain http sites are not accessible. Some http sites work fine. I have put a sniffer on the link and can see that the outbound packets to the effected http sites are being put on the wire.

The normal "IP MTU" on the dialer interface is set to 1492. I have tried to change this size (decrease to 1400) to allow for the additional overhead of the NAT. But the problem still exists. What is going on here ? Has anyone configured a network setup like this ?

Thanks,

1 REPLY
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Re: PPPOE DSL issues with double NAT and 1720 router and MTU

I think the pix is blocking some sites which have java , active x content in them. Check your pix software with the known problems in the cisco website .

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