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ISDN backup to internet

I have just setup an ISDN backup line to our ISP. It connects our public network to the internet.

The ISDN is set up on a different ip subnet to our public address range. When the ISDN line comes up the router can access all resources (because it has a direct connection to the ISDN network), but nothing on our public network can. I have added a satic route to the ISDN net.

Do I need to nat our public addresses behind the ip address on the isdn (to allow the return traffic to be sent to the isdn interface)

I hope I have explained my problem ok.

Thanks for any help

Community Member

Re: ISDN backup to internet

It is best to ask your ISP how the mechanics of there routing are designed to work for there ISDN backup service and check that they (and you) have done everything correctly.

There are several approaches an ISP can take, to solve the routing requirements - so while we could guess - ultimately you'll need to talk to them.

If however you've installed an ISDN backup without involving your ISP (or to another ISP for that matter), you will have to NAT using the ISDN subnet, even if your local address are public since the ISP does route this via the ISDN.

Regards, Jeremy.

Community Member

Re: ISDN backup to internet


Usually the case is that the ISP will have a router

which does not allow any other subnets except its own subnet to send packets to its isdn interface. this is why the router is reaching the internet but the machines on your public network cannot reach.

so you have to use NAT which will nat all your public addresses to the bri interface which has the ip address which you isp provides you when you dial in.

just a sample config

int bri0

ip address negotiated

ip nat outside

int eth0

ip nat inside

ip nat inside source static list 2 int bri0 overload

access-list 5 permit any

I hope this helps

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