ISR Traffic Forwarding

Unanswered Question
Jul 31st, 2007

I wanted to ask your opinion on how to configure the router or the firewall wherein all traffic that comes from the internet through the router?s WAN interface will automatically be forwarded to the firewall.

The goal of this setup is to let the firewall see the incoming traffic?s public IP address.

I have tried to configure a Static NAT on the router for the connection to work, but it defeats the purpose because the firewall will only see the router?s internal translated IP.

Please advise...

I have this problem too.
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rajatsetia Wed, 08/01/2007 - 03:28


Donnt know the IP Addressing and routing that you are using, but taking basic network in mind , here what i can suggest...

Give outside interface of your firewall (connected to router)public ip and there is no need of doing NAT, just route all the traffic meant for your Public IP pool towards firewall and your firewall will filter all the traffic coming from internet...

or you can specify the detail about the kind of ip addressing and routing you are using to get the specific answer ...


brianbono Wed, 08/01/2007 - 05:38

hi rajatsetia,

thanks for taking time to answer my problem. attached is a simple network diagram with a possible public ip address that my ADSL or SDSL Ethernet ISP broadband connection would give me.

my problem is that my ISR router will still connect to the ISP's modem/router as based on the network topology attached...

please advise...


rajatsetia Wed, 08/01/2007 - 09:49


As you have not mention the routing you will be using , i will assume that it will static routing...

Also you have shown the there are two ISPs terminating at your router. Both will give their own different LAN IP pool netblock.

In order to pass the traffic towards firewall as it is coming from router do the following :-

Assign router LAN Interfaces (connected to the firewall) IP Address from their respective LAN netblock.

for example, let us take the data provided for one of the ISP,

In the diagram , you have assigned WAN IPs from the public LAN IP Pool netblock provided by ISP1. i donnt think this will be the case. ISP1 will give you different LAN IP netblock and WAN IP address.

Give your firewall interface (connecting to router) public ip address from your ip pool of

Say router LAN Interface IP is and fierwall ip be All the request coming from internet meant for this LAN IP pool will hit the firewall passing through the router as it will be conneted route in the router.

this way your firewall will see the traffic as it is coming from internet.

similarly other LAN interface connected to the firewall be assigned the IP Address from Netblock assigned by other ISP2.

I hope you have figured out that how route traffic towards internet , as traffic originating from the different LAN pool should be routed on their respective ISP links. May be you have to use policy routing....




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