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what happens if.....?

just a senario i came up with in my head that i was curious about.


s0=ISP internal network



s0= (ip outside)

eth0= (ip inside)

router1 is owned by an ISP. and router2 is owned by xyz.corp.

the ISP gave xyz.corp a public IP of and router2 will only allow an IP from to the internet through its internal network.

before the network used to be on router2's eth0 interface and all clients off of that interface had public ips. now xyz.corp wanted to NAT using a private IP range of

the router is overloading s0 to router1 has a route that says is out its s1 interface, the way it used to be setup that was true. technically it is true now, but isn't bound to an interface its just an ip that 192.168.100.x is NAT/PAT to. what happens? the address is really on the subnet that the subnet is on. clients will send out a request to will be NAT/PAT to and then off to the ISP router, the ISP router will see that is allowed to the internet and will allow it through, will respond to and the response will go back to router2, router2 will see from its table that 200.200.200.x is down s1 and will forward it, router1 will get it and do what? what happens first when the packet comes into the interface?

lists the order of operations for NAT. routing happens first, the default route on router2 says route everything out s0. router2 doesn't have a route to the 200 network because it really doesn't exist and will route it back to router1, router1 will route it back to router2.....till TTL exprires.

would have you have put a NULL interface in, assign it an ip and point a route to that one router2?

anyways just curious what would happen. i was talking with my friends and somehow we ended up with this senario....and wondered what would happen if...



Re: what happens if.....?

I'd imagine that NAT table is checked before the routing table for return traffic. But someone can correct me if this isn't the case.

New Member

Re: what happens if.....?