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ISCW - NAT and default route question

I'm currently studying ISCW and have been reading the official exam guide. In chapter 5, it talks about the setup of NAT, DHCP then config a default route for the SOHO network.

I have an older 2611 router running at home with NAT and DHCP. It is being used to separate my kids network from my own network to implement a time-range blocks to prevent them from spending too much time on the internet.

I remember when I set it up, I didn't put a default route. I went back to my 2611 config today and surely there is no such thing as "ip route 0.0.0.0 0.0.0.0 interface ethernet 0/0". However, show ip route lists a default route as "S*" points to the next hop route to get to the internet.

So, it seems to me that NAT installed this route on the route automatically (and things are working perfectly). If this is the case, why the book (page 119) says to config a static default route on the router??? I have no routing protocol running on this router.

Thanks.

- AC

6 REPLIES
Hall of Fame Super Blue

Re: ISCW - NAT and default route question

AC

You don't need a routing protocol to have a static route. if you do a "sh run | include ip route" do you get any entries back ? such as

ip route 0.0.0.0 0.0.0.0

NAT does not install routes into the routing table.

Jon

Community Member

Re: ISCW - NAT and default route question

Thanks for reply. This is exactly the mystery I want to solve. I wrote the entire config on the router so I know there is absolutely no ip route statement and definitely the "sh run | include ip route" did not return anything. But the default route does show up when I issued "sh ip route". If NAT doesn't put the default route in, who did it? Will a particular IOS version do it automatically? I am using 12.2(17).

-AC

Hall of Fame Super Blue

Re: ISCW - NAT and default route question

The DHCP part. Are you getting a DHCP address for the outside interface and is that setting the default-route.

Could you post config minus any sensitive info.

Edit to your edit :) - looks like we both had the same thought. I suspect this it is DHCP that is installing the route.

Jon

Community Member

Re: ISCW - NAT and default route question

Thanks Jon. I think we crossed mail. I think you and I are thinking the same thing - DHCP for outside IF taking a default gateway from the DHCP server and putting it into the routing table. I think I have overlooked that area. Thanks !

-AC

Community Member

Re: ISCW - NAT and default route question

Thanks for reply. This is exactly the mystery I want to solve. I wrote the entire config on the router so I know there is absolutely no ip route statement and definitely the "sh run | include ip route" did not return anything. But the default route does show up when I issued "sh ip route". If NAT doesn't put the default route in, who did it? Will a particular IOS version do it automatically? I am using 12.2(17).

-AC

p.s. I just think of something after I posted this reply so I'm adding information here.

E0/1 is the NAT inside IF, E0/0 is the NAT outside IF and is set as a "DHCP client". The DHCP lease would include info such a default router (in this case, the next hop IP as shown under "sh ip route"). I wounder if this information from the DHCP lease gets propagated into the routing table?

Community Member

Re: ISCW - NAT and default route question

I also had the same issue when I studies ISCW.

The way I cleared my confusion was after giving inside IF a static address and no DHCP at all.

Do this and find out yourself :-). The best way to do.

Thanks and Regards

OJ

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