[c-nsp] cisco ios + adsl + mixed static ip / nat customer environment

chris tknchris at gmail.com
Mon Dec 26 20:18:02 EST 2011


I am having some trouble getting nat working on my setup. On most examples
I've found online people simply have a /30 or larger on their dialer
interface and can just use that as the nat outside and its just like any
other typical cisco nat. The thing is that in my case the ISP does a /31
ptp on the dialer interface then static routes a larger assignment to me
(in my case a /26). I'm 99% sure I wouldnt want a /31 as my nat outside
which I don't would even think could work at all. If I use public routable
IP's everything works and routes properly, but for some reason nat doesn't.
What I tried to do is create a subinterface with the second /30 of my /26
assignment and use that as nat outside and then created a subinterface for
the lan side of the nat.

My config attempt looks like this:

dialer 1
  ip address
interface fa0/0.10
  description this is the first /30 in my /26 assignement
  ip address x.x.x.x netmask
interface fa0/0.20
  description this is the second /30 in my /26 assignment which i would use
for my nat outside for the first nat client
  ip address x.x.x.x netmask
  ip nat outside
interface fa0/0.30
  description this is the lan side of the first nat
  ip address
  ip nat inside
ip route dialer1
ip nat inside source list 150 interface FastEthernet0/0.20 overload
access-list 150 permit ip any

I hope you get the idea of what it looks like and can maybe shed so light
on where I might be going wrong.


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