Cisco NAT w/ Cable Modem

[sdufeu]

I have a Motorola SB4100 cable modem, attempting to connect to a Cisco 2514 (2 eth, 2 ser) - running NAT - into a Cisco 2924 switch where my PC's connect.

Here's my issue: Cisco IOS will not allow me to NAT between ethernet interfaces as I had planned. (Running 12.2-5) I can't run "encapsulation PPP" or "ip address negotiated" on the outside interface when attempting to use E1. It appears as though I will need to use a Serial interface off of the 2514 to make this work.

So, to the question(s): The Cisco serial interface has a proprietery pin structure, for which I have a cable that terminates with a standard 25 pin male connection. Can I simply use a cable that converts that connector into the RJ45 from the cable modem, or will some type of transceiver need to be used? (Tried a direct adapter - didn't work). Anyone out there had success with connecting a Cisco 2500 series running NAT to a cable modem?

TIA,
Scott

[PhyberOptix]

You won't and can't use your serial interface to attach to your cable modem.

Tell me specifically what you are wanting to accomplish and I'll give you a hand...

[sdufeu]

In a nutshell, I'm trying to use my Cisco 2514 to translate multiple internal IP's (unregistered) on my LAN to the registtered address from my cable modem. I am very familiar with Cisco routers, and NAT concepts, but was not aware that Cisco's IOS will not allow the features necessary to translate internal to external addresses between Ethernet interfaces.

Plan was, quite simply, to connect the cable modem to E0 (running DHCP, PPOE, and NAT OUTSIDE); and my LAN off of E1 (using 10's addressing and NAT INSIDE). I have been able to acquire my outside address in this manner, and can ping everything internally, but cannot successfully NAT between ethernet interfaces. In ALL varying methods of NAT for Cisco (Easy IP, VPDN, etc) you are required to utilize the Serial interface for the outside.

Right now I am experimenting with creating 2 VLANS on my Cisco 2924 and running NAT through it instead. Any insight is appreciated..

Thx,
Scott

[PhyberOptix]

This config should do what you are wanting to accomplish...

int e0
desc inside interface
ip address 10.1.1.1 255.255.255.0
ip nat inside

int e1
desc outside interface
ip address 200.1.1.1 255.255.255.0 (this is your public IP)
ip nat outside

!addresses to be converted
access-list 1 permit 10.1.1.0 0.0.0.255

ip nat inside source list 1 interface ethernet1 overload

I've never heard that you couldn't set up nat from eth to eth. I just used a 2513 to set it up from eth to token ring, so you should be able to do it. Try out the config above and let me know the results...

[PhyberOptix]

Sorry, one other thing...You won't be able to use the 2514 to route btwn the two VLANs if you end up going that route. 10 Mb interfaces can't run ISL or 802.1q. You'll need a 26xx w/ a 100Mb ethernet interface for that...

[sdufeu]

That's pretty much exactly what I was running, but every time I rebooted my PC into the 10's ntwk, I lost outside connectivity. Made the mistake of assuming NAT was failing. I just switched it back over and ran an extended ping to yahoo and cisco's IP's sourced from the INSIDE interface. So NAT is working. Any ideas on what I might be missing? DNS, or some other setting on my PC? (All I have been doing is assigning a static IP)

Again, thanks for the input..
~Scott

[PhyberOptix]

You will definitly need DNS settings on the internal pc's. Give that a try and then send a ping from the pc to www.yahoo.com.
Does it resolve the name to an IP?

[sdufeu]

Comin at 'ya from my NAT'd PC now.. I'm a WAN guy primarily, so it's no surprize that a client setup issue caused my big headache. We're rockin now.. Thanks for the input, Phyber!

Later!
~Scott

[PhyberOptix]

Not a problem. I'm pretty much a WAN guy myself. Just got assigned the project of rolling out a Frame Relay network for 500+ remote retail stores using Cisco 1700's. Should be fun....

Are you going after CCNA/NP/IE?