Cable modem to 3640 without NAT?

[labgrl76]

Hi all,
I've been running a 3640 with dual ethernet interfaces and using NAT for quite some time. When I disable NAT and change the default route to the next hop (my isp's gateway ip) I get nothing. I'm running eigrp and have added the network number my isp has (at the time) assigned me. Anyone have better luck in utilizing a "NATless" environment using a cable modem and cisco router?

 

[burtsbees]

Is the cable modem in bridge mode?

Burt

 

[labgrl76]

That I couldn't tell you since it's a basic model rented from Comcast (RCA). My last one burned out so I've resorted in renting.

 

[burtsbees]

Well, I am not that familiar with the 3640...but is it doing the PPP negotiation, or is the modem? When the modem is in bridge mode, it acts as a bridge and simply handles the atm (or whatever interface connects you to the internet), and the router handles the actual PPP pap/chap. If the modem was previously doing the negotiations, then it was likely handling the NAT. If this is the case and you put in a static route to the next hop, then the next hop would be that of the modem, not the WAN ip address. However if the router was handling all of this, then the best thing to do is to NAT, because then none of the private IP nodes will be able to get out of the network.

Burt

 

[voltron1011]

From my experience with cable-modems (4 years ago) was that they are always in bridging mode. A Cable-modems bridges to your PC's NIC (or router/fwl ethernet interface) and does a DHCP mac-reservation with it. There's no other way to configure the interface manually, even if you copy the exact DHCP assigned stuff directly in. Also, since there the DHCP uses mac-reservation, you have to power-down the cable-modem if you are going to be changing devices (like a different router, pc, etc).
Unless things have changed drastically in the Cable industry, that's how it usually works. Burtsbee, I've actually only seen what you are talking about on DSL lines, where the modem actually acts like a NAT itself and often needs to be put into bridge mode as well as DHCP-server shut down.

 

[voltron1011]

I'm also a bit confused on where you are NATing? As of right now, Comcast should give your router's outside interface (whichever one is plugged into the MODEM) a public ip (perhaps something like 24.xxx.xxx.xxx) and you should be NATing that down into your private network. I'm not exactly sure why you mentioned running EIGRP because Cable and DSL companies aren't going to be running any routing protocols with you (much less EIGRP).

 

[labgrl76]

Well, I'm running EIGRP since there are multiple routers connected into the 3640 via serial wics. I also happen to favor that routing protocol on Cisco equipment. I'm using PPP as my encapsulation since I am running MLPPP in my environment. Voltron, that's exactly what I've experienced with Comcast. Unless you assign DHCP to your outside interface on the router you won't get anything. Appreciate everyone's input on this. It's all clearer now