Weird problem configuring a router with a DSL Modem

Here is a diagram of the described network.

I have a Zyxel 623 DSL modem plugged into Fa0/1 on the 7206. Fa0/1 is configured to obtain its IP via DHCP. Fa0/0 on the 7206 is connected via ethernet to the uplink port on the 1900. Plugged into the switch is a Cisco 1700 router and a laptop PC. The 7206 is configured to be a DHCP server for the 172.16.35.0 /24 network configured on Fa0/0 (connected to the switch).

Problem

I have a default route configured on the 7206 to point all traffic out fa0/1 (connected to the DSL modem) but nothing plugged into the switch can pass traffic out to the internet. On the 7206 I can ping out to the internet just fine, but on anything plugged into the switch I can not. there is connectivity between the switch and the 7206 (I can ping loopbacks on the 7206 from the laptop) but no traffic is hitting the internet.

On the 7206 I have a static default route set to forward everything destined for the internet to go out fa0/1. Hosts plugged into the switch are pulling an IP from the 172.16.35.0 /24 range and DNS server addresses from the DHCP service as well. For DNS I am using my ISP's name servers. The default gateway is 172.16.35.1, the IP address of Fa0/0.

Logically speaking, if I initiate a ping request from the 1700 or the laptop the device should hand the request down to the default gateway (the 7206 fa0/0 interface) then the 7206 should forward the request out fa0/1 based on the static route. This is not occurring for some reason. Can someone possibly give me some insight into why this isn't working?

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Comments

keenon

is the dsl modem acting as a pass through? which means that the 7200 is performing all of the pppoe negotiation. if so your going to need some nat going on.. this is one way of setting it up

also heres something else that may be affecting it. if the dsl modem is the router for the dsl are you using the same ip segment on both sides? ex dsl -ip- 7200-ip- lab

Paul Boz

I'm sorry, it's just a regular dhcp connection. No PPoE, no authentication. For all means and purposes it functions just like a cable modem in bridge mode. Whatever public IP it pulls it assigns to the device plugged directly into it.

dtlokee

My first thought would be NAT, the second would be the default route point to a FastEthernet interface which is multi-access. Try a debug ip packet to see if you're getting "encapsulation failed" messages.

BTW, there are many lesser routers you coul use for your DSL connection

mysql1988

NAT or PAT should solve the problem. Private Ip address are not routable over the internet

Private IP address ranges are

10.0.0.0-10.255.255.255
172.16.0.0-172.31.255.255
192.168.0.0-192.168.255.255

Have a good day

larkspur

BTW, there are many lesser routers you coul use for your DSL connection

yeah Paul, stop showing off!! Everyone knows where you got that from....

trash digger

Paul Boz

I was trying to avoid using nat if possible because the IOS that was on the router didn't support it (which was very odd to me.) I updated the OS and fixed my problem.

I'm sort o surprised that the OS didn't support nat though..

larkspur

I'm sort o surprised that the OS didn't support nat though..

That is odd, what version was it?

Paul Boz

larkspur wrote:

I'm sort o surprised that the OS didn't support nat though..

That is odd, what version was it?

I'll check on Monday when I get back to work and get back to you on that. I believe it was 12.23x but I don't recall the features.

larkspur

paul you got this info ?