Connecting Cisco Router to ISP modem router

[isaiahgoveait]

Hello Everyone,

I am trying to setup my lab here. First I need to connect my Cisco 1841 router to my ISP Arris modem router, the Arris router is routing with NAT. Should I setup Dynamic routing (RIP)?

I want to partition my networks here, and what I mean by that is I want the Arris modem to subnet and give DHCP addresses to my roommates so they can use the internet. Then I want to subnet to my home lab onto my Cisco 1841 router. And then from the 1841 I want to route it to my other routers then onto my switches.

What would I have to be looking up here to accomplish this home lab project?

[dlots]

There is a good chance that the ISP device is to dumb to do a routing protocol, and is already doing NAT.  What I would do if I were you is setup my 1841 router to get an IP address via DHCP and take it's default route out from dhcp

conf t
int fa0/0
no shut
ip address dhcp

ip route 0.0.0.0 0.0.0.0 dhcp

That should get your 1841 on the network, and all your room mates can get on the normal network, then you can do whatever you want to your 1841 and the rest of your lab without screwing up their network (roomies can get REALLY mad when you accidentally take down the internet.)

On your 1841 setup NAT so it looks like everything behind it is using that DHCP address.  With this everything can get to the internet.

[deanwebb]

dlots' suggestion looks good to me. All the router needs is a static outbound route to the ISP device. All return traffic will arrive at the 1841 and the 1841 will be smart enough to hand it back to wherever it needs to go.

No need to make the router do stuff with the ISP device. The ISP prefers it that way, really. They'd rather not have to have customers participate in their routing schemes. Just imagine trying to explain an OSPF or BGP setup to Uncle Leo when he's irritable, and you'll see why they're all glad to keep customer gear for residential accounts all on the other side of the routing table.

[isaiahgoveait]

Okay so go into the Arris modem/router and here I found under the WAN settings the Routed Subnet. Would that be an alternate way of connecting it too?

[dlots]

You don't need any of that.  The traffic behind the NAT will look like it's coming from the IP your 1841 will have, no config on the Arris needed at all.

[isaiahgoveait]

Okay gotcha. Okay so now I have obtained an IP on the 1841 from the arris DHCP. Now when I ping google.com

I get, 'translating "google.com"... domain server (192.168.0.1)' in return

[dlots]

try adding the config
ip name-server 8.8.8.8

This will tell the router where to go to resolve the name with DNS

[icecream-guy]

try adding the config
ip name-server 8.8.8.8

This will tell the router where to go to resolve the name with DNS

'

does ping 8.8.8.8 work ? try that first.

[dlots]

try adding the config
ip name-server 8.8.8.8

This will tell the router where to go to resolve the name with DNS

'

does ping 8.8.8.8 work ? try that first.

Yep
This

[isaiahgoveait]

Okay so I,

conf t'd

int fastethernet0/0

ip route 0.0.0.0 0.0.0.0 dhcp
ip name-server 8.8.8.8
[I checked my Arris I got an IP from the DHCP, Check.]

I now go to ping 8.8.8.8

'Sending 5, 100-byte ICMP Echos to 8.8.8.8, timeout is 2 seconds:
.....
Success rate is 0 percent (0/5)'

What else could I check to see if something is misconfigured?

[isaiahgoveait]

I fixed it by doing this,

(conf t

!

interface e0/0

description ## INTERNET ##

ip access-group MY_WAN in

ip address dhcp

ip nat outside

no shut

!

interface e0/1

description ## MY LAN ##

ip address 192.168.1.1 255.255.255.0

ip nat inside

!)

Source: https://supportforums.cisco.com/discussion/11804476/how-do-you-connect-cisco-router-internet-through-cable-modem-router-gateway

[deanwebb]

Yeah, it needed a DHCP address from the internet router on the external interface. Now you're good to go!

[ggnfs000]

not sure if my situation is applicable, few years back did something similar. But instead of using cisco route, i just used csr1000v and it did not replace the home modem router it was second router at home.  Now problem was from the second subnet I ping the www.google.com then it would not return back. Found out that packet was being dropped when it is travelling back from public IP side of my home router. But if i ping from my original subnet created by the comcast modem it works. So any packet that is traveling back that are not destined for the subnet created by the comcast modem itself was being dropped. So I figured out it has something to do with setting default gateway from the public IP side of my home router which I have no control of. That is what I remember.

[deanwebb]

Tell you what, though, you learn so much from trying stuff out in your home lab. When you have a place you can do stuff and make mistakes, the ability to learn is unparalleled.