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For my new Telstra cable Internet connection I was supplied with a Netgear CG3100D-2 modem/router and I wanted to use it with my existing Cisco 2821 router. The following steps and basic Cisco config will allow you to establish a working Internet connection.

The same config should be compatible with 2921, 2941, 1921 and 1941 routers. For 1821 and 1841 routers you'll need to substitute gi0/0 and gi0/1 with fe0/0 and fe0/1 as those models only had 100mbps network interfaces.

Once you have the basic connection up and running you'll also most likely want to apply your own port forwarding rules and access group to the external interface. N.B. When applying an access group to the external interface you'll need to accept UDP responses from port 67 to allow the DHCP response to be accepted from Telstra.

With my customised NAT and firewall rules applied I still achieved 36mbps/1mbps speed test results on Telstra's speed test which was the same as when tested directly through the Netgear.

I am assuming you have already know how and have configured gi0/1 on the Cisco with an appropriate private IP so that you can telnet to it from your PC. My base config will require customisation for your environment in terms of subnets and internal DHCP.

1. Login to the Netgear web GUI
2. Open Wireless Settings from the left menu and untick Allow Broadcast of Name and Enable Wireless Access Point then click Apply
3. Open Wi-fi Multimedia from the left menu and set all dropdown boxes to Off then click Apply
4. Open NAT Mode from the left menu and select NAT - Disable (Bridge Mode) then click Apply. The router will reboot.
5. Connect the Ethernet 1 port of the Netgear to gi0/0 on your Cisco.
6. Connect your PC to gi0/1 on the Cisco and login.
7. Apply the base config. You'll need to customise the passwords and internal IP/DHCP subnets to match your environment. Also note that I've used Qld name servers so if you're in a different state you'll need to substitute those IPs with your local DNS IPs.
   

   no service pad
   service tcp-keepalives-in
   service tcp-keepalives-out
   service timestamps debug datetime msec localtime show-timezone
   service timestamps log datetime msec localtime show-timezone
   service password-encryption
   service internal
   service sequence-numbers
   !
   hostname router
   !
   boot-start-marker
   boot-end-marker
   !
   enable secret yourpass
   !
   no aaa new-model
   clock timezone AEST 10
   !
   resource policy
   !
   no ip subnet-zero
   no ip source-route
   ip tcp selective-ack
   ip tcp timestamp
   !
   ip cef
   no ip dhcp use vrf connected
   ip dhcp excluded-address 192.168.0.1 192.168.0.50
   ip dhcp excluded-address 192.168.0.200 192.168.0.254
   !
   ip dhcp pool dhcppool
      network 192.168.0.0 255.255.255.0
      default-router 192.168.0.254
      domain-name yourdomain.local
      dns-server 61.9.211.1 61.9.211.33
      update arp
   !
   ip name-server 61.9.211.1 61.9.211.33
   !
   interface GigabitEthernet0/0
    ip address dhcp
    ip nat outside
    duplex auto
    speed auto
    no cdp enable
   !
   interface GigabitEthernet0/1
    ip address 192.168.0.254 255.255.255.0
    ip nat inside
    duplex auto
    speed auto
   !
   ip classless
   no ip forward-protocol nd
   !
   no ip http server
   no ip http secure-server
   ip nat inside source list 1 interface GigabitEthernet0/0 overload
   !
   access-list 1 remark NAT
   access-list 1 permit 192.168.0.0 0.0.0.255
   access-list 2 remark Telnet management
   access-list 2 permit 192.168.0.0 0.0.0.255
   !
   control-plane
   !
   line con 0
    transport output all
   line aux 0
    transport output all
   line vty 0 4
    access-class 2 in
    password yourpass
    login
    transport input all
    transport output all
   !
   scheduler max-task-time 5000
   scheduler allocate 20000 1000
   sntp server au.pool.ntp.org
   

8. Connect the Gi0/1 interface to your switch and also connect your PC to the switch and you should have Internet access.
9. You should also be able to access the Netgear web GUI to adjust some basic settings.