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New Member

C1721 to Motorolla SB4100 Cable Modem

I have not worked with cable modems before and am not sure how the configuration should be.

I have a Cisco 1721 with FastEthernet0 to the LAN (NAT inside), Ethernet0 to the cable modem (NAT outside), & serial0 to a 56Kbps Extranet (Private IP). The clients use DHCP for their configs. Serial0 and FastEthernet0 are configured and working fine. The Motorolla docs say the connection needs to be DHCP client, so I have Ethernet0 config with "ip address dhcp". So my questions are:

1. How do I config the default gateway to be Ethernet0 (ip default-network?)

2. How does NAT need to be configed to work with a dynamic address?

3. If Ethernet0 gets its info from DHCP can I pass the DNS info to the clients?

4. Am I even on the right track?

I've tried to find info on this, but all the stuff I've found on Cisco is using their cable modems.

  • Other Network Infrastructure Subjects
Cisco Employee

Re: C1721 to Motorolla SB4100 Cable Modem

First, i assume that the cable isp will assign the dyncmic public ip to the Ethernet 0 interface of 1721.

1)You can enter command "ip route ethernet 0" on 1721 for a default route to internet

2)YES, you can configure NAT using dynamic address. That way the dynamic ip address allocated to ethernet0 of 1721 will be natted using overload. Actually its PAT. Here is the way

int ethernet 0

ip address dhcp

ip nat outside


Interface fastethernet 0

ip address

ip nat inside


ip nat inside source list 1 interface ethernet 0 overload


access-list 1 permit

with above config, all the network will be natted (PAT) to public ip address of eth0 interface

3) need to configure any public dns manually on 1721 or on the lcients itself.

4) are on the right track..

New Member

Re: C1721 to Motorolla SB4100 Cable Modem


Yes the cable ISP will be providing the public IP to the ethernet0 interface. I've entered 'ip address dhcp' under this interface. However, I am not sure it is receiving an address. How do I check? I've tried all the 'sh ip ...' that I can think of and have entered 'sh int e0' cannot tell if it has an address. I see the packet counters increasing on the interface but am unable to ping, traceroute, etc out the interface.

Thanks again.

Cisco Employee

Re: C1721 to Motorolla SB4100 Cable Modem

You should be able to see the ip address assigned to ethernet interface using "sh interface eth0". What ip address you see assigned to that?

You can also use "sh ip interfaace brief"..Pl. post it here.

New Member

Re: C1721 to Motorolla SB4100 Cable Modem

When I enter 'sh ip int brief' the Ethernet 0 interface shows DHCP unassigned. If the interface is not getting its address from the ISP then I believe I have a problem with the ISP at this point. Thanks for your help.


Re: C1721 to Motorolla SB4100 Cable Modem

Please let me know how this works out. We are having the same issues with out cable modem on Eth0 and NAT addressing. Any configuration documentation that you have woudl be great.

Karl Solid


Re: C1721 to Motorolla SB4100 Cable Modem