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Ping CMTS from Cable Modem Ethernet Secondary Address

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Ping CMTS from Cable Modem Ethernet Secondary Address

I have a basic DOCSIS 2 CISCO ubr. I have modems that come online and everything works properly.
The address of CMTS is is a TFTP server and DHCP server at
The primary address of the cable modem is 10.10.4.x. The secondary address of the cable modem is 10.10.8.x

When an ethernet cable is connected to one of the cable, the PC gets an IP adress through DHCP and the modem will be at
I want to be able to access all the devices in the 10.10.0.x networks( ie the CMTS Server, TFTP server). Currently this is not possible.

I have this problem with Gateway modems that have 4 ethernet ports or non-Gateways. How do i configure it so that this is possible?
I want to measure upstream and downstream speed. That is the reason I need this.

Do i need to make configuration changes on CNR, CMTS? Please let me know and help me out.


Ping CMTS from Cable Modem Ethernet Secondary Address

John -

I believe that's not possible. Check out, "Assigning IP Addresses" on the net.


I have read your description a few times, but your situation still isn't super clear to me

From what I can tell, your CMTS configuration should look something like this :

interface fast0
! local LAN
ip address

interface bundle1
! CPE addresses :
ip address secondary
! CM addresses :
ip address
cable dhcp-giaddr policy
cable helper-address

Then the CNR should be configured to give 10.10.4.x IPs to the CMs, and 10.10.8.x IPs to the CPEs

If you use a gateway, it's WAN will be 10.10.8.x, and it do NAT and usually hand out 192.168.x.x IPs to the LAN.

If you use a basic CM :
* the customer's router will be assigned a 10.10.8.x IP. The router will do NAT and typically give 192.168.x.x IPs to the LAN.
* if they plug a PC direct to cable modem, the PC will get a 10.10.8.x IP.

All of the above should be OK (as long as the CMTS or router upstream of it is doing NAT on the 10.10.8.x range)

Now you say you want to measure upstream/downstream speed. Can you provide some more detail? Are you trying to do speedtests from a computer connected to a cable modem? If so you should be able to setup a HTTP/FTP server on the 10.10.0.x range. Perhaps install the Ookla Speedtest Mini app on this web server. Give it a DNS name like and then customers should be able to browser to that to check the up and down speeds.


yes that is correct. Everything you say is correct.

Now for the Routers will do Nat give I.P's 192.168.x.x to the LAN and PC. For whatever reason, I cannot ping the or IP addresses from the PC.
Is there a echo ping reply option that I have to turn on the CMTS or something?

Is there any other additional configuration that I have to do so that I can ping the or If i I setup a HTTP/FTP server on 10.10.0.x range, i might not be able to access it seeing that I cannot even ping the 10.10.0.x network right now.

It should be possible to ping

It should be possible to ping the 10.10.0.x range from the 10.10.8.x or 192.168.x.x PC

Do you have any ACLs attached to the bundle1 or fast0 on the CMTS?

Is the CMTS doing NAT for the CPE ranges, or is there an upstream router doing the NAT? If the CMTS is doing the NAT you might need to make a no-nat rule for 10.10.8.x <-> 10.10.0.x

Keep in mind that some routers/gateways block all RFC-1918 private IP space from traversing their NAT/firewall. To protect yourself from being bitten by such a problem, do your testing from a PC connected directly to CM to start with. And once you have got that sorted, then move on to trying to do it from a PC connected to gateway.


I dont have any ACL's

You might have hit jackpot there - some routers/gateways block all RFC-1918 private IP space from traversing their NAT/firewall.
Now I when I look up , i see the ranges that are reserved for private ips.

What IP's are public IP's? I will recreate my IP range to those IP's immediately and do a test. I will do some research on this also.

I believe its the router doing the NAT, not the CMTS. How do I find out?


had the same problem

I had the same problem, if your client is a regular cable modem like DPC2100R2 or SB5102 you will be able to ping the CMTS ip from the ethernet port of the CM. However the 4 port wireless modems such as SMC8014WG have a build in firewall that blocks traffic from the ethernet port to the RF port. There is no way around that.

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