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soadlink

join:2002-11-17
USA

Cable modems - CPE MAC address vs HFC MAC Address

Hello,

I was just curious in how the typical Cable ISP uses the various MAC addresses given by the modem. For example, on my SB5100 there are a number of different MAC addresses listed on the config page (192.168.100.1). There is the HFC mac address which is the actual address burned into the modem, and then there are various CPE MAC Addresses that are learned by the modem (various customer devices that may have been connected to the modem since it was last powered on, such as my router.)

I am curious about how it's 'supposed' to work... because all I need to do to change my public IP address is 'spoof' the mac address of the CPE device, and I am almost instantly given a new IP. Wouldn't the ISP wan't to authenticate and assign the IP address based upon the HFC MAC address (the actual modem's MAC address), and not the CPE devices? How is the HFC MAC address actually used... or is it used at all? I have a feeling I am a little ignorant to how it all works, and there is a reason for the HFC Mac, so I would like to know

This topic came up on another site when we were talking about changing public IP addresses. I explained that I am able to change my IP address on the fly just by spoofing the MAC address of the WAN interface in my m0n0wall box. But another user was confused and brought up the question about why my ISP isn't assigning/authenticating based on the HFC Mac address instead. I thought this was a good question, and it came across my mind before, but I had never given it any deep thought. So hopefully someone can explain it in detail. Thanks!



UglyDork
Premium
join:2002-01-09
Buffalo, NY

Every device on a network gets an IP address.

The HFC MAC address relates to the modems IP address. The CPE MAC address relates to the device (computer, router, etc.) that is connected to the modem.



soadlink

join:2002-11-17
USA

said by UglyDork:

Every device on a network gets an IP address.

The HFC MAC address relates to the modems IP address. The CPE MAC address relates to the device (computer, router, etc.) that is connected to the modem.
What is the purpose of the HFC's mac address in the whole process of obtaining an IP address if the CPE's mac address is the one that is taken into consideration for IP address determination? Don't cable ISPs authenticate cable modems based on the HFC mac address? This is where I start to get confused. I know what each of the MAC addresses relate to, but I don't know how they are actually used within the cable network. That is what I was curious about .

Maybe if you could give a step-by-step of what is actually happening from when the modem powers on to when an IP address is actually assigned to the CPE device, and how the HFC and CPE mac's were used in the process. Thanks for the help so far


jsz0
Premium
join:2008-01-23
Jewett City, CT

said by soadlink:

said by UglyDork:

Every device on a network gets an IP address.

The HFC MAC address relates to the modems IP address. The CPE MAC address relates to the device (computer, router, etc.) that is connected to the modem.
What is the purpose of the HFC's mac address in the whole process of obtaining an IP address if the CPE's mac address is the one that is taken into consideration for IP address determination? Don't cable ISPs authenticate cable modems based on the HFC mac address? This is where I start to get confused. I know what each of the MAC addresses relate to, but I don't know how they are actually used within the cable network. That is what I was curious about .
A modem has two distinct interfaces: the cable interface and the ethernet interface. The HFC MAC is on the cable/DOCSIS side and used to determine what bits are for your modem and which ones are not. The CPE MAC is the same exact thing except on the Ethernet side.

As far as how an IP gets assigned and the relationship to the CPE MAC; the CMTS relays the CPE MAC to the DHCP server and tells the DHCP server that is requesting an IP. The DHCP server will check to see if you have a static IP set to that CPE MAC, if not it will give you an address out of its DHCP pool but first it looks in its lease database to see if it has already allocated an IP for this CPE MAC -- if so it will send the associated IP out as a DHCP OFFER -- which gets sent out the Ethernet side of your modem, the CPE device gets the offer and assigns itself all the lease information. From there you have functional IP networking from the internet to your CPE.


soadlink

join:2002-11-17
USA

said by jsz0:

A modem has two distinct interfaces: the cable interface and the ethernet interface. The HFC MAC is on the cable/DOCSIS side and used to determine what bits are for your modem and which ones are not. The CPE MAC is the same exact thing except on the Ethernet side.

As far as how an IP gets assigned and the relationship to the CPE MAC; the CMTS relays the CPE MAC to the DHCP server and tells the DHCP server that is requesting an IP. The DHCP server will check to see if you have a static IP set to that CPE MAC, if not it will give you an address out of its DHCP pool but first it looks in its lease database to see if it has already allocated an IP for this CPE MAC -- if so it will send the associated IP out as a DHCP OFFER -- which gets sent out the Ethernet side of your modem, the CPE device gets the offer and assigns itself all the lease information. From there you have functional IP networking from the internet to your CPE.
Alright that makes sense that the CMTS relays the DHCP requests from the cpe to the dhcp server. So I would assume from all this (and doing a little googling on my own), that the typical cable modem boot up goes like so. And please, feel free to correct me if I am mistaken anywhere:

1) Modem powers on, looks for the CMTS, downloads the modem configuration file from the isp's TFTP server.
2) Depending upon the HFC Mac, the modem may end up downloading a particular configuration file (residential customers, business customers, etc.), but there may only be 1 too. It depends on what your ISP has decided. If a modem's HFC mac is allowed to talk on the network, the config file will contain the entry "NetworkAccess 1;" in it. If the value is 0, no further communication can take place (such as DHCP requests) on the ISPs network. However, some ISPs may allow all cable modems access (even non customers)... but simply set an additional setting called "MaxCPE 0;" in a particular config file so that non/blocked customers can talk to the ISPs internal network, but cannot get public IP addresses.
3) If the cable modem has passes authentication via it's HFC MAC (it received a config file with MaxCPE >= 1 and NetworkAccess = 1), then it can pass on DHCP requests for it's CPE devices.

So that's how I think it works, and please correct any mistakes, or feel free to add anything that I may have missed. To sum it all up:

- HFC mac determines whether or not your modem gets access to the isps network. It can also determine which config file your modem will download.
- CPE mac is sent to the dhcp server for a public ip address. CPE mac may also be tied to a static IP if your ISP has it set up like that.

It was hard to find the really technical information about how it works. I couldn't find any official documentation, so much of the information comes from googled websites, forums, etc. I think that is because CableLabs has an NDA with particular vendors on some of the very technical aspects of DOCSIS. I could have been looking in the wrong place too


UglyDork
Premium
join:2002-01-09
Buffalo, NY

When I worked at Adelphia I was told that this process is called 'DORA'.

Discover
Offer
Request
Acknowledge

»www.google.com/search?q=DORA+DHC···-us&sa=2



soadlink

join:2002-11-17
USA

said by UglyDork:

When I worked at Adelphia I was told that this process is called 'DORA'.

Discover
Offer
Request
Acknowledge

»www.google.com/search?q=DORA+DHC···-us&sa=2
Yea, but that's just the DHCP process. I'm looking at more than just DHCP here.