Technical Backgrounder On PPPoE:3Com

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Here is short description of the problem that you can send to your equipment vendor when requesting a firmware upgrade.

When operating in PPPoA mode, the 3Com Dual Link ADSL Modem does not terminate the PPP session with your ISP. It extends the PPP session to your PC and/or router, using a non-compliant (RFC 2516) dialect of PPPoE ("PPPoE:3com"). The 3Com bundled EnterNet300 PPPoE client software terminates the PPP session and provides an Internet connection.

It's important to understand that the PPPoE session is only between PC/router and the Dual Link modem. The Dual Link modem negotiates the PPPoE Discovery Phase without ISP involvement. The link between modem and ISP is pure PPPoA. After that, the PPPoE Session Phase takes over and that is when the ISP negotiates PPP parameters with the PC (or router).

So that is why asking a router vendor to "support PPPoA" is asking the wrong question. What you need to ask is "do you support PPPoE:3com?"

The router will do the job of the EnterNet300 software, if only it could talk to the 3Com modem. So the good news is that once the router is talking with the 3Com Dual Link, you won't need any special software to connect to the Internet.

The 3Com modem talks to your ISP using PPPoA and then converts protocols to PPPoE:3com (3Com's non-standard dialect of PPPoE) in order to continue packet transmission over your local Ethernet network. This is necessary because PPPoA won't work over Ethernet.

So the goal is for your router to support the following Internet connection:

Non-Standard Dialect: PPPoE:3com

The 3Com Dual Link uses a NON-COMPLIANT version of PPPoE (per RFC 2516). The PPPoE protocol specifies 2 new values for the EtherType field in the Ethernet MAC frame. 

RFC 2516 specifies that the EtherType for "PPPoE discovery" is 0x8863, and "PPPoE session" is 0x8864.

BUT... 3Com changed these values to 0x3c12 and 0x3c13 (shame on them).

Here is a summary:

3Com bundles EnterNet300 to be used as your connection client on the PC (but private labels it as 3Com software). If you locate the enternet.ini file, you'll see these NON-COMPLIANT values being set.

I'm using a Netgear 314, and observed the exact same issue when attempting to get a Linux computer to talk directly to the 3Com Dual Link.

The 3Com simply won't respond to any RFC 2516 PPPoE packets. That means your router will simply timeout when attempting to establish a PPPoE connection with your 3Com modem.

You can test this yourself by removing the router, connecting the PC directly to the Dual Link,  and then find enternet.ini and change the two lines from above to look like:
ETYPEDISCOVERY=0x8863
ETYPEPAYLOAD=0x8864

Next, run the 3Com/EnterNet300 dialer application. I've watched the network traffic with a packet sniffer and can verify that the 3Com modem won't reply at all. After a short time the dialer application will report a problem.

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Last modified: April 26, 2001