The tale of a single MAC

Ian Henderson ianh at ianh.net.au
Sun Jan 2 00:00:49 CST 2011


On 02/01/2011, at 2:33 PM, Graham Wooden wrote:

> I encountered an interesting issue today and I found it so bizarre – so I
> thought I would share it.

Had a fun one with D-Link ADSL modems a few years ago. The MAC address used to source PPPoE frames from the ADSL interface was the same in a batch of modems. This wasn't a problem when using our wholesaler's ATM based DSLAMs, but when we moved users to our own Ethernet based systems, we ran into some fun. 

The DSLAMs were configured to rewrite the user MAC address into a constructed address including their DSLAM port and PVC details toward the BRAS. Even though this rewrite was occurring correctly, within a single DSLAM unit two users with the same real MAC address would have five minutes of connectivity each, alternating between the two ports.

Rgds,



- I.



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