How to change Host-Uniq tag in a PADI (PPPoE Discovery)?

felipesolis · ‎02-14-2014

Hello,

I'm going to be quick, I want to replace my ISP's router at home with a Cisco 2901. The problem is the Cisco router sends PADI but doesn't receive PADO. I checked the PADI from the ISP's router and compared it to the Cisco one, and the only difference is the "Host-Uniq" tag:

(left: ISP, right: Cisco)

Does anyone know how to change this tag?

Thanks

Peter Paluch · ‎02-14-2014

Hi Felipe,

I do not believe the Host-Uniq tag can be modified, and according to the following excerpt from RFC 2516, it does not seem it should be of any meaning to the ISP:

   0x0103 Host-Uniq

      This TAG is used by a Host to uniquely associate an Access
      Concentrator response (PADO or PADS) to a particular Host request
      (PADI or PADR).  The TAG_VALUE is binary data of any value and
      length that the Host chooses.  It is not interpreted by the Access
      Concentrator.  The Host MAY include a Host-Uniq TAG in a PADI or
      PADR.  If the Access Concentrator receives this TAG, it MUST
      include the TAG unmodified in the associated PADO or PADS
      response.

Would it perhaps be possible to call your ISP and ask about this? I do not really think the ISP should be discriminating PPPoE clients based on Host-Uniq field. As you have changed the MAC address, this is really a curious issue as the ISP should indeed not see a difference between your current router and the Cisco.

Best regards,

Peter

sonorman · ‎11-24-2022

I had exactly the same issue. The reason I was not getting any PADOs in response was because of the missing VLAN tag (in my case, VLAN 2). I ran a packet capture on the link between my Router WAN and my Cable Modem LAN, but the missing 802.1q tag was not obvious because it turns out my laptop was stripping the tags from all inbound frames before the Wireshark driver was seeing them. A few registry tweaks later (to stop the VLAN tag stripping - http://forum.gns3.net/topic7559.html), and I saw the tags on the old working ISP router. Once I knew that was the issue, I changed the config to add the VLAN tag, et viola! PPPoE established.