AM300 PPPoA Half Bridge

Discussion in 'Other Linksys Equipment' started by mattsin, Mar 11, 2007.

  1. mattsin

    mattsin LI Guru Member

    Linksys New Zealand has released firmware for the AM300 modem to enable PPPoA half bridge support.

    I have tried using version 1.19 in hafl bridge mode with the Linksys RV082 VPN router and have found that the AM300 modem is passing a external subnet mask of to the RV082 router, whereas the subnet mask that the AM300 is receiving from my ISP is

    Because the full subnet mask is being passed back to the RV082, the RV082 is reporting destination unreachable for any external IP that I try to access.

    Has anyone else used the AM300 in half bridge mode and got it working?
  2. mstombs

    mstombs Network Guru Member

    I don't have an AM300 or RV082, but I do have an adsl modem running D-links version of PPPOA half-bridge 'zipb'. This also passes the mask of, it also gives the WAN IP address as the gateway and short 30 second dhcp leases. I believe it does this because the real gateway (at the other end of the ppp tunnel) may be in a very different subnet and some systems can't cope with this. This mask tries to establish a 1-1 link from modem to router.

    My D-Link zipb has a bug in that it only passes the primay DNS server by dhcp, not the secondary. I have also seen some source for US Robotics Broadcom variant of half bridge 'IP extension' which does the same. Is the AM300 Broadcom chip based?
  3. mattsin

    mattsin LI Guru Member

    I'm not sure what chip the AM300 is based on.

    I can understand a subnet mask of if the gateway is the same as the WAN IP, but in this case with the AM300, the gateway IP being passed back from the modem to the router is different to the WAN IP being passed back. This will mean the router will never send anything to its gateway coz of the full subnet mask, would it not?
  4. mstombs

    mstombs Network Guru Member

    I have to admit I don't know enough tcp/ip and ip routing to understand what is going on. I would have expected the modem to act as a transparent bridge, looking like the ISP gateway from the router side, and like the router from the ppp ISP side.

    I have so far failed to manually setup up transparent bridging rules in my adsl modem (that likes to be a router). I have found that windows refuses to accept manually entered settings that mimic the configuration that D-link zipb uses!

    My understanding of the net mask is that packets are sent to the external gateway if the masked part of the target address is different from the source. Therefore in my case everything should be routed to the gateway which is back to itself! -Fortunately this is not how it works!
  5. mattsin

    mattsin LI Guru Member

    Have got the PPPoA half bridge working no problem now after installing the latest frimware from Cisco support (v 1.19.00 T01). I suggest anyone else having problems with the AM300 should update to this firmware first.

    This firmware can't yet be downloaded from their site but can be downloaded from this forum:

  6. mstombs

    mstombs Network Guru Member

    Is the AM300 opensource? - I'd really like to see how they do this! With my D-Link zipb the full netmask makes my client PC/Router think the whole internet is local so every IP address connected to gets an entry in the 'arp' table. The PC/Router 'arps for everything' the modem must answer every broadcast with 'send it to me', all seems pretty inefficient to me - but there must be a good reason why!
  7. mattsin

    mattsin LI Guru Member

    I'm not sure if the AM300 is open source. Theres some more info here about what chipset etc its running.

    I noticed after the lastest firmware update, the AM300 no longer returns a full subnet mask, instead its returning so maybe its working in a different way to the D-Link zipb.
  8. mstombs

    mstombs Network Guru Member

    I've looked at the binary - its not open source it uses

    realtime OS (SOHO.BIN) and BRN Boot Loader from the Broad Net Technology, In

    I'm really encourage by the change in the subnet mask - it makes much more sense if the gateway is in the same network defined by subnet - I will try that this weekend on my ADSL2MUE
  9. creeping

    creeping Guest

  10. mstombs

    mstombs Network Guru Member

    I did finally understand this and can emulate most of the half-bridge modes by script in my modem. I also found the problem with my Linux router when windows PC is fine

    The script in there works fine with Hyperwrt Thibor and Tomato. I've seen on my Fonera that Openwrt and dd-wrt default installs also need help.

    You need to either do the equivalent of the 2 route commands, or 'spoof' the netmask so that the router sees the gateway in its local lan defined by IP address and netmask, how in a WRV200 I don't know!
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