Router Addendum: Dynalink RTA1320

By | November 15, 2006

I forgot to mention the other piece of my epic ADSL Bittorrent setup: my modem.

I use a weeny little Dynalink RTA1320.  At first glance it probably looks a bit small and pathetic, but don’t be fooled.  It has a full-blown ADSL2+ compatible modem in it, and quite a nice router system, but I’m cold and hard and don’t care about the router system.  I only care that it supports PPP half bridge mode.

I’m gonna get a bit nerdy on you here.  Most sane countries use PPPoE (Point to Point Protocol over Ethernet) to authenticate their ADSL clients, meaning any device that can carry an ethernet signal can carry the encapsulated authentication packets.  Good idea.  This means you can set up an ADSL modem in raw bridge mode, and have your router pass the authentication information down the wire.  The modem only has to deal with modeming (modulating and demodulating)* the ADSL signal.

In silly old New Zealand, we use PPPoA (the A is for ATM, which is does not stand for Automated Teller Machine).  I believe this is because large parts of our backend infrastructure are, ummm, crap.  This causes issues because the modem has to setup the PPP connection (because routers without modems can’t talk ATM), and in most cases this means modems also have to be routers, and combined modem-routers are almost universally shite.

Enter our saviour: PPP half bridge mode.  If I understand correctly (and the chances of that are relatively low), then the modem/router running in PPP half bridge mode acts as follows:

  1. The modem sets up the PPPoA connection using the authentication information you give it.
  2. It then it grabs the MAC address from the internal connection (in my case the ‘internet’ port of the WRT54GL) and exposes it to the intertr0n. 
  3. Any packets received on that MAC address are forwarded without further consideration to the internal connection. 

So once the connection is established, the ‘true’ router effectively sees all the internet traffic, and the crappy modem-router doesn’t have to do any hard work dealing with NAT or firewalling.  Sweet!

So in summary, Ben’s recipe for ADSL Bittorrent successTM:

  • Dynalink RTA1320 in half-bridge mode
  • Linksys WRT54GL running DD-WRT, with config changes to NAT behaviour

*So why isn’t it a moddem then?  Eh?

[tags]bittorrent, networking, personal[/tags]

6 thoughts on “Router Addendum: Dynalink RTA1320

  1. Johann Lo

    Hey mate I am running pretty much exactly that setup (with a billion modem not Dynalink) and it works a treat.

    Though like you I am not 100% sure what ‘half bridge’ really is, my understanding is that it handles the layer 2 but passes layer 3 (ie IP) straight through.

    However I am sure above is not 100% technically correct as you still have to enter the username/password on your router i.e. the PPP authentication, which would mean its still layer 2???

    This is really annoying as I am a networking guy so I WANT OT KNOW, unfortunately in the corporate world you don’t see such setups as you’re dealing with proper Cisco gear all the time where the ADSL goes straight into the ADSL WIC of the router. I might do some tinkering with hooking up a Cisco router to the half bridged modem and seeing how to configure it to work might shed some light.

    Reply
  2. Roddyboy

    Hi,

    I realize this is an old article, but I’m trying to achieve a fairly similar set-up to yours, I’m using an Linksys WRT54G with an old DSE modem. I’ve enabled DMZ with the host IP being my Linksys router, and enabled half bridge mode. Do I enable or disable NAT on the modem? Or how can this be achieved using DD-WRT?

    When I try using it, the PPP connection seems to continuously drop on the modem…

    Thanks,

    Roddy.

    Reply
  3. Ben Post author

    You want NAT turned off, and “PPP IP Extension” or “PPP Half Bridge” turned on.

    If you have a setting for “Awlays on” or “Dial on demand” under the connection settings, choose “Always on”.

    If you’ve got half-bridge on, then you don’t need the DMZ setting, because EVERYTHING will be forwarded to the router.

    Reply
  4. Roddyboy

    Hi,

    I realize this is an old article, but I’m trying to achieve a fairly similar set-up to yours, I’m using an Linksys WRT54G with an old DSE modem. I’ve enabled DMZ with the host IP being my Linksys router, and enabled half bridge mode. Do I enable or disable NAT on the modem? Or how can this be achieved using DD-WRT?

    When I try using it, the PPP connection seems to continuously drop on the modem…

    Thanks,

    Roddy.

    Reply
  5. Peter

    Half bridge is unfortuantely a hack. Sometimes called dhcp spoofing, and gives eratic results depending on what you have downstream.

    My 3c is:
    -if using pppoa when you enable half bridge, check that it hasnt altered (wrongly) the encapsulation to pppoe
    -Check out this: Draytek Vigor 110
    – see these links for more info.

    http://whirlpool.net.au/forum-replies-archive.cfm/806160.html
    http://phirate.exorsus.net/wiki/doku.php?id=nz_dsl_modem_networking
    http://www.speedtouch.net.nz/forum/topic.asp?TOPIC_ID=1002
    http://forums.whirlpool.net.au/forum-replies-archive.cfm/941840.html

    Reply
  6. SneakyWho_am_i

    I use the words “modem” and “router” interchangably, because the unit contains both and I am never quite sure what I really think it is exactly. beware of this strange way of referring to the box, if you read my comment.

    After what feels like a year of high speed, half-bridged goodness, my modem has started to die, and it’s thrown a little bit of light on how it works. For anybody strolling past who wonders how the half bridged stuff works, it worked for me thus:

    – modem powers up and connects to internet
    – computer powers up and requests an ip address
    – modem offers 192.168.1.2
    – computer accepts 192.168.1.2 (but it expires instantly)
    – computer offers external ip (125.236.xxx.xxx or whatever)
    – computer accepts external ip

    now you still have to renew the lease on the address periodically, but you don’t request it from your isp’s dhcp server – no, you request it from your modem/router thingy.

    beyond that i never thought to look at whether the modem is invisible (a la squid) or a proper gateway or what. it’d be interesting to find out from someone who still has a working one.

    My first problem is that every once in a while the modem thing resets.
    My second problem is that sometimes the lease expires and I can’t renew it. My third problem is that normally if I submit a form in the web interface (changing settings), it can reset or even factory reset the router, without processing my input.

    I’ve been unable to get half-bridged mode working without dhcp. The MAC addresses must be gathered through the DHCP conversation, then, and it mustn’t be able to do it by ip, only MAC. I tried with the router as a gateway, and asking directly for the isp’s gateway.

    My problems were alleviated by keeping the router COOL and well VENTILATED, and not loading it up too much.
    Right now because of the DHCP problems, I’m using it with DMZ and static ip.
    YUCK.
    Every time I try to ring in through the www subdomain, I get a 401 from the router. Why? I don’t get it! It’s no great concern. DMZ is totally unsuitable cause it means DNS hacking anyway. I guess I just need to go buy a new modem. Maybe I’ll get a big expensive one so I can’t afford food for seven years.
    Some of those expensive new ones come with switches instead of mere routers. I like that.

    My RTA1320 has served me SOOOOOO well It was a beautiful and power little thing, and I’m gonna miss it :)

    Reply

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