On 24 August 2012 07:40, <linux@???> wrote:
> Hi all
>
> This is a bit convoluted and a bit techie but please bear with me. My question boils down to: Can a BT vision box find the BT Home hub it needs if there is another router on the same LAN and that other router is not part of the btinternet network?
>
> For various reasons, I've been trying to build a home network that has a backup if one of our broadband connections fails. As such, we have two phone lines and a different broadband connection down each - therefore two routers: one btinternet, one zen.
>
> A mate gave me a free BT vision box yesterday and said all I needed to do was plug it in and, providing I have bt total broadband (which I do), all would be well - not so. When I plugged it all in, it didn't work (error code C03 fwiw which is some sort of connection problem from googling) but my first suspicion was that it had picked up the zen router not the bt home hub - which was exactly what had happened. Having quickly separated the two, I got it
> connected directly to the bt home hub and indeed it picked up its IP address from there - but still wouldn't go any further (error C03 again). It turns out that I need to subscribe to BT Vision (so my mate was wrong!) which I don't mind at £4 per month but without (presumably? disabling dhcp server on the zen router) I can't force the bt vision+ box to "find" the bt home hub as it doesn't seem
> to have a manual IP configuration. Unless, that is, the bt vision+ box is clever enough to find the bt home hub in another way. The fact that my first test last night saw it pickup an ip address from my zen router makes me think the vision box expects only one router on the network and
> that router has to be the bt home hub.
>
> Anybody know anything on this one?
>
> In principle, I don't mind disbaling dhcp on the zen router - I think I know how to get most devices to have static ip addresses and gateways (ps3, ipad, laptops) but not sure about things like kindles. The minor issue might be that zen is Fibre so disabling dhcp on the zen router would mean the only dhcp server would be on the bt homehub - which is a slower connection (and I'm not paying for
> two fibre connections - you can take single point of failure too far!!).
>
> Sorry I've waffled a bit - couldn't really see how to explain any more quickly.
>
It would help a bit more if you understood how IP networks work.
One way to approach this is to ask the question:
1) How does a IP packet get from the BT vision box to the Internet?
Once you understand that, you will have the answer to your question.
The steps towards understanding it is understanding protocols like
DHCP, ARP, and understand what the machine does with routing tables.
Summary:
The setup you have is not going to work as is.
1) You have 2 DHCP servers. This is unwise on modern Switched networks
because one DHCP server will not properly detect the other one so if
they give out overlapping IP address ranges, problems will happen.
2) The BT router will give out an IP address and a default gateway in
DHCP. That default gateway will be the BT router.
3) The Zen router will give out an IP address and a default gateway in
DHCP, That default gateway will be the Zen Router.
4) So, depending on which router responds first, we determine which
route to the internet the packets take. If the DHCP came from BT
router, all packets will go via the BT Router, if the DHCP came from
the Zen Router, all packets will go via the Zen router.
Solution:
1) Implement your own more advanced DHCP server, using something like
ISC-DHCP.-SERVER.
You can then configure it to give out different IP addresses and
different default gateways depending on the MAC address of the
requesting device.
So, then if the BT Vision box request DHCP, it would give the BT
Router as the default gateway, if any other device requested DHCP, it
would give the Zen router as the default gateway.
Notes:
If you actually want some sort of load sharing/redundancy between the
two ADSL links, you should put a load sharing box in front of them.
As a short test, switch off the Zen router and the BT Vision box.
Switch the BT Vision box back on, get it working, and then switch the
Zen router back on.
It should work for a bit, if it stops working, switch the Zen router
off again and repeat.
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