We have an ADSL connection at work, with a 3G backup.
The ISP gives us a number of fixed IPs, which are supposed to get rerouted to the 3G if the ADSL fails.
If I power down the ADSL modem, it all works fine. The router detects the loss of the WAN1 ("ethernet") service and switches over to WAN1 ("3G") which is a USB 3G adaptor. When the ADSL has been back up for 2 minutes, the 3G is shut down and it switches back to the ADSL.
We pay just £2.50/month for the 3G backup so this is pretty good.
However we have had several losses of ADSL where the 3G backup did not work. The ISP is pretty arrogant about it and blame it on our equipment. However on the last-1 time it happened (weekend as usual) I did go to the office and found that the IP had changed... this is OK for outgoing stuff but obviously any incoming connections will fail.
The ISP claims this cannot happen with their equipment because they allocate fixed IPs. But this is self evidently bollocks. The IP is allocated (DHCP) by the DHCP server in the ISP's router, and customers who pay for a fixed IP just happen to get the same IP every time. Or maybe not....
The Q is how could one possibly detect such a scenario i.e. where you have one of these cases
the ADSL connection seems OK (how the Draytek router detects the failure of WAN1 is undocumented) and the break in connectivity is further upstream (usually blamed on British Telecom) or
the ADSL connection works but the IP has been changed
The router can't be configured to periodically check an external website and check it's own IP.
We could dump this ISP but they all seem to have these problems from time to time.
You can use an external service like this one to set up monitoring on your public ip addresses...
I've seen those, but how can one use that to switch over the router to use the 3G adapter?
Especially when the router is then not accessible remotely, due to the loss of connectivity 
I imagine there are high-end solutions for this, but what's really needed (for cheap people like me) is some means whereby the router can detect that full connectivity has been lost, or the IP has changed.
I think all the Draytek routers do is they look for a loss of some kind of regular keep-alive packet which passes between the ADSL gear in the telephone exchange, and the router. All that will detect is a physical loss of the ADSL connection to the exchange.
One gets a related sort of issue with say a smartphone with the WIFI enabled and configured to connect automatically, and also 3G enabled. All the time it is picking up unencrypted WIFI access points, it won't use the 3G - because it is too stupid to actually check connectivity all the way out by e.g. checking if google.com can be accessed. And because nearly all the WIFI APs are commercial "pay" ones, nothing works...
I can get two different ISPs to give us two ADSL services, and the router will switch between the two, but (a) the criteria for the fallover won't be any more intelligent and (b) we obviously can't have the same IPs from both.
I wonder how this is solved professionally. An external monitoring service, sending an SMS to the router and switching it over?
I wonder how this is solved professionally. An external monitoring service, sending an SMS to the router and switching it over?
In professional environments they have multiple lines and use multipath routing for fault tolerance.
As for the Draytek router, I doubt it supports multipath over a 3G connection. It looks like the Draytek Vigor 2920 does support multipathing over 2 adsl lines. That should give you a better redundancy.
I spent some time at last year's CES trying to get an answer on this from various router suppliers (including Draytec) without getting a lot of sense in return. It certainly appears that some of them simply rely on the 'ADSL' level of feedback without having the means to check if the Internet is actually present. (This is the same as the 'ADSL' light on the router, as opposed to the 'Intenet' light). Some salesmen were keen to suggest that they 'ping Google' or something but their Chinese engineers were quick to contradict this without actually admitting how it really works.
The upshot was that (in 2012) none of them do what you describe. This includes specialists like Cradlepoint and I came away thinking that none of them had thought it through. Most SME's would benefit from this kind of 3G failover but it is deeply typical of the router industry not to understand it's customer's most modest objectives. We have installed some of the Draytec failover modems but I don't think there's been a case of it being useful in practice. The stupid external dongle with it's valuable SIM card is far too much of a temptation to staff for one thing.
A question is if your ISP somehow maps the 3G to your fixed IP, what IP is the ADSL router using in the meantime? How can it get back online? It's a clever idea mapping the IP to 3G though. Who is this?
When I pressed this exact issue with Draytec, they introduced their engineers and started talking about 'custom development'! This was especially in respect of my request for a large flashing light bolted to the top of the router to warn the user when it's failed over, because otherwise a typical SME won't notice until the credit on their SIM card runs out!
Some salesmen were keen to suggest that they 'ping Google' or something but their Chinese engineers were quick to contradict this without actually admitting how it really works.
It would be easy to ping some site, or the usual 4.4.4.4 etc, but that will fail to pick up the case where the ISP's "fixed IP" has been changed, which is fairly common IME when something goes wrong. Outgoing connections still work...
We have installed some of the Draytec failover modems but I don't think there's been a case of it being useful in practice.
It works well in case of a physical wire break to the exchange (perhaps a common fault in the countryside) or the failure of the ADSL modem (if it is a separate modem, which it will be with most "better" routers).
A question is if your ISP somehow maps the 3G to your fixed IP, what IP is the ADSL router using in the meantime? How can it get back online? It's a clever idea mapping the IP to 3G though. Who is this?
I don't know how it works but it works. The ISP is Andrews & Arnold. I have found ZEN to be much more reliable but we have a backup site (my house) using ZEN so I can't use them on this one.
otherwise a typical SME won't notice until the credit on their SIM card runs out!
Indeed. However A&A supply the SIM card (on the UK "3" network) for just £2.50/month on a contract. The data rate is very cheap, but as you say you need to know when it has fallen over to 3G. The only current way to tell is that everything runs a bit slower.
I chose A&A largely for the 3G backup. Unfortunately it fails to protect against most real-world ADSL outages.
Peter, did you rule out A&A's 'Firebrick' gizmo on cost grounds? Would that handle it any better?
In professional environments they have multiple lines and use multipath routing for fault tolerance.
There are some quite reasonably priced devices out there that will do this nicely on ADSL and 3G - e.g. Juniper's SRX range, but the cost in terms of of setting up and managing these devices is quite steep: either you need to become a competent network engineer, or you need to pay a good one quite a bit of money.
If you need the static IP for incoming services (websites etc), you may find it's cheaper to buy a virtualised server from somewhere like rackspace. If you need it for VPNs, then multipathing is IMO the only way to keep things running - and even then it can be a right PITA.
I've been having this discussion in Usenet with some more specialists and it looks quite difficult (to do on the cheap).
Basically if there is a break in the BT network, then BT allocate you with some random IP, so you get apparent outbound connectivity and the router can't tell the connection has gone by looking for a failure of PPPoE.
I don't want to go down the Cisco sort of thing route. I used to run some Cisco routers and the config was an absolute bastard - the biggest job creation scheme after EASA, ISO9000, ROHS and REACH... I had to use others to set them up all the time.
It turns out the Drayteks do have two ways to detect WAN1 failure for a switchover to WAN2: ARP or a ping. I was using the ARP option, which detects a fail of the ADSL modem or a break in the BT copper to the local exchange, no problem. To detect anything further out one can use the ping option, but that won't detect a change of the IP, which only affects incoming stuff (VPN, RDP, etc).
I am told that the BT-allocated random IP might have very limited DNS options, so that might work if I ping a named site which it isn't going to carry. But this is a long shot.
What I might do is use some site to test the incoming connectivity and send me an SMS on failure. It won't be free but it should be cheap.
Hosting has been suggested but frankly I have seen as much downtime from hosted virtual servers as with a PC (with a SSD) sitting at home on ADSL... until one gets to really proper commercial sites (like the one hosting EuroGA ) and they cost real money.
