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I've got 2 IP over ADSL boxes (Orange & OVH) and 1 TP-LINK router to manage the possible failover of one link. When the Ethernet link between the primary box falls or when the bow itself is down, the router manages correctly the backup to the other wan link.

LAN backup illustration

But, when the WAN link of the primary box is down, the router doesn't see the error and still try to send on the primary WAN line.

WAN backup illustration

How can I solve this issue ?

... I've updated the links. Images can be viewed now, I hope...

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  • Don't use dropbox for img hosting. Getting to the actual image is a serious pain
    – Ricky
    Oct 10, 2014 at 17:02
  • Cannot see the attached diagrams, could you repost the pictures and also the router models (if not already described in the diagrams).
    – user4565
    Oct 10, 2014 at 22:22
  • Does the router have two WAN interfaces? If not, how is your router connected to the modems?
    – user2964971
    Oct 13, 2014 at 7:52
  • Did any answer help you? if so, you should accept the answer so that the question doesn't keep popping up forever, looking for an answer. Alternatively, you could provide and accept your own answer.
    – Ron Maupin ♦
    Aug 10, 2017 at 15:50

2 Answers 2

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Currently, your router monitors WAN connections only on layer 1, i.e. if a link is up or down. In order to react to a broken WAN connection, your router should monitor on a higher layer.

There are 3 approaches:

a) You use a router with two interface directly conntected to the splitter or telephone jack. Your router could still be monitoring a WAN link on layer 1. But your router would dial up via PPPoA. You could monitor both the line and the registration of your router by each ISP's RADIUS or TACACS server. Either your DSL line is run by a protocol such as ADSL2+ and authentication is done by user/password or by a protocol such as VDSL2 and authentication is done by modem MAC address. In case of user/password all you need are login credentials. In case of MAC address you have to personally negiotiate the registration of your router with both ISPs.

b) Your router dials up via PPPoE without the need of using new WAN interfaces. This would allow you to monitor each WAN connection on layer 2 which is sufficient enough. Your router would become the gateway and you could reduce the modems to managing DSL lines. This requires both modems to run a firmware supporting dialup by another device.

c) Your router monitors by probing a host on the WAN. Usually, there is at least one core router. which replies to ICMP Echo. Try traceroute -I 8.8.8.8 with both connections. It changes the Type of Service field to request Echo-Reply messages from each hop during the trace. You should at least find one core router for each ISP which you can probe on a regular basis. Then, you have to reduce the TTL of that ping to prevent reaching that destination on detours when the monitored WAN link fails.

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  • Thanks for your answer ! The third solution should be the right one for us. But, I would like to find a router able to manage this alone. The solution that you describe, if my understanding is correct, requires a monitoring outside the router and, if possible, a re-configuration of the router in case of problem. But my router can't be configured through SNMP for example, but only through its own managemetn interface... I guess that some routers are able to manage such monitoring ?
    – pat_dmllr
    Oct 13, 2014 at 13:40
  • I agree. What you need is a monitor and a trigger. Solution c) requires the router to have a utility which does send ICMP Echo permanently and allows to set the packet interval and the Time to Live field. Typically those built-in ping utilities only serve somebody watching the whole process. Can you access the router via CLI? If so, how about setting up a low-cost, low-power device such as a Raspberry Pi? You could make use of GNU/Linux utilities and script a response to the ICMP trigger. Remote administration would not be an issue anymore either (advanced OpenSSH configuration recommended).
    – user2964971
    Oct 13, 2014 at 14:02
  • Finally, the solution suggested by cpt_fink below is the right one ! The TP-link ER5120 router provides the same features as the 6120: online detection (manual mode => ping for example). It works for the backup, not for returning to the primary line when it comes on again, yet, but we're working on this point.
    – pat_dmllr
    Oct 15, 2014 at 7:20
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In general you need to use some type of probe/monitoring to detect the failure of any path past the physical layer of the device you are configuring.

Also - http://www.tp-link.com/en/article/?faqid=529 - see note #2.

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  • Yes, I've already configured the backup on the router. The problem is when the wan link falls down after the box...
    – pat_dmllr
    Oct 13, 2014 at 9:37
  • Thanks for this answer. This is the right way for us. Just have to find how to manage the coming back to the primary line after a backup (when it becomes on again).
    – pat_dmllr
    Oct 15, 2014 at 7:21
  • From the doc it should auto-recover, but I don't have any actual experience with TP-Link equipment. You could try their support and see if they have an answer, otherwise you can see if they will let you file a bug report/feature request
    – cpt_fink
    Oct 17, 2014 at 0:30
  • No, it doesn't auto-recover. We have already tested that. Unfortunately, their support seams to be mute... We'll try by ourselves. However, the major issue was the first backup to the second line. This remaining question is not critical now. Thanks for your help !
    – pat_dmllr
    Oct 18, 2014 at 9:26

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