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Let's say i have two linux boxes, RouterA and RouterB, both with two NICs (eth0 and eth1).

RouterA/eth0 is connected to an internet uplink. RouterA/eth1 and RouterB/eth1 are directly connected.

What I'd like to do is plug the uplink to both RouterA/eth0 and RouterB/eth0, the latter being down (and without using any SPoF component like a switch, juste something passive). If RouterA goes down, RouterB will know it, and so it can bring it's eth0 NIC up.

But it seems such a passive link sharing device doesn't exist, according to the following comment of another thread (Connect two routers to a single uplink for full redundancy) :

"A passive Ethernet ATS does not exist because there mandatorily must be intelligence inside the device to determine whether the carrier signal of Ethernet is present or not. (It's not like a electrical ATS that just sees voltage.) That intelligence then introduces new electronics and software that can fail, thus canceling out any added benefit of the ATS. This type of ATS could be created, but nobody would buy it, therefore it is not created, manufactured, distributed, and sold at stores."

So, my question : does everybody agree with that ? Is there really no passive way to distribute an uplink between two routers, one being passive (NIC down) ? Something like a link duplicator, that sends the uplink to both RouterA/eth0 (up) and RouterB/eth0 (down). No risk of collision, RouterB/eth0 being down. And if RouterA/eth0 goes down, then RouterB/eth0 goes up

Right now, if RouterA goes down, I have to physically go the there to unplug the uplink cable and plug it into RouterB.

  • The single link is the SPoF. It is far more likely that the link will go down than enterprise-grade equipment (what is on-topic here) fails. In any case, your servers are off-topic here because the manufacturer of a network device must offer optional, paid support for the device to be on-topic. – Ron Maupin Mar 25 at 15:53
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    If your router is likely to fail you should replace it. Adding a second router is only possible with putting a switch in front (or an obsolete repeater hub). – Zac67 Mar 25 at 15:57
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You can do this with 10base2, but I'm sure you're talking about equipment which is using twisted pair.

The main problem is how to detect that Router A has failed. Many failure modes -- eg CPU overloads -- leave the NIC fully enabled, so you can't use anything like "link light". Many failure modes have continuous transmit, so you can't use the Tx light.

Switches are in general much more reliable than routers, which are much more reliable than computers-operating-as-routers. So I wouldn't worry about the switch.

If you absolutely are looking for such a thing, you can use a "watchdog relay", which is a device which looks for pulses periodically and keeps an electromechanical relay energised. It's a standard device in control engineering. If you can get watchdog pulses out of Router A, that would do it. Obviously you'd need at least 4-pole changover contacts to switch between two cables. You'd have to wire it carefully or you'll have out-of-specification cabling, which is more-or-less guaranteed to give problems. You could do it in a lab for fun, don't do it in real life unless you've got a way to test and warrant all the parts.

In real life you do all that resilience with multiple switches, certified cables, STP, and routing protocols. And multiple external links. And of course appropriate UPS.

PS: first principles suggest it should be possible with fibre optics, but I know of no standard splitter equipment with this kind of functionality.

  • Your answer seems to imply that I was looking for an equipment that's able to detect that RouterA has failed, which isn't the case. I know how to detect if RouterA has failed, that's RouterB's job (both are directly connected ). What I AM looking for is a PASSIVE piece of hardware that sends the UPLINK (yes, twisted pair) to both RouterA/eth0 and RouterB/eth0, knowing that at a give time only one of the two will be listening. Not sure if such an equipment exists, though – ChennyStar Mar 26 at 16:34

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