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I have been working at layer 2 for a while now, but I might have to solve a layer 3 problem for an application I am currently working on. A client has a very specific need based on some networked equipment, which requires redundancy and some other subnet requirements that I omit.

Assuming that I have a subnet that is connected to two (OSPF) routers like this:

Router -------- Switch ------ Host ------ Switch ------ Router

and those two routers (which are not connected to each other directly) are connected by other routers to my computer, I am curious about the fail over behavior of the network. If, by default, the route from my computer to the host is routed through the left router and the cable from the left switch to the host fails, will the route fail over and use the right router? Or will the cable from the left router to the left switch have to fail for that to occur?

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  • First hop redundancy protocols (HSRP, GLBP, VRRP, etc.) require that the routers be able to communicate with each other over the subnet for which redundancy is required.
    – Ron Maupin ♦
    Aug 11, 2015 at 0:34
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    Does the host have two NIC cards -one connected to each switch?
    – Ron Trunk
    Aug 11, 2015 at 0:34
  • The host is actually a management interface to a SONET network, so it has as many interfaces as the SONET nodes. Ignoring the details, the host acts like a switch with each node as a host with an IP address to access it from. Therefore, the routers must be on the same subnet as the host. This part of the network is OT and the routers and the rest of the network are IT.
    – user17160
    Aug 11, 2015 at 14:50

2 Answers 2

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As far as the router knows it is still connected to the host network when the cable between host and switch fails, so it will continue to advertise itself as a way to that network. So your computer will continue to use the left router unless the link betwen the switch and router fails aswell meaning that the router has no path to that network.

Although, if you have a default route on your computer to the left router it won't matter as all traffic will be routed to it and traffic for host network will simply be dropped if no path exists from left router.

So to achieve that redundancy you would probably need to run a routing protocol on your computer e.g. RIP to get specific information and be able to choose which way to go.

I'm assuming you have two NICs since your computer are connected to both routers indirectly but they are not connected to each other.

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  • This is what I thought might be the case. We have some simple routers that doesn't support dynamic routes and if the route to host fails, I get an ICMP message back to the host. From what little I know about routers, this ICMP message will not be used by routers. If I use a Windows Server computer can I do fail over based on unreachable host without configuring the NMS software or having the user manually change the routing?
    – user17160
    Aug 11, 2015 at 16:03
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If you can't stack your switches and you don't have enough ports to plug each router into each switch you might be stuck, especially if you can't run VRRP which is pretty much designed for just this. I'm guessing thats the case given your device doesn't support dynamic routes.

You may be able to configure two default gateways on your host perhaps, but thats not ideal i guess.

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