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I have following network setup.

diagram

When the link marked by red cross is up, I can ping from first machine (10.1.1.7) to second machine (10.1.1.3) without any problem. However, if I take down that link and try to ping from first machine to the second, I see that arp queries come for ip address 10.1.1.3 on the second machine's second interface (10.1.3.2) but it does not reply to those queries. This is expected I think as that interface does not know about other interface's ip address.

But if I still want to make them connect, I think I need to set some value in one of the: /proc/sys/net/ipv4/conf/(default|all)/arp_*

But which one? I played around setting some of them but did not work. Any help related to this would be greatly appreciated.


** UPDATE **

Actually let me clarify the problem. What I really want to do is start a file transfer from first machine to the second. The transfer should happen over the link marked by cross. In the middle of the transfer, I will take down that link. Transfer should automatically switch to other links. You can suggest me any changes to IP addresses etc.

What I tried is: On machine 2, I kept both MAC interfaces on the same subnet. That is NIC1: 10.1.1.3 NIC2: 10.1.1.17

Then I created a software bridge on machine 2, gave it IP address 0 and added NIC1, NIC2 under it. My idea is bridge will flood the queries received on NIC2 to NIC1.

Now I have a new problem. When I add both NICs to the bridge, they stop responding to ARP queries even if the link is up.

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Those router like blobs are in fact switches. –  Methos Nov 18 '09 at 15:40
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This is expected I think as that interface does not know about other interface's ip address.

Yes.

But which one? I played around setting some of them but did not work. Any help related to this would be greatly appreciated.

I, frankly speaking, couldn't understand what you mean, but may be you are asking, how to be able to ping that ip, if crossed link fails. If I am right, than you have to put two static routes on routers, something like:

route add 10.1.1.3/32 _ip_of_second_router (on the left router )
route add 10.1.1.3/32 10.1.3.2 (on the right)

UPDATE:

In case of switches, that won't work:

  1. 10.10.0.3 is in the same subnet as 10.1.1.7, so it will talk to him directly.
  2. When link fails, PC will talk to 10.1.1.7 interface mac
  3. Until expiration of it
  4. PC will ask who-has 10.1.1.7
  5. Even, if second PC will answer, that it has it on 10.1.3.2 interface
  6. First PC will start to talk with 10.1.3.2 interface's mac
  7. But when link comes back again, first pc will continue to talk with 10.1.3.2 interface's mac, and won't choose 10.1.1.7 interface's mac, until second link will be down.
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Assuming that those router-like blobs are, in fact, supposed to be routers... –  womble Nov 18 '09 at 15:38
    
Ah I don't want a static route solution as that needs manual configuration. Isn't there any option that would make linux understand IP addresses assigned to other interfaces? –  Methos Nov 18 '09 at 15:39
    
If I add above mentioned static routes, then that link would always be chosen even if "crossed" link comes up once again. I want packets to get transferred over "crossed" link if that is up. Otherwise over other link. –  Methos Nov 18 '09 at 15:42
    
Why always? You can add priority of static route, and when interface is up, route will be chosen. Than you can try "proxy arp" (routing on that PC must be ON). But anyway, when link goes down- IP won't respond, so I am a bit wrong too). –  TiFFolk Nov 18 '09 at 16:04
    
Set that IP to host's loopback, that must help. –  TiFFolk Nov 18 '09 at 16:05
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