RE: 2 NIC's on same network, possible?

["Max Harvey" <IT () smc ac nz>]

> From: Vineet Mehta [mailto:vineet () linux com kw]
> My collegue has a Linux machine which has 2 NIC's on it. What 
> he did was assign the IP's 192.168.0.6/24 and 192.168.0.7/24 
> to the NIC's. And he was trying to ping the network but was 
> getting errors (i dont know the errors).
>
>                -----------------
>               | Switch         |
>               |_________________|
>                 |             |
>                 |             |
>                 |             |
>           -------------------------------
>           |  NIC1            NIC2     |
>           |192.168.0.6/24 192.168.0.7/24|
>           |        Machine            |
>           |-----------------------------|



> From: Burton M. Strauss III [mailto:BStrauss () acm org] 
> Now it gets weird if somebody is trying to reach YOU on the 
> 2nd NIC.  Why? Because the routing decision is address based, 
> not NIC based.
>
> So a packet TO the address of the 2nd NIC (192.168.0.7) is 
> received on the 2nd NIC.  The reply, addressed say to 
> 192.168.0.5 again, is sent VIA the 1st NIC (1st match in the 
> routing rules wins!).  Unless the sender also has this kind 
> of funky routing table.
>
> THIS is what leads to the 'rule' that you can't have two NICs 
> with the same network portions, because if they're really NOT 
> connected identically, you'll lose traffic, and if you're not 
> really, really careful with routine rules (wait for it) (yes) 
> you'll lose traffic.


Does this mean, that if both eth0 and eth1 are connected directly to the
same switch as in the original post I believe, therefore having the same
connection to the network, that it would work?

I am wondering as I had a network cable die on one of my servers
yesterday, so I am considering sticking a backup Ethernet card in the
server, and having it connected into the exactly same switch that the
current card/cable connects to. I am just trying to confirm that I have
read this right, and that since they are connected to the same
connection, that it will work.

Hopefully this means that if I lose a cable/card, it will determine that
the route isn't working, and move onto the next match?


Max.


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