two interfaces one subnet

Ben Scott mailvortex at
Mon May 11 16:40:33 CDT 2009

On Mon, May 11, 2009 at 5:28 PM, Hector Herrera <hectorherrera at> wrote:
> On Mon, May 11, 2009 at 2:22 PM, David Devereaux-Weber
> <ddevereauxweber at> wrote:
>> ... both> interfaces are on the same subnet, the OS sees the same router (gateway)
>> address on both interfaces, and the results are sub-optimal ... around 50%
>> packet loss.
> packet loss is probably due to the network switch having to re-learn
> the location of the MAC address constantly as it sees packets on two
> or more ports with the same MAC address (think STP loops).

  My understand of the scenario is: Two physical interfaces, each with
a unique IP address, in the same Ethernet broadcast domain, on the
same IP (sub)network.

  If that's the case, the MAC address won't change.  The cards stay
put.  So a layer two switch will be none the wiser.

  The reason this doesn't work (for most implementations) is that most
IP routers look only at the destination IP address, and keep no state.
 (Here, I'm using "router" to include the routing engine built-in to
any full IP implementation, not just dedicated equipment from Cisco,
et. al.)

  So we have a host with IP addresses A and B on the same subnet.  A
packet comes in from some other host X.  The application software does
whatever it does, and sends a response.  The router looks at the
destination IP address X, and sees that it has two routes, A and B.

  Depending on implementation, the router may send everything out the
first interface it finds in the routing table (e.g., use A and ignore
B), or round-robin between the two, or who-knows-what.  Either way, if
the packet *from* X was addressed *to* B but the response comes back
from *A*, then host X is going to drop the packet as

  With Linux, at least, you *can* use the routing policy database to
configure the kernel router to pay attention to more than just the
destination IP address.  For example, you can have it look at the
source IP address (A or B), and route out the appropriate interface.
However, IIRC, this only works if the application software binds to
individual network interfaces.  If the app software just listens for
anything (, then the kernel gets to pick the source IP address
for any response.

  I can post examples with gory details from our firewall, if anyone needs them.

-- Ben

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