two interfaces one subnet

Per Heldal heldal at
Tue May 12 03:30:18 CDT 2009

David Devereaux-Weber wrote:
> I work with iHDTV <>, a project that sends uncompressed high
> definition television (1.5 Gbps) as UDP over two 1 Gbps interfaces.  If 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.

Check closes and you may discover that the loss isn't on the path
host->router, but on the reverse.

Some hosts/interfaces are optimised for L2 link-aggregation, in which
case they by default present the same MAC (host MAC-address) on multiple
interfaces. Those will fail when configured with separate IP-addresses
on the same VLAN/subnet.  L2 switches that support link-aggregation will
try to spread the traffic across the available ports on one VLAN on
which it sees the same MAC. 2 ports gives 50% loss, 3 ports 67% and so
on. Most host interfaces will not pick up IP-packets which destination
doesn't match it's assigned address.


More information about the NANOG mailing list