Problems with two interfaces on the same subnet?

[Ivan Voras]

Hello,

I have a machine with two interfaces, igb2 and igb3 on the same subnet
but with different IP addresses, e.g. igb2 has 192.168.1.221, igb3 has
192.168.1.222. Firstly, is there anything which would preclude this from
working? As I see it, these are two different MAC addresses and to the
outside world should look as two different hosts, right?

I currently don't have the right tools to diagnose this, I'm using
nas4free which is a 9.1-based "embedded" system lacking tcpdump, so here
are some raw observations:

* With both NICs connected to the same switch, everything appears to
work, but an *upstream* router *seems* to have strange issues which
manifest in it replicating the traffic coming from this machine to
unrelated ports. This router is an old, unsupported Cisco router which
should have long be replaced and no-one here knows how to debug it. The
network admin says that he sees that the "Cat4k Mgmt LoPri" counter
shows high CPU usage, but cannot help other than that. This also may be
due to weirdness in other parts of the network, we don't know if it's
caused by my machine.

* With both NICs connected to different switches, everything appears to
work, BUT, if igb2 cable is disconnected, pings to igb3 simply stop,
even though its cable *is* still connected.

This sort of looks like incoming and outgoing traffic to the same IP
address (that of igb3) are arriving and departing on separate ports,
which I could accept as the packets are unrelated on the IP level and
the route says that the subnet is reachable only through the first
interface, if the kernel does nothing when the first NIC's link goes
down. Is this what's going on?

The reason why I have two addresses on the same subnet is for poor-man's
load-balancing, this is a NFS server and I'd like to use both NICs at
the same time, accessed from different client machines. I'm suspiocious
that the network equipment I'm using does not support LACP. Is there
another way to do it? Would it help to have two different private subnets?

And the big question on my network admin's mind: would it help to switch
to Linux?


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