OK, I'll narrow this down:
We will have a router serving the same subnet in wireless and wire.
We'll have a laptop with 2 interfaces, wifi0 and eth0.
We'll not do any routing configuration beyond a default.
Finally, this explanation is watered down to dilution because I don't have a 
lot of time right now. 

In a nutshell, when the protocol is sending a packet, it will look for an 
interface that matches the subnet of the packet, and if it finds it, it will 
send a "who has" request over that interface. 

Otherwise, the packet is handed to the "gateway" (default routing) which is 
not what we are discussing here. 

There is no guarantee (that I know of) that the kernel will search the 
network interfaces in any particular order for a matching subnet, and the 
search will stop as the first one is found. 

That creates a race condition with the ARP table where a packed may be sent 
while the ARP table gets refreshed and moved to the other interface, and 
those packets will die a slow death.  That will create random connection 
drops and transmission slowdowns. 

I've seen it...
tctpdump(it), and you'll see it too.
ET 

 

Matt Graham writes: 

> On 2019-12-07 14:20, kitepilot@kitepilot.com wrote:
>> Mark Phillips writes:
>>> dd-wrt router (ASUS RT_N16) would do this. I then
>>> noticed that the firmware was over 2 years old, so I thought, I should
>>> upgrade the firmware. Long story short, I may have bricked my router.
>>> My question is, can I run the wifi on SUBNET (192.168.25.x) and my wired
>>> connection on another SUBNET
>> You *HAVE* to configure different subnets in each interface or you'll
>> have a chaos.
> 
> Not necessarily.  I have a bog-standard Netgear consumer grade 
> wireless/wired gateway.  It serves up addresses in 192.168.2.0/24 to wired 
> and wireless clients.  The option for having a separate subnet for 
> (whatever) is called "guest network" in this, consult your man page for 
> dd-wrt for what that's called there. 
> 
> My device is probably doing something funky involving bridging in its guts 
> so that it allows 192.168.2.1 to be accessible over wired and wireless 
> interfaces.  I think I turned on both wired and wireless networking on my 
> laptop at some point, and it didn't break everything.  I'll have to wait a 
> few hours to try that out again though.  This is *not* recommended, but it 
> should not be the horrible failure you got in the 2000s if you had 2 wired 
> Ethernet devices on the same machine in the same subnet.  ICBW though. 
> 
> -- 
> Crow202 Blog: http://crow202.org/wordpress
> There is no Darkness in Eternity
> But only Light too dim for us to see.
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