[Bridge] pinging other side (normal bridge ok, anonymous bridge not ok)

[richardvoigt at gmail.com]

On Tue, Oct 14, 2008 at 1:08 PM, Alexis Huxley <ahuxley at gmx.net> wrote:
>> Adding a dummy device to a bridge doesn't do much for you. The dummy
>> device never has any incoming packets, and any outgoing packets are
>> discarded.
>
> I understand that. But I still want to do it like this. As I mentioned,
> I'm trying to make a setup that mirrors a physical setup; i.e. two
> physical computers, each with one NIC plugged into a hub which itself
> has no IP address.  The closest I can get to this with my (kvm VM)
> is: a physical machine with a dummy0 interface plugged into br0
> and a VM with a virtual twisted cable with its eth0 (which via the
> VM:eth0<->PM:tap0 virtual crossover cable) plugged into br0.

That's not close at all to representing a physical ethernet switch,
which is why you're having difficulty.
What you're doing is putting an IP address on a switch port, not on a
device connected to the port via a cable.

>
>> The veth driver might be more useful for you. If you have the veth
>> module buillt (CONFIG_VETH), you can create an interface pair using:
>> $ ip link add name veth0 type veth peer name veth1
>> This gives you a pair of ethernet devices (veth0 and veth1) set up so
>> that a packet sent out on any one is received on the other. With this,
>> you can make one of these a bridge port and give the other end the IP
>> address.
>
> So you mean without br0 at all? If so then, no, I really want to do
> it with br0.

I don't know why you'd think "make one of these a bridge port" means no bridge.

veth is the representation of a physical cable, what Srinivas
suggested is you plug one end of this cable into your switch (via
brctl addif) and put the IP on the *other* end, that corresponds
exactly to the physical case.

>
> Simply: why, when this works:
>
>>> brctl addbr br0
>>> ifconfig br0 192.168.1.42 up
>>> ifconfig dummy0 0.0.0.0
>>> brctl addif br0 dummy0
>>> brctl addif br0 tap0
>
> does this not work:
>
>>> brctl addbr br0
>>> ifconfig dummy0 192.168.1.42 up
>>> brctl addif br0 dummy0
>>> brctl addif br0 tap0

Because dummy0 is now a port of the ethernet switch, not a device
attached to the ethernet switch as you persist in claiming.  You
already have trouble believing an ethernet switch can have an IP
address (although managed switches most certainly do), why would you
try to put an IP on each and every port of the switch?

>
> ? Thanks!

Hope that helps.

>
> Alexis
>
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