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Setup: Assume I have 3 VMs. VM1 VM2 and VM3. VM2 has two interfaces, say eth1 and eth3. eth1 is connected to VM1 physically and eth3 to VM3. I create a bridge on VM2 using brctl with eth1 and eth3.
When VM1 pings VM3, it needs MAC address of VM3. It will send ARP request to VM2. According to this, a bridging decision must be taken based only on the basis of destination MAC address (which will be roadcast ff:ff:ff:ff:ff:ff...). What should happen now. Why does it go on the forward chain?
Welcome to Network Engineering. Bridges forward broadcasts, so VM2 will forward the broadcast to VM3.
The ARP request is using a broadcast, so VM2 forwards it across its interfaces.
Broadcasts are always forwarded across a bridge. Unicasts are only forwarded when the destination is located on the other side (or to the port the destination is located on, for a multi-port bridge).
