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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?

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Welcome to Network Engineering. Bridges forward broadcasts, so VM2 will forward the broadcast to VM3.

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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).

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