Thursday, June 23, 2016

[Linux] Why does Linux require moving IP from eth interface to bridge interface?

This could be a common problem if you have KVM ( or other hypervisor in Linux ) on your physical server and want to use bridge mode with your VMs. At the same time, you also want to let your physical server has the network that can be accessed from other hosts at the same network subnet. At this moment, when a network interface (e.g., eth0) is added to a Linux bridge (e.g., br0), the IP address must be removed from eth0 and added to br0 for the networking to function properly.

I find some answers as follows:

The NIC represents the uplink cable. A cable is layer 1, not layer 3. Now the Bridge works as the device that is being addressed for network traffic (incoming) on the server - either on layer 2 (Ethernet/MAC) and/or layer 3 (IP). So the device that responds to ARP-requests is the bridge - which is good, since it needs to distribute the traffic to the other interfaces on that bridge. If the responding device were the NIC, traffic would not be passed further on to the bridge.

Normally it does not make sense to put any L3 protocol address on port interfaces - because incoming packets are diverted to the bridge interface before the L3 protocol is examined. This means the L3 protocol running on the port interface will never see any incoming packets.
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