Understanding Linux Network Internals

Merging LANs with Bridges

Let's take the scenario in Figure 14-2 as an example and see how a bridge can be used to merge two LANs and make them look like one. Let's assume the hosts in the two LANs were configured to be part of the same IP subnet. We do not need to include the IP addresses in the figure because we will focus on what happens at the link layer.

You should note that a LAN is nothing but a multiport bridge.

Any frame transmitted on a LAN by any host is received by all other hosts. So when Host A sends out a frame, both the other hosts of LAN1 and the bridge receive it. A bridge copies its ingress frame out on all the other ports (there is just one other port, in this case). At the end, therefore, all hosts of both LAN1 and LAN2 receive a copy of the frame generated by Host A. This means that thanks to the bridge, there is only one big LAN from the perspective of the hosts of LAN1 and LAN2. Bridges are commonly employed to merge physical LANs whose hosts are configured on the same IP subnet, because they give the hosts the illusion of a single LAN.

Note that the bridge forwards the ingress frames as they are received. It does not add, remove, or change anything on them: the hosts in LAN2 receive an exact copy of the original frame generated by Host A.

You may argue that a packet from Host A addressed to Host B is needlessly forwarded to LAN2, which is a waste of bandwidth on LAN2 and a waste of CPU

Figure 14-2. Two LANs merged with a bridge

power for the hosts of LAN2 (since ...