Understanding Linux Network Internals

Address Learning

We saw in the previous section that a bridge that blindly copies ingress frames to all the ports except the one that received the frame may lead to a waste of resources. Fortunately, bridges are not as blind as that. They actually are able to learn the location of hosts (their L2 addresses, to be more exact) and use that knowledge to selectively copy ingress frames only to the right port. This process is called passive learning, because it is handled by the bridge alone, without any need for user configuration or help from a protocol. Let's see how it works with the help of Figure 14-4. To make the figure more readable, the figure uses the "Host N" notation to refer to L2 addresses (i.e., it does not show real L2 addresses, as Figure 14-2 does).

Let's see what happens when the hosts of Figure 14-4 exchange a few frames. Keep in mind that the addresses discussed here are link layer addresses (i.e., Ethernet MAC addresses):

Figure 14-4(a)

Host A transmits a frame addressed to Host B. Host B receives it, because it sits on the same LAN, and the bridge receives a copy as well. Because the bridge does not know where Host B is located, it copies the frame on LAN2. But because the bridge has received a frame from Host A on the bridge's LAN1 port, it now knows that Host A is located in LAN1. Note that this is possible because Ethernet headers include both the source and destination addresses.

Figure 14-4(b)

Host B transmits a frame addressed to Host A. Both Host A and the ...