Understanding Linux Network Internals

Multiple Bridges

So far, we have seen only simple scenarios with just one bridge. However, because transparent bridges are transparent to each other, as well as to the hosts and the routers, you can create a larger L2 domain (i.e., a bigger LAN) by employing multiple bridges, as shown in Figure 14-6.

The figure also shows the list of addresses that is learned by each interface of the two bridges, assuming each host has spoken at least once and thus given the bridges a chance to learn their locations. Note, for example, that from Bridge 1's perspective, Hosts A, B, C, and D are all located in LAN2, or in other words, are reachable via Bridge 1's interface on LAN2. After all, a bridge does not really care exactly where a host is located; all it needs to know is what port to use to reach it.

The use of multiple bridges, however, requires care to be taken when designing the topology. Let's see why with the example in Figure 14-7.

Figure 14-6. Topology with two bridges

Figure 14-7. Redundant bridges

Multiple bridges on the same LAN can be useful, for instance, to increase the availability of the connectivity between the LANs on which the bridges have interfaces. If one bridge becomes unusable for some reason, the other ones will be able to keep up connectivity. The figure shows a topology with ...