Chapter 16. Building bridges, and pseudo-bridges with Proxy ARP
Bridges are devices which can be installed in a network without any
reconfiguration. A network switch is basically a many-port bridge. A bridge
is often a 2-port switch. Linux does however support multiple interfaces in
a bridge, making it a true switch.
Bridges are often deployed when confronted with a broken network that needs
to be fixed without any alterations. Because the bridge is a layer-2 device,
one layer below IP, routers and servers are not aware of its existence.
This means that you can transparently block or modify certain packets, or do
shaping.
Another good thing is that a bridge can often be replaced by a cross cable
or a hub, should it break down.
The bad news is that a bridge can cause great confusion unless it is very
well documented. It does not appear in traceroutes, but somehow packets
disappear or get changed from point A to point B ('this network is
HAUNTED!'). You should also wonder if an organization that 'does not want to
change anything' is doing the right thing.
The Linux 2.4/2.5 bridge is documented on
this page.