At first approximation,
a bridge between two coaxial trunks is trivial. It appears as a station on each trunk;
every frame it receives from trunk 1, it relays to trunk 2, and vice-versa.
One obvious optimization, not relaying every frame, was made in the first bridge standard.
For example, suppose stations A and B are both on trunk 1. If A sends a frame with
B as its destination address, there is no point in the bridge relaying that frame to trunk 2.
But how does the bridge know which trunks A and B are connected to? They could somehow
register with the bridge, but one of the goals of IEEE Std 802.1D was to make the
bridge invisible to the stations. That is, no station (and as important, no software in a
station) needed to be modified when a coaxial trunk was extended with a bridge.
When first initialized, therefore, the bridge does relay every frame between the two
trunks; it doesn??™t know any better. Soon, however, the bridge receives a frame, with A
in its source address (the address of the station sending the frame), on trunk 1. The
bridge remembers that it received a frame from A on trunk 1 while it relays that frame
to the other trunks.
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