??? Now that the
common carrier world, whose networks are traditionally composed of switches, is utilizing Ethernet
technology, there are standards emerging from the ITU-T that define Ethernet switches. This chapter uses
the term bridge when referring to an IEEE 802-defined device and switch when referring to an ITU-T-defined
circuit switching device.
378 Chapter 13
immediately. But, if A remains silent waiting for another station to talk to it, and if that
other station is on, say, trunk 1, it might wait forever; the bridge, having learned that
A is on trunk 1, will not relay frames addressed to A from trunk 1. Therefore, there is
a timeout on every entry in the bridge??™s filtering database. After five minutes without
receiving a frame with A in its source address, the entry for A is removed. The next
frame sent to A will be relayed to all trunks because the bridge no longer knows what
else to do. As soon as A responds, its location is learned.
Note that in the worst case, A responds on some other medium entirely, so the bridge
never learns A??™s Ethernet MAC address. In that case, the bridged LAN is no worse off
than if it were all one coaxial trunk.
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