Because there is no geographical hierarchy to MAC addresses, there are
no subnetworks.
?– Routes are established among the bridges, irrespective of the addresses of the
stations. These routes are spanning trees. Each spanning tree is ???spanning??? in
the sense that there is a path between every pair of LANs, and it is a ???tree??? in the
sense that there is only one such path. Routes are not computed to stations or to
subnetworks.
?– Given that there is only one path between any pair of LANs, A and B, and that the
path from A to B is identical to the path from B to A, each bridge is able to learn
the direction in which each station lies by observing the flow of traffic. Unlike routing,
no station addresses (or subnetwork addresses) are conveyed by the control
protocols.
?– The spanning tree protocols are mathematically incapable of forwarding frames in
a closed loop, even momentarily. Therefore, except at the entrance to or exit from a
service boundary, a bridge does not alter the frames it forwards. Unlike with routers,
Ethernet Bridging 381
with bridges, there is no time-to-live (TTL) decremented as a frame is forwarded from
bridge to bridge for two reasons: no routing loops are possible, and bridges must be
transparent to the stations they serve.
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