Again, a loop-avoidance mechanism eliminates
this problem by preventing one of the four interfaces from transmitting frames during
normal operation, thereby breaking the loop.
Resolving Issues with STP
STP provides loop resolution by managing the physical paths to given network segments.
STP allows physical path redundancy while preventing the undesirable effects of active
loops in the network. STP is an IEEE committee standard de?¬?ned as 802.1D. Figure 2-21
illustrates how a blocked port would prevent traf?¬?c ?¬‚ow between the segments.
Figure 2-21 Blocking on a Port
STP behaves as follows:
?– STP forces certain ports into a standby state so that they do not listen to, forward, or
?¬‚ood data frames. The overall effect is that there is only one path to each network
segment that is active at any one time.
Server/Host X Router Y
Port 1
Port 2
Port 1
Port 2
Unicast
Segment 1
Segment 2
Unicast
Switch A Switch B
Block
50 Chapter 2: Medium-Sized Switched Network Construction
?– If any of the segments in the network have a connectivity problem, STP reestablishes
connectivity by automatically activating a previously inactive path, if one exists.
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