Avoiding Loops Using Spanning Tree Protocol
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The third primary feature of LAN switches is loop prevention, as implemented by Spanning
Tree Protocol (STP). Without STP, any flooded frames would loop for an indefinite period
of time in Ethernet networks with physically redundant links. To prevent looping frames,
STP blocks some ports from forwarding frames so that only one active path exists between
any pair of LAN segments.
The result of STP is good: Frames do not loop infinitely, which makes the LAN usable.
However, STP has negative features as well, including the fact that it takes some work to
balance traffic across the redundant alternate links.
A simple example makes the need for STP more obvious. Remember, switches flood
unknown unicast frames and broadcast frames. Figure 7-8 shows an unknown unicast frame,
sent by Larry to Bob, which loops forever because the network has redundancy but no STP.
Note that the figure shows one direction of the looping frame only, just to reduce clutter,
but a copy of the frame would also loop the other direction as well.
Figure 7-8 Network with Redundant Links but Without STP: The Frame Loops Forever
The flooding of this frame would cause the frame to rotate around the three switches,
because none of the switches list Bob’s MAC address in their address tables, each switch
floods the frame. And while the flooding process is a good mechanism for forwarding
unknown unicasts and broadcasts, the continual flooding of traffic frames as in the figure
can completely congest the LAN to the point of making it unusable.
A topology like Figure 7-8, with redundant links, is good, but we need to prevent the bad
effect of those looping frames. To avoid Layer 2 loops, all switches need to use STP. STP
causes each interface on a switch to settle into either a blocking state or a forwarding state.
Blocking means that the interface cannot forward or receive data frames, while forwarding
means that the interface can send and receive data frames. If a correct subset of the interfaces
is blocked, only a single currently active logical path exists between each pair of LANs.
NOTE STP behaves identically for a transparent bridge and a switch. Therefore, the terms
bridge, switch, and bridging device all are used interchangeably when discussing STP.
The Cisco CCNA Routing and Switching ICND2 200-105 Official Cert Guide book covers
the details of how STP prevents loops.