1. Look at nature, think carefully about their principles and sources of
2. Why 2 switching network produces the physical loop?

For redundancy, bandwidth between switches to ascend, or wrong connection will inevitably produce a closed loop of physics, and Ethernet store-and-forward mechanism again decided to not have a physical loop, there is a loop, those sent to all hosts Broadcast, Unknown Unicast Frame will be unscrupulous on the loop around the circle endlessly: mountain without edges, heaven and earth together, will dare to and loop off! These frame never reach the destination, to switch the CPU is a fatal blow at the same time, if there is a loop, you may not be able to local, or remote login to your switch, only to restart or drawn wire, isn't it a bit of a mess...
3. The layer 2 switch forwarding mechanism

Switch from a port on the N incoming frame, learning the source MAC X, generate the MAC Address Table is as follows:

MAC X & lt; -- -- -- -- -- - & gt; The Port N

So that it generates the MAC Address and Port mapping Table, if you receive a Frame, it will query the Frame's Destination MAC and the MAC Address Table matching, matching to send out from the corresponding Port,

If you don't match to considered unknown Unicast or broadcast, no choice but to remove it from all the Port (besides receives the Frame of interface) to send out, to another switch, so then the Frame has been in a closed loop in infinite loop,


4. Spanning Tree Spanning Tree

Network scientists discovered the rule, has a bold vision, since there are physical loop network, on the second floor in a logical way that physical loop cut, chop into a divergent tree architecture, can we won't be the infinite loop Frame? The answer is yes, did the same,

If the tree topology used for layer 2 switching network, on the second floor in the network to select a root (root bridge), the other switches as tree branches, each branch has a natural root tip (root port), this is the upstream interface switch, in addition to the root tip, other interfaces, the interfaces are downstream of the downstream interface is smooth, or block, depends on the path to the root cost cost, who is closer to the root, who's smooth (Forwarding), is often said that the Designated port. Who stay away from the root, who will need to be Blocked (Blocked), is often said that the Non Designated Port, through the bionic mechanism, can effectively avoid the loop network,

Election of the spanning tree mechanism
As to who will do the root bridge, the high priority to do, it is the boss, the boss of all port is designated, no words, then choose the closest to the leader of each switch root port, closest to the quantitative measure? Use cost, cost of small wins, then the younger brother in interconnection designated port choice, also simple, use cost to than size, small who wins, designated,