Introduction to Spanning-Tree

Lessons Discussion
67

Alan,

  1. The cost is 19 because the line speed connecting the switches is fast ethernet (100 meg), which is assigned a cost value of 19 by STP. Port 18 (FastEthernet 0/16) is the port that connects Switch B to the root bridge. To be honest, I haven’t figured out why it is assigned a value of “18” instead of “16” (the actual physical interface port).

  2. A port moves out of the BLOCKING state once it stops hearing BPDUs coming in on that interface for the MaxAge amount of time (20 seconds by default). In other words, BPDUs must continue to be received in order for a port to remain in BLOCKING

  3. Aging Time 300 refers to the MAC entry timeout value. Once a entry has been created in the MAC address table, a 300 second counter starts. If traffic hasn’t been received to/from that MAC address within the 300 second timer, the entry is flushed from the MAC address table.

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