In the topology, SW-A is the Root.
So E0/1 of SW-B & E0/2 of SW-C are Root Ports..
E0/3 of SW-C is the Alternate Port..
All are Fast Ethernet ports..
Let's see what happens when a Root Port is down..
I have enabled debug spanning-tree events on SW-C..
Removing the SW-A to SW-C link..
You can see it went through the STP states of Blocking > Listening > Learning > Forwarding which took 50 seconds by default..
Now let's enable UplinkFast and see what is the difference..
After reconnecting the link;
SW-C(config)#spanning-tree uplinkfast
It is a global command, cannot be implemented in interface level..
Removing the SW-A to SW-C link..
Debug output prompt, but believe me, wow! there was no down time :O
What will happen when I reconnect the cable?
It waits for a while and immediately swaps back to the previous port roles without a delay..
What really happened was when the UplinkFast is enabled and when SW-C opens the Alternate Port, it creates a dummy multicast frame. The source MAC address of this Ethernet Frame will be all the MAC addresses that it has in its MAC address table. The destination multicast address is a proprietary Cisco MAC address. This multicast frame will be flooded to all other switches so they can update their MAC address tables right away..
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