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Friday, December 1, 2017

Eliminating the Direct Root Port Down Time of STP by UplinkFast

This feature is to re-converge immediately using an Alternate Port in a case of a direct Root Port failure.. What does that even mean?? Let's see..

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 happened at the back end?

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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