STP UplinkFast

Lessons Learned:

Direct root port failure should reconverge immediately if alternate port available

Used when you have one primary interface ( the root port) and other upstream interfaces that are blocking but hey could be used as alternate ports to reach the root.

The UplinkFast feature will start forwarding to an Alternate port without recalculating because STP has already determined there is a loop free path.

It does this by implementing two additional features behind the scenes.

1.       It does this my increasing the cost of all its links to try and make sure it will not be used as transit for someone else to reach the root. Basically if you have a very high cost on the links you will most likely be excluded from someone else’s path selection.

2.       The switch will also take the entire CAM table and spoofs the source from the original Root port to the new uplink. It will send the MAC to the upstream switch – which will clear its local arp for the original interface and updates the source to the new uplink. This results in sub-second convergence when the directly root port goes down.

NOTE: Uplinkfast is ONLY for your Directly Connected port failure!!

TO configure from Global config mode:

# Spanning-tree uplinkfast

This will change your priority to a higher numerical value – this will make sure you’re not root of able to be transit to reach root.

Useful commands:

#Debug Spaning-tree uplinkfast

You will see the TCN’s and the port transitions. This Debug is very chatty because it does it for all configured VLANS.

#Service timestamp log uptime. – Will help with viewing logs.

Note: When the original link return to service, This will force the switch transition back to the original root port.

This feature MUST be used in either the Access or Distribution layer. You cannot use in the Core because the switch is increasing it cost and priority values.