Configuring UplinkFast
What is UplinkFast?

Cisco’s Spanning Tree UplinkFast provides fast convergence after a direct link failure. This immediate convergence is facilitated through the creation of an uplink group; a set of Layer 2 interfaces on a single switch, only one of which is forwarding at any given time. An uplink group consists of the root port (which is forwarding) and a set of blocked ports. The uplink group provides alternate failover paths in event that the root port link fails.

UplinkFast can failover to a backup link very quickly, therefore the MAC address tables of other network switches must in turn be updated quickly to account for data traffic that should now traverse the backup path. To accomplish this, the UplinkFast switch will begin flooding frames with a source MAC address of all the entries in its CAM table to a destination Cisco proprietary multicast MAC Address. These frames will be sent out the backup port. This will in turn populate the CAM table of switches on the backup path with MAC addresses that were previously learned through the failed link.

The figure shows an example of a topology in which switch A is deployed in the Building Access submodule with uplink connections to the root switch over link 2 and the backup root switch over link 3. Initially, the port on switch A connected to link 2 is in the forwarding state, and the port connected to link 3 is in the blocking state.

When switch A detects a link failure on the currently active link 2 on the root port (a direct link failure), UplinkFast unblocks the blocked port on switch A and transitions it to the forwarding state without going through the listening and learning states. This switchover occurs within 5 seconds. UplinkFast is implemented on an access switch with at least one forwarding port and one blocked port toward the root.