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.