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| 3.3 |  | STP Enhancements |
| 3.3.3 |  | UplinkFast |

During the time it takes for STP to converge, some end stations might become inaccessible, depending on the STP state of the switch port to which the station is attached. This disrupts network connectivity, it would be advantageous to decrease STP convergence time and reduce the length of the disruption. UplinkFast was developed to facilitate fast STP convergence. UplinkFast is a Catalyst feature that accelerates the choice of a new Root Port when a link or switch fails.

Switches in hierarchical networks can be grouped into core layer switches, distribution layer switches, and access layer switches. Figure shows a portion of a campus network in which distribution switches and access switches each have at least one redundant link that STP blocks to prevent loops. If a switch loses connectivity on a link, it begins using the alternative paths as soon as STP selects a new Root Port. When STP reconfigures the new Root Port, other inter-faces flood the network with multicast packets, at least one for each address that was learned on the interface.

STP UplinkFast accelerates the choice of a new Root Port when a link or switch fails or when STP reconfigures itself. The Root Port transitions to the Forwarding state immediately without going through the Listening and Learning states, as it would with the usual STP process. UplinkFast also limits the burst of multicast traffic by reducing the max-update-rate. For IOS the default for this parameter is 150 packets per second.

UplinkFast is most useful in wiring-closet switches at the edge of the network. It is not appropriate for backbone devices. UplinkFast provides fast convergence after a direct link failure and achieves load balancing between redundant links using uplink groups. An uplink group is a set of interfaces (per VLAN), only one of which is forwarding at any given time. Specifically, an uplink group consists of the Root Port (which is forwarding) and a set of blocked ports.

The uplink group provides an alternative path in case the currently forwarding link fails. Figure shows a sample topology with no link failures. Switch A, the Root Switch, is connected directly to Switch B over link L1 and to Switch C over link L2. The interface on Switch C that is connected directly to Switch B is in a Blocking state. Switch C is configured with UplinkFast. If Switch C detects a link failure on the currently active link L2 on the Root Port (a direct link failure), UplinkFast unblocks the blocked port on Switch C and transitions it to the Forwarding state without going through the Listening and Learning states. This change takes approximately 1 to 5 seconds. As soon as a switch transitions an alternative port to the Forwarding state, the switch begins transmitting dummy multicast frames on that port, one for each entry in the local bridge table (except entries that are associated with the failed Root Port). By default, approximately 15 dummy multicast frames are transmitted per 100 milliseconds.

Each dummy multicast frame uses the station address in the bridge table entry as its source MAC address and a dummy multicast address (01-00-0C-CD-CD-CD) as the destination MAC address. Switches receiving these dummy multicast frames immediately update their bridge table entries for each source MAC address to use the new port, allowing the switches to begin using the new path almost immediately.

In the event that connectivity on the original Root Port is restored, the switch waits for a period equal to twice the Forward Delay time plus 5 seconds before transitioning the port to the Forwarding state in order to allow the neighbor port time to transition through the Listening and Learning states to the Forwarding state. Configuring UplinkFast on Catalyst switches is straightforward.