There are three major enhancements available for Spanning Tree, as it is applied on Cisco devices:
PortFast - By default, all ports on a switch are assumed to have the potential to have bridges or switches attached to them. Since each of these ports must be included in the STP calculations, they must go through the four different states whenever the STP algorithm runs (when a change occurs to the network). Enabling PortFast on the user access ports is basically a commitment between the Network Architect and the switch, agreeing that the specific port does not have a switch or bridge connected, and therefore this port can be placed directly into the Forwarding state; this allows the port to avoid being unavailable for 50 seconds while it cycles through the different bridge states, simplifies the STP recalculation and reduces the time to convergence. UplinkFast - Convergence time on STP is 50 seconds. Part of this is the need to determine alternative paths when a link between switches is broken. This is unacceptable on networks where real-time or bandwidth-intensive applications are deployed (basically any network). If the UplinkFast feature is enabled (it is not by default) AND there is a least one alternative path whose port is in a blocking state AND the failure occurs on the root port of the actual switch, not an indirect link; then UplinkFast will allow switchover to the alternative link without recalculating STP, usually within 2 to 4 seconds. This allows STP to skip the listening and learning states before unblocking the alternative port. BackboneFast - BackboneFast is used at the Distribution and Core layers, where multiple switches connect together, and is only useful where multiple paths to the root bridge are available. This is a Cisco proprietary feature that speeds recovery when there is a failure with an active link in the STP. Usually when an indirect link fails, the switch must wait until the maximum aging time (max-age) has expired, before looking for an alternative link. This delays convergence in the event of a failure by 20 seconds (the max-age value). When BackboneFast is enabled on all switches, and an inferior BPDU arrives at the root port - indicating an indirect link failure - the switch rolls over to a blocked port that has been previously calculated. The primary difference between UplinkFast and BackboneFast is that BackboneFast can detect indirect link failures, and is used at the Distribution and Core layers; while UplinkFast is aware of only directly connected links, and is used primarily on Access layer switches. If UplinkFast is turned on for the root switch, it will automatically disable it. Since BackboneFast is an enhancement strictly for Core and Distribution layer devices, and these are all Set-based switches, there is no command to enable it for IOS based switches. Hope that helps... -=- Dennis -----Original Message----- From: William [mailto:[EMAIL PROTECTED]] Sent: Wednesday, November 14, 2001 8:11 AM To: [EMAIL PROTECTED] Subject: Uplink fast and Port fast [7:26236] Dear all, Any one know what is uplink fast and port fast? Thanks a lot!! Message Posted at: http://www.groupstudy.com/form/read.php?f=7&i=26249&t=26236 -------------------------------------------------- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
