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Can anyone explain in details port fast, uplink fast &
backbone fast features introduced by CISCO (The GOD FATHER
of NETWORKING.... Hehehehehe).
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Answer Posted / rajat kumar dwivedi
Portfast, Uplinkfast, Backbonefast
In switching there are some terms that we must know for advanced spanning tree protocol settings which are portfast, uplinkfast and backbonefast.
Portfast is one of the simplest implementation and is usually studied when you take your CCNA exam. Portfast is used when a switch port is directly connected to a single host (e.g. PC). This feature allows a port which runs STP to go from Blocking mode (BLK) to Forwarding mode (FWD). Remember not to use this feature on connecting hubs, switches etc since it can cause a switching loop. You can enable portfast on an interface with the "spanning-tree portfast" command or you can enable it globally on the config mode of the switch and enter the "spanning-tree portfast default" command.
Uplinkfast is used when a switch is connected to another bridging device such as to another switch. If the root bridge goes down, the port that goes to the alternate port goes from BLK mode to FWD mode in a few seconds. Remember that uplinkfast cannot be configured on a ROOT switch. Besides that, uplinkfast is enabled globally, so you cannot enable it on a port only. If the original root port goes up again, then the switch uses the formula : (2xFwdDelay + 5 seconds) before the original root port goes up again.
A non root switch cannot be a root switch in times when the root switch goes down because of 2 reasons which are:
1. The switch priority will be set to 49.152 (remember the default priority is 32768)
2. STP Port Costs to the particular switch will be increased by 3000
Backbonefast is used on the core switches because it can recover the switches from an indirect link failure. This means that if the switch detects a failure on a link that is NOT directly connected to the core switch, this feature goes on. Backbonefast applies when the core switch receives an inferior BPDU. An inferior BPDU is a BPDU that is received by the secondary root switch on a inferior switch telling that he is the actually root switch. (it can be known that the link between the route switch and the secondary root switch is broken).
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