Setting uplinkfast on a Cisco switch

Written by on Tuesday, July 22, 2008 22:02 - 0 Comments

Setting uplinkfast on a cisco switch and what does it mean?

Cisco switches use uplink fast to improve the convergence time of the STP due to an uplink failure. The UplinkFast feature is supported on Cisco Catalyst 4500/4000, 5500/5000, and 6500/6000 series switches running CatOS. This feature is also supported on Catalyst 4500/4000 and 6500/6000 switches that run Cisco IOS® System Software and 2900 XL/3500 XL, 2950, 3550, 3560 and 3750 series switches. Uplink fast is only recommended to be used for switches with blocked ports, typically at the access-layer because it is designed to be run for a switch that has at least one backup root port. This feature is not for use in switches that do not have a backup root port. To enable Uplink fast on a CatOS switch use the A>(enable) set spantree uplinkfast enable command. To enable Uplink fast on an IOS use the A> (config)#spanning-tree uplinkfast command.

According to the Cisco website ( UplinkFast has the
following limitations:
Uplink fast does not do the fast transition during a High Availability supervisor switchover on 6500/6000 switches that run CatOS. When the root port is lost on failed-resetting supervisor, the situation after a switchover is similar to when the switch boots up the first time because you do not sync the root port information between Supervisors. High Availability (HA) maintains only spanning tree port state, not the root port information, so when the HA switchover occurs, the new sup has no idea that it has lost a port on one of the uplink ports of the failed supervisor. A common workaround is the use of a port channel (EtherChannel). Root port status is maintained when a Port Channel is built across both supervisors, 1/1-2/1 or 1/2-2/2, for example, or root port is on the port of any Line Card. As no spanning tree topology change occurs when failing-resetting the active supervisor, no UplinkFast transition is necessary

Uplink fast does not do the fast transition during an RPR or RPR+ switchover on a 6500/6000 switch that runs Cisco IOS System Software. There is no workaround because Layer 2 port must go through spanning tree convergence states of listening, learning, and forwarding.  Cisco IOS Software on 6500/6000 does not implement generation of dummy multicast packets after uplinkfast transition for the update of CAM tables, refer to Cisco bug ID CSCdm65881 ( registered customers only) .

Uplink fast implementation on gigastack of 2900/3500XL/2950/3550/3560/3750 is called Cross Stack Uplink Fast Feature (CSUF), general UplinkFast feature on gigastack setup is not supported.  CSUF does not implement generation of dummy multicast packets after UplinkFast transition for the update of CAM tables.

Do not change spanning tree priority on the switch when UplinkFast is enabled because, it depends on the platform, and it can cause UplinkFast feature to be disabled, or it can cause a loop as the UplinkFast feature automatically changes the priority to a higher value in order to prevent the switch from becoming Root Bridge. ')}

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