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UplinkFast is a Cisco specific feature that improves the convergence time of the Spanning-Tree Protocol (STP) in the event of the failure of an uplink. 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. The UplinkFast feature is designed to run in a switched environment when the switch has at least one alternate/backup root port (port in blocking state), that is why Cisco recommends that UplinkFast be enabled only for switches with blocked ports, typically at the access-layer. Do not use on switches without the implied topology knowledge of a alternative/backup root link typically to distribution and core switches in Cisco multilayer design.
This diagram illustrates a typical redundant network design. Users are connected to an access switch. The access switch is dually attached to two core, or distribution, switches. As the redundant uplink introduces a loop in the physical topology of the network, the Spanning-Tree Algorithm (STA) blocks it.
In the event of failure of the primary uplink to core switch D1, the STP recalculates and eventually unblocks the second uplink to switch D2, therefore it restores connectivity. With the default STP parameters, the recovery takes up to 30 seconds, and with aggressive timer tuning, this lapse of time can be reduced to 14 seconds. The UplinkFast feature is a Cisco proprietary technique that reduces the recovery time further down to the order of one second.
This document details how the standard STP performs when the primary uplink fails, how UplinkFast achieves faster reconvergence than the standard reconvergence procedure, and how to configure UplinkFast. This document does not cover the basic knowledge of STP operation. Refer to Understanding and Configuring Spanning Tree Protocol (STP) on Catalyst Switches in order to learn more about STP operation and configuration:
Switch A considers its link to D2, which still receives BPDUs from the root, as an alternate root port. Bridge A can start to transition port P2 from the blocking state to the forwarding state. In order to achieve this, it has to go through the listening and learning stages. Each of these stages last forward_delay (15 seconds by default), and holds port P2 blocking for 30 seconds.
The minimum value allowed for the forward_delay timer is seven seconds. Tuning the STP parameters can lead to a recovery time of 14 seconds. This is still a noticeable delay for a user, and this kind of tuning should be done with caution. This section of this document shows how UplinkFast dramatically reduces the downtime.
The UplinkFast feature is based on the definition of an uplink group. On a given switch, the uplink group consists in the root port and all the ports that provide an alternate connection to the root bridge. If the root port fails, which means if the primary uplink fails, a port with next lowest cost from the uplink group is selected to immediately replace it.
On a given bridge, the root port and all blocked ports that are not self-looped form the uplink group. This section describes step-by-step how UplinkFast achieves fast convergence with the use of an alternate port from this uplink group.
Note: UplinkFast only works when the switch has blocked ports. The feature is typically designed for an access switch that has redundant blocked uplinks. When you enable UplinkFast, it is enabled for the entire switch and cannot be enabled for individual VLANs.
Once UplinkFast has achieved a fast-switchover between two uplinks, the Content-Addressable Memory (CAM) table in the different switches of the network can be momentarily invalid and slow down the actual convergence time.
The backup link is brought up so quickly, however, that the CAM tables are no longer accurate. If S sends a packet to C, it is forwarded to D1, where it is dropped. Communication between S and C is interrupted as long as the CAM table is incorrect. Even with the topology change mechanism, it can take up to 15 seconds before the problem is solved.
In the event of failure of the primary uplink, a replacement is immediately selected within the uplink group. What happens when a new port comes up, and this port, in accordance with STP rules, should rightfully become the new primary uplink (root port)? An example of this is when the original root port P1 on switch A goes down, port P2 takes over, but then port P1 on switch A comes back up. Port P1 has the right to regain the root port function. Should UplinkFast immediately allow port P1 to take over and put P2 back in blocking mode?
The best solution is to keep the current uplink active and hold port P1 blocked until port P3 begins forwarding. The switchover between port P1 and port P2 is then delayed by 2*forward_delay + 5 seconds (which is 35 seconds by default). The five seconds leave time for other protocols to negotiate, for example, DTP of EtherChannel.
When the primary uplink comes back up, it is first kept blocked for about 35 seconds by uplinkfast, before it is immediately switched to a forwarding state, as was explained previously. This port is not able to do another uplinkfast transition for roughly the same period of time. The idea is to protect against a flapping uplink that keeps triggering UplinkFast too often, and can cause too many dummy multicasts to be flooded through the network
In order to be effective, the feature needs to have blocked ports that provides redundant connectivity to the root. As soon as Uplink Fast is configured on a switch, switch automatically adjusts some STP parameters in order to help achieve this:
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.
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.
At this point, the primary uplink is manually plugged in and put back up. You can see that the UplinkFast feature forces the port into a blocking mode, whereas usual STP rules have put it in listening mode. At the same time, port that connects to D2, which should go immediately into blocking mode according to the standard STP, is kept in forwarding mode. UplinkFast forces the current uplink to stay up until the new one is fully operational:
Use the set spantree uplinkfast disable command in order to disable UplinkFast. Only the feature is disabled when this command is issued. All the tuning that is done on the port cost and switch priority remains unchanged:
Use the no spanning-tree uplinkfast command in order to disable UplinkFast. In Cisco IOS switches, unlike CatOS switches, all the tuning that is done on the port cost and switch priority revert to the old values automatically at this point:
The UplinkFast feature dramatically decreases the convergence time of the STP in the event of the failure of an uplink on an access switch. UplinkFast interacts with other switches that have a strict standard STP. UplinkFast is only effective when the configured switch has some non-self-looped blocked ports. In order to increase the chances to have blocked ports, the port cost and bridge priority of the switch are modified. This tuning is consistent for an access switch, but is not useful on a core switch.
UplinkFast only reacts to direct link failure. A port on the access switch must physically go down in order to trigger the feature. Another Cisco proprietary feature, Backbone Fast, can help to improve convergence time of a bridged network in case of indirect link failure.
The optical ground station (OGS) is connected either to one arm of a source of polarization entangled photon pairs (E91) or to a pulsed laser with randomly chosen polarization and mean photon number for each pulse (DSP). The signal photons are transmitted to the CubeSat in a 500 km low-earth orbit (LEO) via a free-space link. OGS and CubeSat point beacon lasers at each other for precise attitude control. The quantum signal is analyzed on board the CubeSat using a randomly switched half-wave plate (HWP) and a polarizing beam splitter (PBS). Measurement outcome, basis choice and time tag of each event are recorded. Information about the latter two is transmitted to the OGS using a classical radio frequency (RF) link. The OGS identifies the matching bits using a cross-correlation analysis (\(g^(2)\)) and tells the CubeSat which ones to use. Both disregard the other bits, perform post-processing and then share a secret key. 041b061a72