Uplink Fast Access Layer Uplinks

Consider an access-layer switch that has redundant uplink connections to two distribution-layer switches. Normally, one uplink would be in the Forwarding state and the other would be in the Blocking state. If the primary uplink went down, up to 50 seconds could elapse before the redundant uplink could be used.

The UplinkFast feature on Catalyst switches enables leaf-node switches or switches at the ends of the spanning-tree branches to have a functioning root port while keeping one or more redundant or potential root ports in Blocking mode. When the primary Root Port uplink fails, another blocked uplink immediately can be brought up for use.

TIP Many Catalyst switches have two built-in, high-speed uplink ports (Gigabit Ethernet, for example). You might get the idea that UplinkFast can only toggle between two leaf-node uplink ports. This is entirely untrue. UplinkFast keeps a record of all parallel paths to the Root Bridge. All uplink ports but one are kept in the Blocking state. If the Root Port fails, the uplink with the next-lowest Root Path Cost is unblocked and used without delay.

To enable the UplinkFast feature, use the following global configuration command: Switch(config)# spanning-tree uplinkfast [max-update-rate pkts-per-second]

When UplinkFast is enabled, it is enabled for the entire switch and all VLANs. UplinkFast works by keeping track of possible paths to the Root Bridge. Therefore, the command is not allowed on the Root Bridge switch. UplinkFast also makes some modifications to the local switch to ensure that it does not become the Root Bridge and that the switch is not used as a transit switch to get to the Root Bridge. In other words, the goal is to keep UplinkFast limited to leaf-node switches that are farthest from the Root.

First, the switch's bridge priority is raised to 49,152, making it unlikely that the switch will be elected to Root Bridge status. The port cost of all local switch ports is incremented by 3,000, making the ports undesirable as paths to the root for any downstream switches.

The command also includes a max-update-rate parameter. When an uplink on a switch goes down, UplinkFast makes it easy for the local switch to update its bridging table of MAC addresses to point to the new uplink. However, UplinkFast also provides a mechanism for the local switch to notify other upstream switches that stations downstream (or within the access layer) can be reached over the newly activated uplink.

The switch accomplishes this by sending dummy multicast frames to destination 0100.0ccd.cdcd on behalf of the stations contained in its Content-Addressable Memory (CAM) table. The MAC addresses are used as the source addresses in the dummy frames, as if the stations actually had sent them. The idea is to quickly send the multicast frames over the new uplink, giving upstream hosts a chance to receive the frames and learn of the new path to those source addresses.

These multicast frames are sent out at a rate specified by the max-update-rate parameter in packets per second. This limits the amount of bandwidth used for the dummy multicasts if the CAM table is quite large. The default is 150 packets per second (pps), but the rate can range from 0 to 65,535 pps. If the value is 0, no dummy multicasts are sent.

TIP You can use the following command to display the current status of STP UplinkFast:

Switch# show spanning-tree uplinkfast

UplinkFast is enabled

Station update rate set to 150 packets/sec. UplinkFast statistics

Number of transitions via uplinkFast (all VLANs) : 2

Number of proxy multicast addresses transmitted (all VLANs) : 52 Name Interface List

VLAN0001 VLAN0010 VLAN0100 Switch#

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