Hello Florian!
Let’s begin with Cisco’s explanation and we’ll go from there. Cisco says that this command:
Sets the priority level used to select the active router in an HSRP group. The level range is from 0 to 255. The default is 100. Optionally, sets the upper and lower threshold values used by vPC to determine when to fail over to the vPC trunk. The lower-value range is from 1 to 255. The default is 1. The upper-value range is from 1 to 255. The default is 255.
An example of this command is the following:
switch1(config-if-hsrp)# priority 60 forwarding-threshold lower 40 upper 50
(Please note, when I refer to “switch” in the following paragraphs, I am referring to an L3 switch.)
Keep in mind that the forwarding-threshold keyword is used as part of the priority command. The priority command is used to determine which router will be the active router. The addition of the forwarding-threshold keyword is used in conjunction with vPC (Virtual Port Channel). vPC is a feature that is available on the Cisco Nexus series switches and allows the creation of a “Virtual” port channel where the physical ports of the port channel can span two switches that are functioning as an HSRP group. In such a virtual port channel, under normal conditions, vPS forwards traffic to both the active and standby switches with ports participating in the vPC.
The purpose of the forwarding-threshold keyword and its configuration parameters is to determine when a switch participating in HSRP/vPC is considered “down” so that the ports in the vPC will forward traffic only to the “good” switch. If the standby router priority falls below the lower threshold, HSRP sends all standby router traffic accross the vPC trunk to forward through the active HSRP router. HSRP maintains this scenario until the standby HSRP router priority increases above the upper threshold.
Keep in mind that the priority of an interface on a switch can dynamically change based on the Object Tracking functionality of HSRP. Take a look at this for more information: http://www.cisco.com/c/en/us/td/docs/switches/datacenter/sw/5_x/nx-os/unicast/configuration/guide/l3_cli_nxos/l3_hsrp.html#17650
I hope this has been helpful!
Laz