Hello Sebastian
The delay down timer will begin counting from the first failed ICMP echo. The IP SLA will consider an SLA failed if either the configured timeout or the configured threshold are exceeded. See the NetworkLessons Note on IP SLA parameters for more information about that.
So to be precise, the down timer will begin counting after the timeout or threshold values have been exceeded. If they are set to 1 second for example, then the delay down timer will start counting 1 second after the initial ICMP echo request was sent.
Based on this, it makes sense for you to set the delay timer to be 11 seconds since you want three echoes to fail before the SLA is considered failed. Let’s take a look at the process second by second, assuming a threshold of 1 second.
0 - echo request sent
1 - echo reply was not received, SLA fails, delay down timer is started
2
3
4
5 - echo request sent, delay down timer is at 4
6 - echo reply was not received, SLA fails, delay down timer is at 5
7
8
9
10 - echo request sent, delay down timer is at 9
11 - echo reply was not received, SLA fails, delay down timer is at 10
12 - delay down timer is at 11, the down delay has expired, SLA now requires a reaction event (i.e. any tracking associated with SLA is triggered).
You could theoretically set it to 10 seconds, but that would make the timer expire exactly the same time as the third echo reply fails, so leaving an extra second is helpful.
Note that if your threshold/timeout values are larger, then you will have to appropriately change your delay down timer as well to accommodate the slightly larger times needed to define an SLA as down.
I hope this has been helpful!
Laz