Delay down timer & Frequency timer interaction with IP SLA during a failed SLA check

[Dean Romanelli]

Hi All,

So I have the following IP SLA configured:

ip sla 2
icmp-echo 75.xxx.xxx.82
timeout 1000
threshold 1000
frequency 5
ip sla schedule 2 life forever start-time now

track 101 ip sla 2 reachability
delay down 7 up 180

ip route 192.168.90.0 255.255.255.0 198.168.3.1 track 101

ip route 192.168.90.0 255.255.255.0 192.168.3.2 251

Let's assume the SLA check fails. The delay down timer then starts, which I have configured to fail over in 7 seconds if no packet replies are seen. I have my frequency timer in the SLA set to check status every 5 seconds.  How do the status pings work during those 7 seconds of delay? Will my device continuously ping the target every second (since my timeout is 1000ms) during that 7 second duration, or will it only send out another single status ping when the frequency timer comes up again in 5 more seconds? If it is the latter, then essentially the above configuration means if my SLA fails 2 consecutive sanity checks (i.e. 2 ping fails in a row total), then it will fail over.  Is that correct? Below I've typed both theories out by the second to help with the concept:

Option A (In seconds):

1
2
3
4
5 ----> Check SLA status
6 ----> SLA Ping failed (on second 6 because timeout is 1000ms)
1 ----> Delay down timer of 7 seconds starts
2
3
4
5 ----> Check SLA status
6 ----> SLA Ping failed (on second 6 because timeout is 1000ms)
7 ----> Delay down timer expires ---> failover to backup route

-or-

Option B (In seconds):

1
2
3
4
5 ----> Check SLA status
6 ----> SLA Ping failed (on second 6 because timeout is 1000ms)
1 ----> Delay down timer of 7 seconds starts
2 ----> Ping target.
3 ----> No reply.
4 ----> Ping target.
5 ----> No reply.
6 ----> Ping target.
7 ----> No reply. Delay down timer expires. Failover to backup route.

[Jon Marshall · Accepted solution]

Dean

Without actually testing it my understanding is that when the ping fails you wait 7 seconds because of your delay timer.

Your frequency is 5 seconds so after the first ping fails you wait 5 seconds to send the next ping.

The timeout is not how often pings are sent, it is simply how long you are prepared to wait for a response to an ICMP request.

As far as I know whether the ping is successful or not it does not change the frequency of pings it sends and that is the key value here.

Jon

[Dean Romanelli]

Thanks Jon.  I had thought that to be the case as well.  Appreciate your reply.