IP SLA Timers/Flapping

[Joshuabowers]

Hello,

Today I was expermenting with IPSLA in GNS3.

During my intial experment my IP SLA kept flapping during failover. I fixed that with a source interface command. Then I noticed IP SLA was passing/failing/pass/fail/pass at 60 second intervals. I fixed that with the following screenshot. Can someone explain line by line what the Threshold/timeout/ and other timers do exactly? It seems like I fixed my issue, but if I put this in production I dont want to cause a mini Ping DDOS attack on my ISP. In the end, my SLA should ping every 60 seconds without fail. The defaults seem to timeout my primary route before that happens and fails.

[Julio E. Moisa · Accepted solution]

Hi Joshua,

Threshold: is used to calculate the monitoring statistics configured for a SLA, the time is in miliseconds (1000 miliseconds = 1 seg). Basically it is used to monitor the behavior and store information about the events. 

Timeout: a time in milisecons as well, where the SLA is waiting for a response of a requested packet, it should not be a higher value than the frequency.  It is basically the maximun time to complete a SLA operation. You can see the round trip time through show ip sla monitor statistics

Frequency: A time in seconds, Sets the rate at which a specified IP SLAs operation repeats. 

You can use show ip sla statistics to see the SLA behavior. 

It will not generate any problem you could use a frequency of 60 in order to be for example making ping each 60 seconds.

The following is recommended by Cisco:

• (frequencyseconds ) > ((timeoutmilliseconds ) + N)

• (timeoutmilliseconds ) > (thresholdmilliseconds )

where N = (num-packetsnumber-of-packets ) * (intervalinterpacket-interval ).

Reference: https://www.cisco.com/c/en/us/td/docs/ios-xml/ios/ipsla/command/sla-cr-book/sla_s2.html#wp3249187301

Hope it is useful.

:-)

[chad patterson]

From that same document, those are the recommend values for IP SLAs UDP jitter operation. But for all other IP SLA the recommended value does not take value of N into account, and it is simply:

(frequency seconds ) > (timeout milliseconds ) > (threshold milliseconds )

Same resource:

 https://www.cisco.com/c/en/us/td/docs/ios-xml/ios/ipsla/command/sla-cr-book/sla_s2.html#wp3249187301 

Consider the following guidelines before configuring the frequency (IP SLA), timeout (IP SLA), andthreshold (IP SLA) commands. For the IP SLAs UDP jitter operation, the following guidelines are recommended:

• (frequency seconds ) > ((timeout milliseconds ) + N)

• (timeout milliseconds ) > (threshold milliseconds )

where N = (num-packets number-of-packets ) * (interval interpacket-interval ). If you are running Cisco IOS IP SLAs Engine 3.0, use the num-packets command and the interval (params) commands to configure the values that define N. Otherwise, use the udp-jitter command to configure the num-packets number-of-packets and interval interpacket-interval values.

For all other IP SLAs operations, the following configuration guideline is recommended:

(frequency seconds ) > (timeout milliseconds ) > (threshold milliseconds )