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Reliable Backup Static Routing using Object Tracking

Hi All,

I've been tasked with making a proper failover on the a router running Cisco IOS 12.4

The connectivity consists of a T1 (serial) primary connection, and another firewall hosting a VPN over an internet connection for the backup connection.

I've built it out, and it all works great... except....

This T1 is a little schitzophrenic.  Every now and then it drops a packet or two (like 3 times a minute.)  This has no effect whatsoever on our primary use of this connection, which is telnet traffic for an AS/400.  

The problem is, the tracked SLA changes from up to down and back three times a minute.  Meaning it changes the routing three times a minute.  This kind of behaviour is VERY disruptive to the AS/400 traffic.

Here's my wish - I want the SLA to ONLY switch state if it loses say 10 consecutive pings.  I thought the answer was the "threshold" quantity on the SLA, but it not only seems to have no effect (state still changing) but much of my reading says it's connected to a "hysterisis" function - which I don't really understand.  Even if I crank the "threshold" up to ridiculous quantities (30000 say) it still logs the tracked object as changing state just as frequently.

The "frequency" is just how often the SLA pings.  I've increased this quantity too, but really it's like Russian roulette as to whether it gets a good ping or a bad ping when it goes off.

Can anyone tell me what I'm missing here?


1 Solution
what is your current config for the SLA now?

petranator2011Author Commented:
ip sla monitor 1
 type echo protocol ipIcmpEcho source-ipaddr
 timeout 1000
 threshold 30000
 frequency 15
ip sla monitor schedule 1 life forever start-time now

As I understand it, that should set the timeout on each ping to 1000ms, it should repeat every 15 seconds, and as I mentioned before - changing the frequency seems to have no affect on how the trackable object works at all.  Currently I have it set for 30000, whatever unit that is in.

either increase the frequency of the ping or the amount of the threshold

the threshold is in milliseconds

I'd try a frequency of 1, or a threshold of 150,000

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for clarity...

take the number of failed pings you want to trigger failover, multiply it by the ping frequency, then multiply is by 1000

so for 10 failed pings at 15 second intervals, 10 * 15 * 1000 = 150,000

This means you could be down for 150 seconds before failover

for your particular requirement, I would have a more frequent ping (1 per second) and have it lose no more than 30

so for 30 failed pings at 1 second intervals, 30 * 1 * 1000 = 30,000 this should be more appropriate for your T1

I would have called out a fault on your T1 a long time ago...

There is a better way of handling this that is slightly less dirty. Usually, your SLA is being used by a track object that you then apply to your static route.

On that track object, you can specify a delay down and delay up value.

Therefore, you can leave it pinging every second, with a 1 second timeout, and let your sla go down if it misses a ping. If you configure delay down 20 on your track object, if the SLA does not respond, the track object (and thus your route) will stay up unless the SLA continues to fail the ping for a concurrent period of 20 seconds.

If it comes back, the timer is reset back to 20.


ip sla monitor 1
 type echo protocol ipIcmpEcho source-ipaddr
 timeout 1000
 threshold 30000
 frequency 1
ip sla monitor schedule 1 life forever start-time now
track 1 ip sla 1 reachability
delay down 20
delay up 0

ip route track 1

This config will mean that the route will only disappear if the SLA is down for a concurrent 20 seconds
petranator2011Author Commented:
Thank you all.
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