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New Member

IOS SLB and probe failure


we use server-load-balancing with IOS 12.1(19)E1

We have a problem if the server receives more connections following error messages “REAL (HSSAT1-LX) has changed to PROBE_FAILED” and few seconds later “REAL (HSSAT1-LX) has changed to OPERATIONAL” appears and so on.

We checked the server and they works proper.

What could be the reason for probe failed?

My configuration:

ip slb probe HS-PROBE tcp

interval 5

ip slb serverfarm HSSAT1-LX

nat server

predictor leastconns

failaction purge

probe HS-PROBE

real 99

reassign 2


real 99

reassign 2


ip slb vserver HS.SAT1.DE

virtual tcp www

serverfarm HSSAT1-LX

advertise active

inservice standby allvips

How does a TCP probe works? – I could not find more exact information in the documents to configure probes.

Is it better to use another probe (icmp)? – or without any probe?

When does it make sense to use probes?

Best regards



Re: IOS SLB and probe failure

HI Stefan,

tcp probes do a complete TCP 3-way handshake and normaly terminate the session. A problem which I had some times timeout for a session to be established might be to short if the server is "heavy" loaded.

Probing on a specific method (TCP HTTP ...) is most of the times the better solution. Imagine a WEB-Server which is properly pingable but the httpd died due to some internal error. If you would probe on a per ping basis the loadbalancer will never notice this but if you monitor tcp-port 80 by a tcp probe or better a http probe you will notice this and the server would be taken out of the serverfarm. Even better but afaik not possible in IOS SLB is to probe a certain page e.g. index.html. As you know that the httpd is up and running and pages can be displayed.

Regarding the probing issue it might be usefull to read the follwing link describing healthmonitoring with the CSM

Hope that helped.

Best Regards,


New Member

Re: IOS SLB and probe failure

Hi Joerg,

Thanks for information, we will try a TCP HTTP probe.

Best regards


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