IOS SLB and probe failure

Hello,
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 192.168.197.8 (HSSAT1-LX) has changed to PROBE_FAILED” and few seconds later “REAL 192.168.197.8 (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 192.168.197.8 99
reassign 2
inservice
real 192.168.197.9 99
reassign 2
inservice
ip slb vserver HS.SAT1.DE
virtual xxx.xxx.xxx.xxx 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
Stefan

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
http://www.cisco.com/en/US/products/hw/switches/ps708/products_installation_and_configuration_guide_chapter09186a00801c5899.html#1024967
Hope that helped.
Best Regards,
Joerg

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