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Rajesh_07_16489's avatar
Rajesh_07_16489
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Jan 12, 2015

What happens when clients send request to downed DC

My scenario is, we have 2 DCs with pair of LTM at each DC and single GTM is deployed for load balancing the traffic across both the DCs and we configured Round Robin at GTM end to manage both the DCs in parallel way. Assume, we have configured http service in LTM and when clients do DNS query, GTM responded with 2nd DC’s LTM IP, which was healthy. However, after responding to clients, 2nd DC is down completely due to some reason. As clients have valid DNS responses, it would forward http request to 2nd DC’s LTM, which is down now. Moreover, http request also would be failed due to unavailability of 2nd DC. Do we have any mechanism at GTM end to effectively manage this scenario. Thanking you in advance for your help

 

application delivery
BIG-IP DNS
LTM

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  • StephanManthey's avatar
    StephanManthey
    Icon for MVP rankMVP
    Jan 12, 2015

    There is an option in GTM to send multiple A-records in a sorted order upon incoming DNS requests.

     

    Same mechanism is used by others for high availability. The client receives multiple answers, tries one and fails back to another after timeout.

     

    Please be aware, that the sorted order of records (having a prioritization of a DC in mind) is kind of useless, as they may be re-ordered by the client´s DNS resolver or in the client.

     

  • Rajesh_07_16489's avatar
    Rajesh_07_16489
    Icon for Nimbostratus rankNimbostratus
    Jan 12, 2015

    Thank you Stephan. In this case, we have to create multiple A records in Zone runner explicitly rather than creating Wide-IP.

     

    As client or client's LDNS choose active A-record by themself, Wide-IP level Intellegent DNS is not needed and creating multiple A records in Zone runner is only needed.

     

    Please advise, if I am wrong. Thank you again for your suport.

     

  • StephanManthey's avatar
    StephanManthey
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    Jan 12, 2015

    Hi Rajesh,

     

    new records in ZoneRunner will be created automatically for each new virtual server. No need to configure them manually.

     

    To return multiple resource records you modify the GTM pool configuration (pool has to hold at least two poolmembers for your configuration).

     

    Please navigate to DNS >> GSLB:Pools:Pool List and select the pool to be modified. Enable the "advanced" settings and modify the parameter "Maximum Addresses Returned" to the value of "2".

     

    From now both A-records (reflecting the 2 virtual servers in the 2 different data centers) will be returned as long as they are available. If GTM monitoring is recognizing a virtual server as unavailable, it will not longer be referenced.

     

    Thanks, Stephan