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ltp_55848's avatar
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Dec 09, 2010

Non-Redundant GTM Pair and DNS

Forgive me if this is a stupid question, but after reading and re-reading the manuals and searching through the forums I'm still stumped.



I currently have four BIGIP 3900 units, two each at separate sites configured as a HA LTM pairs and one unit at each site has been licensed with a GTM module (I'm well-aware of the specific advice against asynchronously licensed HA pairs). The GTMs have been configured as a ConfigSync group.



The objective is to have wideip requests routed to the topologically closest resource to the requester and also to fail over to the remote site. I've managed to configure two separate wideips to this end and both are working as expected, but I've hit a stumbling block in that the SOA for the delegated subdomain can specify only a single server. I understand that normally this wouldn't be a problem for a HA GTM pair as the SOA would refer to a floating IP address, but in this scenario where the GTM's are not in a HA pair, it's proving to be a single failure point.



Does anyone have a suggestion how it may be possible to work around this problem or should I just accept that this is an unavoidable limitation of the design?





1 Reply

  • I think we need to clarify a bit what you are attempting to accomplish.



    If your desire is to have the DNS requests routed to a specific GTM then that will pose a difficulty.




    If, however, you want to have the resources (the actual website, for example) to which the DNS records point distributed by topology with the possibility of failover to the other side in the event of failover then you will have exactly the same wideip at both sites.




    For example if I have two datacenters with the same website, one in the UK ( and one in the US ( and wanted to distribute them by topology so that EMEA users go to the UK and all others go to the US as long as both sites are up, but fail to the other site in the event of failover then I would use the following configuration:






    (set up the topology record for EMEA following the documentation)




    primary lb method: topology


    secondary method: global availability


    virtual server <-- the US virtual server will be used by all who do not match the EMEA topo record


    virtual server






    This wideip would be exactly the same on both GTMs.




    Now, when a user makes a DNS request they will be directed to the the UK resource if they are from EMEA but all others will use the US virtual. If for some reason either virtual server is unavailable, all users will be directed to the remaining available site.




    This DOES have the implication that US users may make their DNS requests against the UK GTM. That is unavoidable. But it is a necessary evil if one considers that if the US GTM is down they need to be able to resolve against the one in the UK or your site will be entirely unavailable.