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WWT_BIGIP_99228's avatar
WWT_BIGIP_99228
Icon for Nimbostratus rankNimbostratus
Nov 14, 2013

Insight into GTM Preferred vs. Alternate

I have a configuration under the GTM > Topology section in which I have two topologies each to one DC. I have DC1 weighted higher than DC2 as it is our primary datacenter. Under the GTM Pool, the load balancing algorithm for preferred is set to Topology based so I would expect, when using nslookup, to always see myself getting sent to DC1. The alternate is set to Round Robin and Fallback is return to DNS. Under statistics and using nslookup i see myself sporadically being sent to DC2 and under statistics I do see that the Alternate method is being triggered. My question is why? What does preferred actually mean? Is it a definite? If I set both preferred and alternate to Topology I get the desired behavior of always being sent to DC1 (Unless it is down in a worst case scenario). If someone could give me some insight into this, that would be great! Thanks, WWTBIGIP

 

availability
big-ip global traffic manager
big-ip gtm
design
  • Matt_Dierick's avatar
    Matt_Dierick
    Icon for Employee rankEmployee
    Nov 18, 2013

    Hi,

     

    Preferred: You can select either a static or a dynamic load balancing mode from this list. Topology in your case.

     

    Alternate: You can select only a static load balancing mode from this list, because dynamic load balancing modes, by definition, rely on metrics collected from different resources. If the preferred load balancing mode does not return a valid resource, it is likely that Global Traffic Manager was unable to acquire the proper metrics to perform the load balancing operation. By limiting the alternate load balancing method to static load balancing modes only, Global Traffic Manager can better ensure that, should the preferred method prove unsuccessful, the alternate method returns a valid result.

     

    If you see stats on Alternate, that means your topology does not match your Topology records. Topology is based on querier Ip address --> your local DNS.

     

    Are U sure you enter in the same Pool every time ??? You can have several pools in one WideIP.