GTM - Topology load balancing failover when one pool is down

Cumulonimbus

Dec 09, 2022

give the BIGIP version, and the configuration of GTM wideip and gslb pool. In wideip Advanced>>Load Balancing Decision Log, Check all the options. You can see why this pool member is selected in the /var/log/ltm

After testing, the reason why wideip cannot rollback and skip tcp down gslb pool is related to the settings of Preferred, Alternate, and Fallback in gslb pool.
1. If gslb pool_ A is manually disabled by the administrator, wideip will skip disabled gslb pool_A and automatically selected Status "up" gslb pool_ B
2. If gslb pool_ A all pool members are disabled by the administrator or all pool members tcp monitor down, wideip will automatically fallback to the Status "up" gslb pool_ B need to do this steps:

wideip name Load Balancing Method set "Topology", Pools is ["gslb pool_A", "gslb pool_B"]

gslb pool_A,pool_B Load Balancing Method, "Preferred" set Round Robin, "Alternate" set None, "Fallback" set Topology

root@(f5)(cfg-sync Standalone)(Active)(/Common)(tmos)# list gtm wideip a www.bestpay.com.cn
gtm wideip a www.bestpay.com.cn {
    aliases {
        mapi.bestpay.com.cn
    }
    load-balancing-decision-log-verbosity { pool-selection pool-traversal pool-member-selection pool-member-traversal }
    pool-lb-mode topology
    pools {
        gslb_pool_bestpay_ctc_v4 {
            order 0
        }
        gslb_pool_bestpay_cuc_v4 {
            order 1
        }
    }
    topology-prefer-edns0-client-subnet enabled
}



root@(f5)(cfg-sync Standalone)(Active)(/Common)(tmos)# list gtm pool a gslb_pool_bestpay_ctc_v4 
gtm pool a gslb_pool_bestpay_ctc_v4 {
    alternate-mode none
    fallback-mode topology
    members {
        DC-2-GTM-ipv4:/Common/vs_ctc_97_22 {
            disabled
            member-order 0
        }
        DC-2-GTM-ipv4:/Common/vs_ctc_97_23 {
            disabled
            member-order 1
        }
    }
}
root@(f5)(cfg-sync Standalone)(Active)(/Common)(tmos)# list gtm pool a gslb_pool_bestpay_cuc_v4 
gtm pool a gslb_pool_bestpay_cuc_v4 {
    alternate-mode none
    fallback-mode topology
    members {
        DC-2-GTM-ipv4:/Common/vs_ctc_98_22 {
            member-order 0
        }
        DC-2-GTM-ipv4:/Common/vs_ctc_98_23 {
            member-order 1
        }
    }
}

Mike757

MVP

Dec 09, 2022

Hi xuwen,

So fallback-mode within the pools as "none" replicates the issue?

To me this makes no sense. We're talking about the LB mode within the wide-ip to choose the correct pool. The LB mode within the pools (and the fallback mode, at that!) should have no effect on that. Do you agree?

/Mike

xuwen

Cumulonimbus

Dec 09, 2022

In wideip Advanced>>Load Balancing Decision Log, Check all the options. You can see why this pool member is selected in the /var/log/ltm

Overview of BIG-IP DNS Topology records (11.x - 17.x) (f5.com)

After testing with BIGIP VE, when the wideip LB mode is topology, it only considers the following bound gtm pool to calculate the corresponding score according to the topology algorithm. gtm pool A is 200 points, and gtm pool B is 20 points, so the gtm pool A with 200 points is highest selected, regardless of the up/down status of the gtm pool at wideip level(when the administrator disables the gtm pool, will skip the gtm pool disabled by the administrator and directly select the next gtm pool)
If the fallback of the gtm pool is set to None or Topology, gtm pool A Preferred Round Robin failed because no active pool members are available at the gtm pool level, and "Alternate" set None, "Fallback" set None or Topology, it will fallback to topology choose lower points status "up" gtm pool pool_B

If the fallback is set to Return to DNS, it will fallback to dns bind to resolution, and all pool members in gtm pool A and pool B will be returned as A records(such as A1,A1, B1, B2)

fallback set "None", log:

[mapi.bestpay.com.cn A] 
[matched topology record (ldns:(region:/Common/all_ctc), server:(region:/Common/Region_CTC_GSLB_Pool), score:2000) to pool (gslb_pool_bestpay_ctc_v4)] 
[matched topology record (ldns:(region:/Common/Other_Region), server:(region:/Common/Region_CUC_GSLB_Pool), score:5) to pool (gslb_pool_bestpay_cuc_v4)] 
[topology selected pool (gslb_pool_bestpay_cuc_v4) - topology score (5) is higher] 
[topology selected pool (gslb_pool_bestpay_ctc_v4) - topology score (2000) is higher] 
[topology selected pool (gslb_pool_bestpay_ctc_v4) with the highest topology score (2000)] 
[topology selected pool (gslb_pool_bestpay_ctc_v4)] 
[pool member select check failed (vs_ctc_97_23:58.213.97.23) - pool member is disabled] 
[pool member select check failed (vs_ctc_97_22:58.213.97.22) - pool member is disabled] 
[round robin failed to select a pool member] 
[failed to select pool member by preferred load balancing method] 
[Using none load balancing method] 
[failed to select pool member by alternate load balancing method] 
[Using none load balancing method] 
[failed to select pool member by fallback load balancing method] 
[topology selected pool (gslb_pool_bestpay_cuc_v4) - topology score (5) is higher] 
[topology selected pool (gslb_pool_bestpay_cuc_v4) with the highest topology score (5)] 
[topology selected pool (gslb_pool_bestpay_cuc_v4)] 
[pool member check succeeded (vs_ctc_98_23:58.213.98.23) - pool member state is available (green)] 
[round robin selected pool member (vs_ctc_98_23:58.213.98.23)]

fallback set "Topology", log:

[mapi.bestpay.com.cn A] 
[matched topology record (ldns:(region:/Common/all_ctc), server:(region:/Common/Region_CTC_GSLB_Pool), score:2000) to pool (gslb_pool_bestpay_ctc_v4)] 
[matched topology record (ldns:(region:/Common/Other_Region), server:(region:/Common/Region_CUC_GSLB_Pool), score:5) to pool (gslb_pool_bestpay_cuc_v4)] 
[topology selected pool (gslb_pool_bestpay_ctc_v4) - topology score (2000) is higher] 
[topology skipped pool (gslb_pool_bestpay_cuc_v4) - topology score (5) is not higher] 
[topology selected pool (gslb_pool_bestpay_ctc_v4) with the highest topology score (2000)] 
[topology selected pool (gslb_pool_bestpay_ctc_v4)] 
[pool member select check failed (vs_ctc_97_23:58.213.97.23) - pool member is disabled] 
[pool member select check failed (vs_ctc_97_22:58.213.97.22) - pool member is disabled] 
[round robin failed to select a pool member] 
[failed to select pool member by preferred load balancing method] 
[Using none load balancing method] 
[failed to select pool member by alternate load balancing method] 
[pool member select check failed (vs_ctc_97_22:58.213.97.22) - pool member is disabled] 
[pool member select check failed (vs_ctc_97_23:58.213.97.23) - pool member is disabled] 
[topology failed to select a pool member] 
[failed to select pool member by fallback load balancing method] 
[topology selected pool (gslb_pool_bestpay_cuc_v4) - topology score (5) is higher] 
[topology selected pool (gslb_pool_bestpay_cuc_v4) with the highest topology score (5)] 
[topology selected pool (gslb_pool_bestpay_cuc_v4)] 
[pool member check succeeded (vs_ctc_98_22:58.213.98.22) - pool member state is available (green)] 
[round robin selected pool member (vs_ctc_98_22:58.213.98.22)] 
[root@f5:Active:Standalone] config #

fallback set "Return to DNS", log:

[mapi.bestpay.com.cn A] 
[matched topology record (ldns:(region:/Common/all_ctc), server:(region:/Common/Region_CTC_GSLB_Pool), score:2000) to pool (gslb_pool_bestpay_ctc_v4)] 
[matched topology record (ldns:(region:/Common/Other_Region), server:(region:/Common/Region_CUC_GSLB_Pool), score:5) to pool (gslb_pool_bestpay_cuc_v4)] 
[topology selected pool (gslb_pool_bestpay_cuc_v4) - topology score (5) is higher] 
[topology selected pool (gslb_pool_bestpay_ctc_v4) - topology score (2000) is higher] 
[topology selected pool (gslb_pool_bestpay_ctc_v4) with the highest topology score (2000)] 
[topology selected pool (gslb_pool_bestpay_ctc_v4)] 
[pool member select check failed (vs_ctc_97_23:58.213.97.23) - pool member is disabled] 
[pool member select check failed (vs_ctc_97_22:58.213.97.22) - pool member is disabled] 
[round robin failed to select a pool member] 
[failed to select pool member by preferred load balancing method] 
[Using none load balancing method] 
[failed to select pool member by alternate load balancing method] 
[selected configured option Return To DNS]

MH
Altocumulus
Dec 12, 2022
Hello,
Thank you for your help on this. I will test this set-up with three pools to verify that that the topology rules with lower priority are actually being used to select the correct pools.
Thanks,
Michael

MH
Altocumulus
Dec 12, 2022
Hello,
I found documentaiton that if we set the Alternate and Fallback to None, then it will fall back and use the next avaiable pool. This is a situation I am now testing. It woudl work in a situation with only 2 load balancing pool. With more than 3 load balancing pools, this is not ideal, as it does not give you explicit control over the pool that is then selected as it says simply that it selects the next avaiable pool. So this is likely to be the based on the order of the pools associated with the Wide IP.
Thanks,
Michael
- xuwen
  Cumulonimbus
  Dec 12, 2022
  You set the GTM pool fallback mode to topology for test. I did not test more than 3 gtm pools. If the effect is not good, you can use the gtm iRules in the wideip, and select the correct gtm pool according to its status up status

MH
Altocumulus
Dec 12, 2022
Hello,
We had the loggin already enabled and we use dthis to verify that the topology load balancing was selecting the pool with al lmemembers down:
[pool member check failed (Yyyyyyyy_RAS:y.y.y.y)]
[pool member (Yyyyyyyy_RAS:y.y.y.y) deleted persistence (y.y.y.y)]
[matched topology record (ldns:(region:/Common/Japan_RAS_VPN_region), server:(pool:/Common/Geneva_RAS_VPN_Pool), score:20) to pool (Geneva_RAS_VPN_Pool)]
[matched topology record (ldns:(region:/Common/Japan_RAS_VPN_region), server:(pool:/Common/Japan_RAS_VPN_Pool), score:200) to pool (Japan_RAS_VPN_Pool)]
[topology selected pool (Geneva_RAS_VPN_Pool) - topology score (20) is higher]
[topology selected pool (Japan_RAS_VPN_Pool) - topology score (200) is higher]
[topology selected pool (Japan_RAS_VPN_Pool) with the highest topology score (200)]
[topology selected pool (Japan_RAS_VPN_Pool)]
[pool member select check failed (Zzzzzzzz_RAS_VPN:z.z.z.z) - pool member is disabled]
[pool member select check failed (Yyyyyyyy_RAS:y.y.y.y) - pool member is disabled]
[round robin failed to select a pool member]
[failed to select pool member by preferred load balancing method]
[selected configured option Return To DNS]
We were using the default "Return to DNS" as the faillback load balancing method. If the Fallback load balancing method as topology, would this not only apply the topology rules ot the pool members of the pool. But I have not tested what you have suggest. I will test it when I can.
Many thanks,
Michael
- xuwen
  Cumulonimbus
  Dec 12, 2022
  Fallback load balancing method set None or Topology, result is the same(choose a lower points state "up" gtm pool for wideip)
  The system will also check the up status of the gtm pool member when the persistence is enabled for wideip. Like LTM, if the pool member is down, it will be reselected

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