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Redundancy of Servers using nat/SLB by checking server aliveness using track and IP sla

My requirements are:--

1. I want to access Machine_2, Machine_3 i.e., in a redundancy mode i.e. Active and passive mode.

2. But always request goes to active say and once this is down then only requests should go to passive say

3. These servers ( & should be visible to outside world over a virtual IP say

4. Means clients from can access these redundant server using V_IP (


I want some solution/configuration for this problem using cisco L3 switches or routers. Preferably either using nat or SLB.

I had tried, NAT and SLB both but I found some limitations to achieve this requirements.

My observations:--

1. With SLB its difficult to achieve Active/passive kind of redundancy, whereas it's purely working in load balance  fashion. We can only define weight for servers and minimize the requests hitting it. Available Weight value is <1-255> 

2. With NAT I am not able to create nat table for request originating from outside to inside. Once request start from inside to outside then only nat table is creating and lasting for 24 hours i.e default time-out for nat table. If in between primary server goes down, then also nat table is not creating for secondary server unless nat table is forcefully deleted on router.

​So please suggest me some solution to achieve redundancy from outside to inside servers by analyzing their reach-ability in Active/passive mode.