IAS Cluster problem

I have a problem when I install IAS on Microsoft cluster.
When the radius client sends requests to the virtual IP of the cluster
its get the responses from the 'real' IP of the active IAS server.
This couse that the client dont accept the answer.
E.g.:
Access request: 10.10.1.21 --> 10.10.1.100 (100 = virtual IP)
Access response: 10.10.1.101 --> 10.10.1.21 (101 = 'real IP)

Any ideas how to solve it?

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