When a cluster is Active/Passive the passive node interfaces do not pass any traffic. This is by design to prevent creating any layer 2 loops from any alternate paths created by having two devices serve the same traffic. Since the Active/Passive design is that only one firewall at a time is processing sessions, this generally does not create an issue. If you network design requires that traffic pass on the inactive node, then you will need to implement an Active/Active cluster. This is the case if you need dynamic routing protocols to traverse the inactive device or if you have multiple network paths setup by design and want the cluster to handle asymmetrical routing. It sounds like your network design method might require an Active/Active cluster deploy.
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