Sequence of events

Two Node cluster (Node0 and Node1) 
Gateway traffic is served by Node0
Node0 Goes down / manually set to Passive, Now Node1 handles the Gateway traffic
Node0 cones back to Active State, at this point Gateway traffic is affected.

On VMware ESX - Enable MAC spoofing (also make sure “Forged Transmits” is enabled) , or else the packets generated from cluster mac can be dropped by at the hypervisor level.

https://docs.citrix.com/en-us/citrix-adc/12-1/clustering/cluster-setup/cluster-setup-backplane.html

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Problem Cause

Analyzing the packet traces we see that when Node0 transitions to an active state, the client side packets arrived on Node0 and it steers (TX) the packets to Node1, however the trace running on Node1 does not show any of the steered packets on (RX).

Issue Identification: 

Below is the Node0 Trace:

• Looking at the SourceNode and DestinationNode (part of the nstrace header) we can identify a packet in incoming from client or steered.
• Src Node=-1 and Dst Node=0: Meaning incoming traffic to Node0
• Src Node=0 and Dst Node=1: Meaning steered traffic from Node0 to Node1, this traffic was not showing up on the simultaneous trace being run on Node1, this is also evident as the packets are getting re-transmitted on Node0
• Also note the Steered packets will be sourced from cluster MAC(**0x02 0x00 0x6F **)

https://docs.citrix.com/en-us/citrix-adc/12-1/clustering/cluster-setup/cluster-setup-backplane.html
https://docs.citrix.com/en-us/netscaler/12/clustering/cluster-overview/cluster-communication.html