The problems with externally loadbalancing splunk forwarders are tricky and not necessarily obvious.
The mechanism by which forwarders splay their load across N indexers relies upon them periodically reconnecting to different indexers. However if you use a loadbalancer, then the forwarders will not know that there are different indexers and will not reconnect. This means they will effectively remain glued to a single indexer permanently via your F5.
This has problems for balancing, since forwarders do not typically have equal data loads. It also has trouble for indexer lifecycle goals. If you shut down an indexer, forwarders will shift away from it. If you bring the indexer back up, forwarders will not shift back to it, because they do not know it exists.
Additionally, depending upon your configuration, forwarders performing health checks at the time an indexer goes offline or closes its port due to overload may be seen by other forwarders as an indication that their current link is not healthy, potentially leading to an exacerbated problem under high load.
If sending your forwarding data reliably through an external load balancer is important to you, please raise it officially with splunk staff via support channels (Enhancement Request). However for now I suggest you use the Splunk built-in load balancing.
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