In an Instana Custom Edition deployment, the Cassandra data store cluster runs on a dedicated network segment separate from the Instana backend processing nodes. The network team must define firewall rules between these segments to enable backend operation. Which rule is required?

Cassandra uses TCP port 9042 as its native CQL (Cassandra Query Language) client port for all read and write operations. In a distributed Custom Edition deployment, the Instana backend processing nodes are Cassandra clients and must be able to reach the Cassandra cluster on port 9042 to write ingested telemetry data and execute data queries. Without this firewall rule, the backend cannot persist any observability data, resulting in complete data loss and a non-functional Instana installation. The correct answer is 'Allow TCP port 9042 inbound to Cassandra nodes originating from the Instana backend processing nodes'. The incorrect options — 'Allow TCP port 443 inbound to Cassandra nodes originating from monitored agent hosts', 'Allow TCP port 9200 inbound to Cassandra nodes originating from agent hosts for metric submission', 'Allow TCP port 8080 inbound to backend processing nodes originating from Cassandra for result callbacks' — are wrong because they do not align with IBM Instana's architecture or recommended practices for this scenario. Understanding this concept is essential for the Domain 3: Installation domain of the IBM Instana Observability certification.