| # Reference Solution | |
| ## Step 1 — Identify the degraded node | |
| ```bash | |
| nodetool status | |
| # → node1 (10.10.1.2) shows DN (Down/Normal) | |
| ``` | |
| ## Step 2 — Diagnose: cluster NIC is down | |
| ```bash | |
| ssh node1 ip link show enp6s0f1 | |
| # → state DOWN — the cluster-internal NIC is offline | |
| ``` | |
| The management NIC (`enp1s0f0`) is still up (SSH works), but node1 lost its cluster gossip link on `enp6s0f1`. | |
| ## Step 3 — Restore the NIC | |
| ```bash | |
| ssh node1 sudo ip link set enp6s0f1 up | |
| ``` | |
| Cassandra gossip will detect the NIC coming back and automatically mark node1 as UN within ~30–60 seconds. | |
| ## Step 4 — Confirm node rejoined | |
| ```bash | |
| nodetool status | |
| # → All 3 nodes UN | |
| ``` | |
| ## Step 5 — Repair stale data | |
| While node1 was isolated, writes at QUORUM consistency went to node0 and node2. node1 missed those mutations. | |
| ```bash | |
| ssh node1 nodetool repair bench | |
| # Wait for: "Repair completed successfully" | |
| ``` | |
| ## Step 6 — Verify consistency | |
| ```bash | |
| cqlsh 10.10.1.2 -e "CONSISTENCY ONE; SELECT * FROM bench.kv;" | |
| # → Should show 5 rows (ids 1–5, including delta and epsilon written during outage) | |
| ``` | |
| ## Root Cause Summary | |
| Physical NIC failure on node1's cluster interface (`enp6s0f1`) caused the node to become unreachable on the Cassandra gossip/storage network (10.10.1.x). The node continued to function (OS up, management SSH accessible) but was invisible to the cluster. Writes at QUORUM consistency proceeded with 2/3 replicas (node0 + node2), leaving node1 with a stale copy. Bringing the NIC back up restores gossip, and `nodetool repair` performs Merkle-tree-based anti-entropy sync to reconcile the divergent data. | |