---
title: "Kafka network split inside one DC: ISR drops to the floor"
description: "A link fails inside DC-A, cutting broker-2 off from its peers. The Kafka ISR drops to exactly min.insync.replicas=3 and writes continue while DC-B stays whole."
url: https://monedula.dev/kafka-simulator/scenarios/intra-dc-partition-split/
---

# Kafka network split inside one DC: ISR drops to the floor

Scenario 09.1.6 · Intra-DC partition split. Disaster recovery · Advanced · about 3 min · Apache Kafka 4.3.

A link fails INSIDE one DC, splitting that DC’s own brokers across the partition - the ISR sheds the cut replica to the min.ISR floor while the other DC stays whole.

Run it: https://monedula.dev/kafka-simulator/playground/?scenario=09.1.6

## What you start with

Reusing the 2.5-DC MRC layout, this time the network failure lands inside one data centre: a single broker is cut off from its own DC peers and everything else. The controller quorum and the leader stay together in the majority, so only the isolated broker drops out of the ISR. We watch the ISR fall to exactly the min.insync.replicas floor while the other DC stays whole, then heal.

## Step by step

1. **Stretched cluster ready - DC-A & DC-B each 2 sync + 1 observer, witness DC-C** (t+0.000s) [Open at this step](https://monedula.dev/kafka-simulator/playground/?scenario=09.1.6&t=0)
2. **Record committed across both DCs (ISR = 4 sync replicas ≥ min.ISR 3)** (t+5.000s) [Open at this step](https://monedula.dev/kafka-simulator/playground/?scenario=09.1.6&t=1)
3. **Intra-DC link cut - DC-A splits internally; broker-2 leaves the ISR (now 3 = min.ISR)** (t+8.000s): A network partition lands INSIDE DC-A: broker-2 is cut off from its own DC peers (broker-1, broker-3) and the rest of the cluster, while DC-B stays whole. The fence renders inside the DC-A band, not between DCs. Every controller voter and the leader stay together, so the quorum and leadership are untouched - the active controller simply can no longer reach broker-2, which drops out of the ISR. The ISR falls to exactly the min.ISR floor of 3, so no observer is promoted. [Open at this step](https://monedula.dev/kafka-simulator/playground/?scenario=09.1.6&t=2)
4. **Surviving replicas keep committing - write acked on the shrunk ISR of 3** (t+12.000s) [Open at this step](https://monedula.dev/kafka-simulator/playground/?scenario=09.1.6&t=3)
5. **Healed - broker-2 catches up and rejoins the ISR (back to 4 sync replicas)** (t+14.000s) [Open at this step](https://monedula.dev/kafka-simulator/playground/?scenario=09.1.6&t=4)

## The takeaway

A partition need not fall on a DC boundary - an intra-DC link failure simply sheds the unreachable replica from the ISR. As long as the survivors still meet min.insync.replicas, writes continue at the floor with no observer promotion.

## Configs in this scenario

- [`min.insync.replicas`](https://kafka.apache.org/43/configuration/topic-configs/#topicconfigs_min.insync.replicas): The fewest in-sync replicas, leader included, that must have a write for an acks=all produce to succeed (default 1); with fewer in the ISR the leader rejects the write with NOT_ENOUGH_REPLICAS. [How min.insync.replicas works](https://monedula.dev/kafka-simulator/configs/min-insync-replicas/)

## Further reading

- [Apache Kafka documentation](https://kafka.apache.org/43/design/design/#replication)