diff --git a/tests/v1/ec_connector/unit/cpu/test_ec_shared_region.py b/tests/v1/ec_connector/unit/cpu/test_ec_shared_region.py
index 722c0d89fa..c168970147 100644
--- a/tests/v1/ec_connector/unit/cpu/test_ec_shared_region.py
+++ b/tests/v1/ec_connector/unit/cpu/test_ec_shared_region.py
@@ -13,6 +13,8 @@ from unittest.mock import MagicMock, patch
 import pytest
 import torch
 
+from vllm.config import ParallelConfig
+from vllm.distributed.ec_transfer.ec_connector.cpu import common
 from vllm.distributed.ec_transfer.ec_connector.cpu.ec_shared_region import (
     ECSharedRegion,
     _wait_for_file_size,
@@ -95,6 +97,32 @@ def test_second_instance_opens_existing_file_and_shares_memory():
         r1.cleanup()
 
 
+def test_dense_dp_engines_get_distinct_regions(monkeypatch):
+    """Dense DP engines reset data_parallel_rank to 0 but share instance_id,
+    so the region must be keyed by data_parallel_index."""
+    monkeypatch.setattr(common, "_get_encoder_cache_hidden_dim", lambda cfg: 8)
+    instance_id = str(uuid.uuid4())
+    regions = []
+    try:
+        for dp_index in range(2):
+            parallel_config = ParallelConfig(
+                data_parallel_size=2, data_parallel_rank=dp_index
+            )
+            parallel_config.reconfigure_for_independent_dp_rank()
+            cfg = MagicMock()
+            cfg.instance_id = instance_id
+            cfg.parallel_config = parallel_config
+            cfg.model_config.dtype = torch.float16
+            cfg.ec_transfer_config.ec_connector_extra_config = {"ec_cpu_bytes": 64}
+            regions.append(common.create_ec_shared_region(cfg))
+
+        assert all(r._is_creator for r in regions)
+        assert regions[0]._mmap_path != regions[1]._mmap_path
+    finally:
+        for r in regions:
+            r.cleanup()
+
+
 def test_only_creator_unlinks_file_on_cleanup():
     """Critical contract: if the non-creator unlinks, the creator's mmap path
     becomes a dangling backing file and a third opener would create a new one
diff --git a/tests/v1/worker/test_gpu_worker_weight_transfer.py b/tests/v1/worker/test_gpu_worker_weight_transfer.py
index e576ebd031..537653bda3 100644
--- a/tests/v1/worker/test_gpu_worker_weight_transfer.py
+++ b/tests/v1/worker/test_gpu_worker_weight_transfer.py
@@ -11,7 +11,7 @@ import pytest
 import torch
 import torch.nn as nn
 
-from vllm.config import VllmConfig, get_current_vllm_config
+from vllm.config import ParallelConfig, VllmConfig, get_current_vllm_config
 from vllm.lora.layers import BaseLayerWithLoRA
 from vllm.v1.worker.gpu_model_runner import _get_parameter_for_reload
 from vllm.v1.worker.gpu_worker import Worker
@@ -136,12 +136,22 @@ def test_rank_local_update_selects_worker_payload(rank, expected):
     assert worker._weight_update_active is True
 
 
-def test_rank_local_update_includes_data_parallel_rank():
+def test_rank_local_update_uses_data_parallel_index_after_reconfigure():
     engine = _RecordingEngine()
     worker = _make_worker(engine)
     worker.rank = 0
-    worker.vllm_config.parallel_config.data_parallel_size = 4
-    worker.vllm_config.parallel_config.data_parallel_rank = 2
+    parallel_config = ParallelConfig(
+        data_parallel_size=4,
+        data_parallel_rank=2,
+    )
+    assert parallel_config.data_parallel_rank == 2
+    assert parallel_config.data_parallel_index == 2
+
+    parallel_config.reconfigure_for_independent_dp_rank()
+    assert parallel_config.data_parallel_rank == 0
+    assert parallel_config.data_parallel_index == 2
+
+    worker.vllm_config.parallel_config = parallel_config
     Worker.start_weight_update(worker)
 
     Worker.update_weights(
diff --git a/vllm/distributed/ec_transfer/ec_connector/cpu/common.py b/vllm/distributed/ec_transfer/ec_connector/cpu/common.py
index e2c37b72a6..eff16e08b0 100644
--- a/vllm/distributed/ec_transfer/ec_connector/cpu/common.py
+++ b/vllm/distributed/ec_transfer/ec_connector/cpu/common.py
@@ -99,8 +99,8 @@ def create_ec_shared_region(vllm_config: "VllmConfig") -> ECSharedRegion:
     ec_config = vllm_config.ec_transfer_config
     assert ec_config is not None, "ec_transfer_config required to build region"
 
-    dp_rank = vllm_config.parallel_config.data_parallel_rank
-    engine_id = f"{vllm_config.instance_id}_dp{dp_rank}"
+    dp_index = vllm_config.parallel_config.data_parallel_index
+    engine_id = f"{vllm_config.instance_id}_dp{dp_index}"
 
     dtype = vllm_config.model_config.dtype
     hidden_dim = _get_encoder_cache_hidden_dim(vllm_config)
diff --git a/vllm/v1/worker/gpu_worker.py b/vllm/v1/worker/gpu_worker.py
index a288a97cc1..02f599e57e 100644
--- a/vllm/v1/worker/gpu_worker.py
+++ b/vllm/v1/worker/gpu_worker.py
@@ -1449,7 +1449,7 @@ class Worker(WorkerBase):
                 if isinstance(update_info, list):
                     parallel_config = self.vllm_config.parallel_config
                     local_update_info = update_info[
-                        parallel_config.data_parallel_rank * parallel_config.world_size
+                        parallel_config.data_parallel_index * parallel_config.world_size
                         + self.rank
                     ]
                 else: