diff --git a/gns3server/compute/base_node.py b/gns3server/compute/base_node.py
index a4a2ce365..24d7813ae 100644
--- a/gns3server/compute/base_node.py
+++ b/gns3server/compute/base_node.py
@@ -1073,7 +1073,7 @@ class BaseNode:
                 )
                 i += 1
 
-    async def _ubridge_add_marker_filter(self, bridge_name, name, bpf, pcap_path, tag=None):
+    async def _ubridge_add_marker_filter(self, bridge_name, name, bpf, pcap_path, tag=None, link_id=None):
         """
         Attach a `mark` packet filter to a uBridge bridge for traffic insight.
 
@@ -1104,6 +1104,11 @@ class BaseNode:
         )
         if tag is not None:
             cmd += f" tag {tag}"
+        # Per-link attribution (contract §3.2): when one ubridge bridge serves
+        # several GNS3 links (e.g. IOU's per-node bridge), bridge+filter collide,
+        # so the link id is the only way to tell signals — and pcap files — apart.
+        if link_id:
+            cmd += f" link {link_id}"
         cmd += ' pcap "{path}"'.format(path=pcap_path)
         # Let BPF compile errors propagate — the marker is the user's intent, so a
         # bad expression must surface instead of being silently dropped.
@@ -1133,7 +1138,7 @@ class BaseNode:
                 markers_dir, f"{self._id}_{link_id}_{name}.pcap"
             )
             try:
-                await self._ubridge_add_marker_filter(bridge_name, name, bpf, pcap_path, tag)
+                await self._ubridge_add_marker_filter(bridge_name, name, bpf, pcap_path, tag, link_id)
             except UbridgeError as e:
                 # Swallow BPF compile errors (warn + skip) so a single bad
                 # expression can't break link creation / node restart — mirrors
diff --git a/gns3server/compute/iou/iou_vm.py b/gns3server/compute/iou/iou_vm.py
index d36795af3..11bffb69a 100644
--- a/gns3server/compute/iou/iou_vm.py
+++ b/gns3server/compute/iou/iou_vm.py
@@ -1137,6 +1137,11 @@ class IOUVM(BaseNode):
             )
             if tag is not None:
                 cmd += f" tag {tag}"
+            # IOU uses one per-node IOL-BRIDGE for every link, so bridge+filter
+            # are identical across this node's links — `link` is the only way the
+            # controller can tell their signals apart (contract §3.2).
+            if link_id:
+                cmd += f" link {link_id}"
             cmd += ' pcap "{path}"'.format(path=pcap_path)
             try:
                 await self._ubridge_send(cmd)
diff --git a/gns3server/compute/marker/marker_listener.py b/gns3server/compute/marker/marker_listener.py
index d8bfa3303..f22f59bff 100644
--- a/gns3server/compute/marker/marker_listener.py
+++ b/gns3server/compute/marker/marker_listener.py
@@ -79,6 +79,7 @@ class MarkerListener(asyncio.DatagramProtocol):
             return
 
         # "-" means the field was unset on the ubridge side (see contract §3.3).
+        link = kv.get("link")
         tag = kv.get("tag")
         length = kv.get("len")
 
@@ -89,10 +90,16 @@ class MarkerListener(asyncio.DatagramProtocol):
             )
             return
 
+        # `link=` is the authoritative per-link id (opaque, set by gns3server at
+        # filter install time). It disambiguates signals that share a node+filter
+        # across several links; fall back to the registry's link only for legacy
+        # signals that carry no `link=`.
+        signal_link = link if link and link != "-" else None
+
         event = {
             "project_id": project_id,
             "node_id": node_id,
-            "link_id": link_id,
+            "link_id": signal_link or link_id,
             "filter": filter_name,
             # Prefer the value carried in the signal; fall back to the one we registered.
             "tag": tag if tag and tag != "-" else registered_tag,
diff --git a/tests/compute/marker/test_marker_manager.py b/tests/compute/marker/test_marker_manager.py
index dc8e826ff..4a2796002 100644
--- a/tests/compute/marker/test_marker_manager.py
+++ b/tests/compute/marker/test_marker_manager.py
@@ -161,6 +161,28 @@ class TestMarkerListener:
         lis.datagram_received(b"MARK 2.0 node=n filter=f tag=- len=20\n", None)
         assert fmgr.events[0][1]["tag"] == 42
 
+    def test_link_in_signal_overrides_registry_link(self):
+        # Per-link attribution (contract §3.2/§3.3): the signal's `link=` is
+        # authoritative and must disambiguate links sharing a node+filter —
+        # e.g. several links on one IOU node under the same global marker name.
+        fmgr = FakeMarkerManager()
+        fmgr.register("p", "n", "f", "registry-link", tag=1)
+        lis = MarkerListener(fmgr)
+        lis.connection_made(None)
+        lis.datagram_received(
+            b"MARK 3.0 node=n filter=f link=signal-link tag=1 len=42\n", None
+        )
+        assert fmgr.events[0][1]["link_id"] == "signal-link"
+
+    def test_link_dash_falls_back_to_registry_link(self):
+        # Legacy signals that carry no link fall back to the registry's link_id.
+        fmgr = FakeMarkerManager()
+        fmgr.register("p", "n", "f", "registry-link", tag=1)
+        lis = MarkerListener(fmgr)
+        lis.connection_made(None)
+        lis.datagram_received(b"MARK 3.0 node=n filter=f link=- tag=1 len=42\n", None)
+        assert fmgr.events[0][1]["link_id"] == "registry-link"
+
     def test_exception_does_not_kill_listener(self):
         fmgr = FakeMarkerManager()
         lis = MarkerListener(fmgr)