diff --git a/tests/replication/tcp/test_handler.py b/tests/replication/tcp/test_handler.py
index 2db1e4711323..c2bbc75d7383 100644
--- a/tests/replication/tcp/test_handler.py
+++ b/tests/replication/tcp/test_handler.py
@@ -11,20 +11,30 @@
 # WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 # See the License for the specific language governing permissions and
 # limitations under the License.
+from unittest.mock import AsyncMock, Mock
 
 from twisted.internet import defer
 
-from synapse.replication.tcp.commands import PositionCommand, UserIpCommand
+from synapse.replication.tcp.commands import (
+    PositionCommand,
+    RemoteServerUpCommand,
+    UserIpCommand,
+)
+from synapse.server import HomeServer
+from synapse.util import Clock
 
 from tests.replication._base import BaseMultiWorkerStreamTestCase
+from tests.server import ThreadedMemoryReactorClock
+from tests.unittest import override_config
 
 
 class ChannelsTestCase(BaseMultiWorkerStreamTestCase):
     def test_subscribed_to_enough_redis_channels(self) -> None:
-        # The default main process is subscribed to the USER_IP channel.
+        # The default main process is subscribed to the REMOTE_SERVER_UP and USER_IP
+        # channel.
         self.assertCountEqual(
             self.hs.get_replication_command_handler()._channels_to_subscribe_to,
-            [UserIpCommand.NAME],
+            [UserIpCommand.NAME, RemoteServerUpCommand.NAME],
         )
 
     def test_background_worker_subscribed_to_user_ip(self) -> None:
@@ -76,6 +86,59 @@ def test_non_background_worker_not_subscribed_to_user_ip(self) -> None:
             len(self._redis_server._subscribers_by_channel[b"test/USER_IP"]), 1
         )
 
+    @override_config({"federation_sender_instances": ["worker1"]})
+    def test_federation_sender_subscribed_to_remote_server_up(self) -> None:
+        # The default main process and federation senders are subscribed to the
+        # REMOTE_SERVER_UP channel.
+        worker1 = self.make_worker_hs(
+            "synapse.app.generic_worker",
+            extra_config={
+                "worker_name": "worker1",
+                "redis": {"enabled": True},
+            },
+        )
+
+        self.assertIn(
+            RemoteServerUpCommand.NAME,
+            worker1.get_replication_command_handler()._channels_to_subscribe_to,
+        )
+
+        # Advance so the Redis subscription gets processed
+        self.pump(0.1)
+
+        # The counts are 2 because both the main process and the worker are subscribed.
+        self.assertEqual(len(self._redis_server._subscribers_by_channel[b"test"]), 2)
+        self.assertEqual(
+            len(self._redis_server._subscribers_by_channel[b"test/REMOTE_SERVER_UP"]), 2
+        )
+
+    @override_config({"federation_sender_instances": ["worker1"]})
+    def test_non_federation_sender_not_subscribed_to_remote_server_up(self) -> None:
+        # Only the default main process is subscribed to the REMOTE_SERVER_UP channel
+        # because it is the main process. The override above tells 'send_federation' to
+        # be false, so main is not a sender.
+        worker2 = self.make_worker_hs(
+            "synapse.app.generic_worker",
+            extra_config={
+                "worker_name": "worker2",
+                "redis": {"enabled": True},
+            },
+        )
+        self.assertNotIn(
+            RemoteServerUpCommand.NAME,
+            worker2.get_replication_command_handler()._channels_to_subscribe_to,
+        )
+
+        # Advance so the Redis subscription gets processed
+        self.pump(0.1)
+
+        # The count is 2 because both the main process and the worker are subscribed.
+        self.assertEqual(len(self._redis_server._subscribers_by_channel[b"test"]), 2)
+        # For REMOTE_SERVER_UP, the count is 1 because only the main process is subscribed.
+        self.assertEqual(
+            len(self._redis_server._subscribers_by_channel[b"test/REMOTE_SERVER_UP"]), 1
+        )
+
     def test_wait_for_stream_position(self) -> None:
         """Check that wait for stream position correctly waits for an update from the
         correct instance.
@@ -202,3 +265,43 @@ def test_wait_for_stream_position_rdata(self) -> None:
         # Master should get told about `next_token2`, so the deferred should
         # resolve.
         self.assertTrue(d.called)
+
+
+class ChannelsCapabilityTestCase(BaseMultiWorkerStreamTestCase):
+    def make_homeserver(
+        self, reactor: ThreadedMemoryReactorClock, clock: Clock
+    ) -> HomeServer:
+        self.replication_data_handler = Mock(
+            spec=["on_remote_server_up", "on_position"]
+        )
+        self.replication_data_handler.on_remote_server_up = Mock()
+        self.replication_data_handler.on_position = AsyncMock()
+
+        hs = self.setup_test_homeserver(
+            replication_data_handler=self.replication_data_handler,
+        )
+        return hs
+
+    def test_sending_command_while_not_subscribed(self) -> None:
+        # Test that sending a command from a worker that is not on a specific channel
+        # actually allows receiving on a 'worker'(in this case main) that is.
+        #
+        # Proves: a worker doesn't have to be subscribed to a channel to send commands.
+        mock_on_remote_server_up = self.replication_data_handler.on_remote_server_up
+
+        # worker1 is setup to listen on ["test", "test/USER_IP"] and main is on all
+        worker1 = self.make_worker_hs(
+            "synapse.app.generic_worker",
+            extra_config={
+                "worker_name": "worker1",
+                "run_background_tasks_on": "worker1",
+                "redis": {"enabled": True},
+            },
+        )
+        worker_cmd_handler = worker1.get_replication_command_handler()
+        # Pump the reactor, so the replication connections are established
+        self.pump()
+        worker_cmd_handler.send_remote_server_up("test2")
+        # Actually allow the replication to take place
+        self.replicate()
+        mock_on_remote_server_up.assert_called_once()