diff --git a/configure.ac b/configure.ac
index b5116922f9..82af1655d0 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1061,6 +1061,21 @@ AC_COMPILE_IFELSE([AC_LANG_PROGRAM([[#include <malloc.h>]],
  [ AC_MSG_RESULT([no])]
 )
 
+AC_MSG_CHECKING(for compatible sysinfo call)
+AC_COMPILE_IFELSE([AC_LANG_PROGRAM([[#include <sys/sysinfo.h>]],
+ [[
+  struct sysinfo info;
+  int rv = sysinfo(&info);
+  unsigned long test = info.freeram + info.bufferram + info.mem_unit;
+ ]])],
+ [
+  AC_MSG_RESULT(yes);
+  AC_DEFINE(HAVE_LINUX_SYSINFO, 1, [Define this symbol if you have a Linux-compatible sysinfo call])
+ ],[
+  AC_MSG_RESULT(no)
+ ]
+)
+
 dnl Check for posix_fallocate
 AC_MSG_CHECKING([for posix_fallocate])
 AC_COMPILE_IFELSE([AC_LANG_PROGRAM([[
diff --git a/src/init.cpp b/src/init.cpp
index 9974c4fec7..491f1b8949 100644
--- a/src/init.cpp
+++ b/src/init.cpp
@@ -441,6 +441,7 @@ void SetupServerArgs(ArgsManager& argsman)
     argsman.AddArg("-dbcache=<n>", strprintf("Maximum database cache size <n> MiB (%d to %d, default: %d). In addition, unused mempool memory is shared for this cache (see -maxmempool).", nMinDbCache, nMaxDbCache, nDefaultDbCache), ArgsManager::ALLOW_ANY, OptionsCategory::OPTIONS);
     argsman.AddArg("-includeconf=<file>", "Specify additional configuration file, relative to the -datadir path (only useable from configuration file, not command line)", ArgsManager::ALLOW_ANY, OptionsCategory::OPTIONS);
     argsman.AddArg("-loadblock=<file>", "Imports blocks from external file on startup", ArgsManager::ALLOW_ANY, OptionsCategory::OPTIONS);
+    argsman.AddArg("-lowmem=<n>", strprintf("If system available memory falls below <n> MiB, flush caches (0 to disable, default: %s)", util::g_low_memory_threshold / 1024 / 1024), ArgsManager::ALLOW_ANY, OptionsCategory::OPTIONS);
     argsman.AddArg("-maxmempool=<n>", strprintf("Keep the transaction memory pool below <n> megabytes (default: %u)", DEFAULT_MAX_MEMPOOL_SIZE_MB), ArgsManager::ALLOW_ANY, OptionsCategory::OPTIONS);
     argsman.AddArg("-maxorphantx=<n>", strprintf("Keep at most <n> unconnectable transactions in memory (default: %u)", DEFAULT_MAX_ORPHAN_TRANSACTIONS), ArgsManager::ALLOW_ANY, OptionsCategory::OPTIONS);
     argsman.AddArg("-mempoolexpiry=<n>", strprintf("Do not keep transactions in the mempool longer than <n> hours (default: %u)", DEFAULT_MEMPOOL_EXPIRY_HOURS), ArgsManager::ALLOW_ANY, OptionsCategory::OPTIONS);
@@ -586,7 +587,8 @@ void SetupServerArgs(ArgsManager& argsman)
     argsman.AddArg("-datacarriercost", strprintf("Treat extra data in transactions as at least N vbytes per actual byte (default: %s)", DEFAULT_WEIGHT_PER_DATA_BYTE / 4.0), ArgsManager::ALLOW_ANY, OptionsCategory::NODE_RELAY);
     argsman.AddArg("-datacarrierfullcount", strprintf("Apply datacarriersize limit to all known datacarrier methods (default: %s)", DEFAULT_DATACARRIER_FULLCOUNT), ArgsManager::ALLOW_ANY | (DEFAULT_DATACARRIER_FULLCOUNT ? uint32_t{ArgsManager::DEBUG_ONLY} : 0), OptionsCategory::NODE_RELAY);
     argsman.AddArg("-datacarriersize", strprintf("Maximum size of data in data carrier transactions we relay and mine (default: %u)", MAX_OP_RETURN_RELAY), ArgsManager::ALLOW_ANY, OptionsCategory::NODE_RELAY);
-    argsman.AddArg("-mempoolfullrbf", strprintf("Accept transaction replace-by-fee without requiring replaceability signaling (default: %u)", DEFAULT_MEMPOOL_FULL_RBF), ArgsManager::ALLOW_ANY, OptionsCategory::NODE_RELAY);
+    argsman.AddArg("-mempoolfullrbf", strprintf("Accept transaction replace-by-fee without requiring replaceability signaling (default: %u)", (DEFAULT_MEMPOOL_RBF_POLICY == RBFPolicy::Always)), ArgsManager::ALLOW_ANY, OptionsCategory::NODE_RELAY);
+    argsman.AddArg("-mempoolreplacement", strprintf("Set to 0 to disable RBF entirely, \"fee,optin\" to honour RBF opt-out signal, or \"fee,-optin\" to always RBF aka full RBF (default: %s)", "fee,optin"), ArgsManager::ALLOW_ANY, OptionsCategory::NODE_RELAY);
     argsman.AddArg("-permitbaremultisig", strprintf("Relay non-P2SH multisig (default: %u)", DEFAULT_PERMIT_BAREMULTISIG), ArgsManager::ALLOW_ANY,
                    OptionsCategory::NODE_RELAY);
     argsman.AddArg("-minrelaytxfee=<amt>", strprintf("Fees (in %s/kvB) smaller than this are considered zero fee for relaying, mining and transaction creation (default: %s)",
@@ -1488,6 +1490,17 @@ bool AppInitMain(NodeContext& node, interfaces::BlockAndHeaderTipInfo* tip_info)
     }
     LogPrintf("* Using %.1f MiB for in-memory UTXO set (plus up to %.1f MiB of unused mempool space)\n", cache_sizes.coins * (1.0 / 1024 / 1024), mempool_opts.max_size_bytes * (1.0 / 1024 / 1024));
 
+    if (gArgs.IsArgSet("-lowmem")) {
+        util::g_low_memory_threshold = gArgs.GetIntArg("-lowmem", 0 /* not used */) * 1024 * 1024;
+    }
+    if (util::g_low_memory_threshold > 0) {
+        LogPrintf("* Flushing caches if available system memory drops below %s MiB\n", util::g_low_memory_threshold / 1024 / 1024);
+    }
+
+    if (mempool_opts.rbf_policy == RBFPolicy::Always) {
+        nLocalServices = ServiceFlags(nLocalServices | NODE_REPLACE_BY_FEE);
+    }
+
     for (bool fLoaded = false; !fLoaded && !ShutdownRequested();) {
         node.mempool = std::make_unique<CTxMemPool>(mempool_opts);
 
diff --git a/src/kernel/mempool_options.h b/src/kernel/mempool_options.h
index 400d14ab92..20e25be5da 100644
--- a/src/kernel/mempool_options.h
+++ b/src/kernel/mempool_options.h
@@ -16,14 +16,16 @@
 
 class CBlockPolicyEstimator;
 
+enum class RBFPolicy { Never, OptIn, Always };
+
 /** Default for -maxmempool, maximum megabytes of mempool memory usage */
 static constexpr unsigned int DEFAULT_MAX_MEMPOOL_SIZE_MB{300};
 /** Default for -maxmempool when blocksonly is set */
 static constexpr unsigned int DEFAULT_BLOCKSONLY_MAX_MEMPOOL_SIZE_MB{5};
 /** Default for -mempoolexpiry, expiration time for mempool transactions in hours */
 static constexpr unsigned int DEFAULT_MEMPOOL_EXPIRY_HOURS{336};
-/** Default for -mempoolfullrbf, if the transaction replaceability signaling is ignored */
-static constexpr bool DEFAULT_MEMPOOL_FULL_RBF{false};
+/** Default for -mempoolreplacement; must update docs in init.cpp manually */
+static constexpr RBFPolicy DEFAULT_MEMPOOL_RBF_POLICY{RBFPolicy::OptIn};
 
 namespace kernel {
 /**
@@ -55,7 +57,7 @@ struct MemPoolOptions {
     bool datacarrier_fullcount{DEFAULT_DATACARRIER_FULLCOUNT};
     bool permit_bare_multisig{DEFAULT_PERMIT_BAREMULTISIG};
     bool require_standard{true};
-    bool full_rbf{DEFAULT_MEMPOOL_FULL_RBF};
+    RBFPolicy rbf_policy{DEFAULT_MEMPOOL_RBF_POLICY};
     MemPoolLimits limits{};
 };
 } // namespace kernel
diff --git a/src/node/mempool_args.cpp b/src/node/mempool_args.cpp
index 0671fe7714..545223a3ac 100644
--- a/src/node/mempool_args.cpp
+++ b/src/node/mempool_args.cpp
@@ -10,6 +10,7 @@
 #include <consensus/amount.h>
 #include <kernel/chainparams.h>
 #include <logging.h>
+#include <node/interface_ui.h>
 #include <policy/feerate.h>
 #include <policy/policy.h>
 #include <script/standard.h>
@@ -93,7 +94,59 @@ std::optional<bilingual_str> ApplyArgsManOptions(const ArgsManager& argsman, con
         return strprintf(Untranslated("acceptnonstdtxn is not currently supported for %s chain"), chainparams.NetworkIDString());
     }
 
-    mempool_opts.full_rbf = argsman.GetBoolArg("-mempoolfullrbf", mempool_opts.full_rbf);
+    if (argsman.IsArgSet("-mempoolreplacement") || argsman.IsArgSet("-mempoolfullrbf")) {
+        // Generally, mempoolreplacement overrides mempoolfullrbf, but the latter is used to infer intent in some cases
+        std::optional<bool> optin_flag;
+        bool fee_flag{false};
+        if (argsman.GetBoolArg("-mempoolreplacement", false)) {
+            fee_flag = true;
+        } else {
+            for (auto& opt : SplitString(argsman.GetArg("-mempoolreplacement", ""), ",+")) {
+                if (opt == "optin") {
+                    optin_flag = true;
+                } else if (opt == "-optin") {
+                    optin_flag = false;
+                } else if (opt == "fee") {
+                    fee_flag = true;
+                }
+            }
+        }
+        if (optin_flag.value_or(false)) {
+            // "optin" is explicitly specified
+            mempool_opts.rbf_policy = RBFPolicy::OptIn;
+        } else if (argsman.GetBoolArg("-mempoolfullrbf", false)) {
+            const bool mempoolreplacement_false{argsman.IsArgSet("-mempoolreplacement") && !(fee_flag || optin_flag.has_value())};
+            if (mempoolreplacement_false) {
+                // This is a contradiction, but override rather than error
+                InitWarning(_("False mempoolreplacement option contradicts true mempoolfullrbf; disallowing all RBF"));
+                mempool_opts.rbf_policy = RBFPolicy::Never;
+            } else {
+                mempool_opts.rbf_policy = RBFPolicy::Always;
+            }
+        } else if (!optin_flag.value_or(true)) {
+            // "-optin" is explicitly specified
+            mempool_opts.rbf_policy = fee_flag ? RBFPolicy::Always : RBFPolicy::Never;
+        } else if (fee_flag) {
+            // Just "fee" by itself
+            if (!argsman.GetBoolArg("-mempoolfullrbf", true)) {
+                mempool_opts.rbf_policy = RBFPolicy::OptIn;
+            } else {
+                // Fallback to default, unless it's been changed to Never
+                if (mempool_opts.rbf_policy == RBFPolicy::Never) {
+                    mempool_opts.rbf_policy = RBFPolicy::Always;
+                }
+            }
+        } else if (!argsman.IsArgSet("-mempoolreplacement")) {
+            // mempoolfullrbf is always explicitly false here
+            // Fallback to default, as long as it isn't Always
+            if (mempool_opts.rbf_policy == RBFPolicy::Always) {
+                mempool_opts.rbf_policy = RBFPolicy::OptIn;
+            }
+        } else {
+            // mempoolreplacement is explicitly false here
+            mempool_opts.rbf_policy = RBFPolicy::Never;
+        }
+    }
 
     ApplyArgsManOptions(argsman, mempool_opts.limits);
 
diff --git a/src/protocol.cpp b/src/protocol.cpp
index aa59bae6ff..49784ecf14 100644
--- a/src/protocol.cpp
+++ b/src/protocol.cpp
@@ -198,6 +198,7 @@ static std::string serviceFlagToStr(size_t bit)
     case NODE_WITNESS:         return "WITNESS";
     case NODE_COMPACT_FILTERS: return "COMPACT_FILTERS";
     case NODE_NETWORK_LIMITED: return "NETWORK_LIMITED";
+    case NODE_REPLACE_BY_FEE:  return "REPLACE_BY_FEE?";
     // Not using default, so we get warned when a case is missing
     }
 
diff --git a/src/protocol.h b/src/protocol.h
index cbcd400fef..bbce407d75 100644
--- a/src/protocol.h
+++ b/src/protocol.h
@@ -299,6 +299,8 @@ enum ServiceFlags : uint64_t {
     // collisions and other cases where nodes may be advertising a service they
     // do not actually support. Other service bits should be allocated via the
     // BIP process.
+
+    NODE_REPLACE_BY_FEE = (1 << 26),
 };
 
 /**
diff --git a/src/rpc/mempool.cpp b/src/rpc/mempool.cpp
index 29857ab5b1..eaff5abc27 100644
--- a/src/rpc/mempool.cpp
+++ b/src/rpc/mempool.cpp
@@ -793,7 +793,12 @@ UniValue MempoolInfoToJSON(const CTxMemPool& pool, const std::optional<MempoolHi
     ret.pushKV("minrelaytxfee", ValueFromAmount(pool.m_min_relay_feerate.GetFeePerK()));
     ret.pushKV("incrementalrelayfee", ValueFromAmount(pool.m_incremental_relay_feerate.GetFeePerK()));
     ret.pushKV("unbroadcastcount", uint64_t{pool.GetUnbroadcastTxs().size()});
-    ret.pushKV("fullrbf", pool.m_full_rbf);
+    ret.pushKV("fullrbf", (pool.m_rbf_policy == RBFPolicy::Always));
+    switch (pool.m_rbf_policy) {
+    case RBFPolicy::Never : ret.pushKV("rbf_policy", "never"); break;
+    case RBFPolicy::OptIn : ret.pushKV("rbf_policy", "optin"); break;
+    case RBFPolicy::Always: ret.pushKV("rbf_policy", "always"); break;
+    }
 
     if (histogram_floors) {
         const MempoolHistogramFeeRates& floors{histogram_floors.value()};
@@ -876,6 +881,7 @@ static RPCHelpMan getmempoolinfo()
                 {RPCResult::Type::NUM, "incrementalrelayfee", "minimum fee rate increment for mempool limiting or replacement in " + CURRENCY_UNIT + "/kvB"},
                 {RPCResult::Type::NUM, "unbroadcastcount", "Current number of transactions that haven't passed initial broadcast yet"},
                 {RPCResult::Type::BOOL, "fullrbf", "True if the mempool accepts RBF without replaceability signaling inspection"},
+                {RPCResult::Type::STR, "rbf_policy", "Policy used for replacing conflicting transactions by fee (one of: never, optin, always)"},
                 {RPCResult::Type::OBJ_DYN, "fee_histogram", /*optional=*/true, "",
                     {
                         {RPCResult::Type::OBJ, "<fee_rate_group>", "Fee rate group named by its lower bound (in " + CURRENCY_ATOM + "/vB), identical to the \"from_feerate\" field below",
diff --git a/src/test/validation_chainstate_tests.cpp b/src/test/validation_chainstate_tests.cpp
index 2078fcd8f8..b9854ec428 100644
--- a/src/test/validation_chainstate_tests.cpp
+++ b/src/test/validation_chainstate_tests.cpp
@@ -24,6 +24,8 @@ BOOST_FIXTURE_TEST_SUITE(validation_chainstate_tests, ChainTestingSetup)
 //!
 BOOST_AUTO_TEST_CASE(validation_chainstate_resize_caches)
 {
+    util::g_low_memory_threshold = 0;  // disable to get deterministic flushing
+
     ChainstateManager& manager = *Assert(m_node.chainman);
     CTxMemPool& mempool = *Assert(m_node.mempool);
     Chainstate& c1 = WITH_LOCK(cs_main, return manager.InitializeChainstate(&mempool));
diff --git a/src/txmempool.cpp b/src/txmempool.cpp
index b86d133f4d..5b9a16aaf8 100644
--- a/src/txmempool.cpp
+++ b/src/txmempool.cpp
@@ -412,7 +412,7 @@ CTxMemPool::CTxMemPool(const Options& opts)
       m_max_datacarrier_bytes{opts.max_datacarrier_bytes},
       m_datacarrier_fullcount{opts.datacarrier_fullcount},
       m_require_standard{opts.require_standard},
-      m_full_rbf{opts.full_rbf},
+      m_rbf_policy{opts.rbf_policy},
       m_limits{opts.limits}
 {
 }
diff --git a/src/txmempool.h b/src/txmempool.h
index bb73a6463c..1e5f46f627 100644
--- a/src/txmempool.h
+++ b/src/txmempool.h
@@ -464,7 +464,7 @@ public:
     const std::optional<unsigned> m_max_datacarrier_bytes;
     bool m_datacarrier_fullcount;
     const bool m_require_standard;
-    const bool m_full_rbf;
+    const RBFPolicy m_rbf_policy;
 
     const Limits m_limits;
 
diff --git a/src/util/system.cpp b/src/util/system.cpp
index 549eb6955b..fba996f00e 100644
--- a/src/util/system.cpp
+++ b/src/util/system.cpp
@@ -42,6 +42,10 @@
 #include <malloc.h>
 #endif
 
+#ifdef HAVE_LINUX_SYSINFO
+#include <sys/sysinfo.h>
+#endif
+
 #include <univalue.h>
 
 #include <fstream>
@@ -1149,4 +1153,41 @@ std::pair<int, char**> WinCmdLineArgs::get()
     return std::make_pair(argc, argv);
 }
 #endif
+
+size_t g_low_memory_threshold = 10 * 1024 * 1024 /* 10 MB */;
+
+bool SystemNeedsMemoryReleased()
+{
+    if (g_low_memory_threshold <= 0) {
+        // Intentionally bypass other metrics when disabled entirely
+        return false;
+    }
+#ifdef WIN32
+    MEMORYSTATUSEX mem_status;
+    mem_status.dwLength = sizeof(mem_status);
+    if (GlobalMemoryStatusEx(&mem_status)) {
+        if (mem_status.dwMemoryLoad >= 99 ||
+            mem_status.ullAvailPhys < g_low_memory_threshold ||
+            mem_status.ullAvailVirtual < g_low_memory_threshold) {
+            LogPrintf("%s: YES: %s%% memory load; %s available physical memory; %s available virtual memory\n", __func__, int(mem_status.dwMemoryLoad), size_t(mem_status.ullAvailPhys), size_t(mem_status.ullAvailVirtual));
+            return true;
+        }
+    }
+#endif
+#ifdef HAVE_LINUX_SYSINFO
+    struct sysinfo sys_info;
+    if (!sysinfo(&sys_info)) {
+        // Explicitly 64-bit in case of 32-bit userspace on 64-bit kernel
+        const uint64_t free_ram = uint64_t(sys_info.freeram) * sys_info.mem_unit;
+        const uint64_t buffer_ram = uint64_t(sys_info.bufferram) * sys_info.mem_unit;
+        if (free_ram + buffer_ram < g_low_memory_threshold) {
+            LogPrintf("%s: YES: %s free RAM + %s buffer RAM\n", __func__, free_ram, buffer_ram);
+            return true;
+        }
+    }
+#endif
+    // NOTE: sysconf(_SC_AVPHYS_PAGES) doesn't account for caches on at least Linux, so not safe to use here
+    return false;
+}
+
 } // namespace util
diff --git a/src/util/system.h b/src/util/system.h
index eb86da8829..0b058e48d3 100644
--- a/src/util/system.h
+++ b/src/util/system.h
@@ -522,6 +522,10 @@ private:
 };
 #endif
 
+extern size_t g_low_memory_threshold;
+
+bool SystemNeedsMemoryReleased();
+
 } // namespace util
 
 #endif // BITCOIN_UTIL_SYSTEM_H
diff --git a/src/validation.cpp b/src/validation.cpp
index e7538e752e..5ac5a87cde 100644
--- a/src/validation.cpp
+++ b/src/validation.cpp
@@ -802,7 +802,15 @@ bool MemPoolAccept::PreChecks(ATMPArgs& args, Workspace& ws)
                 //
                 // If replaceability signaling is ignored due to node setting,
                 // replacement is always allowed.
-                if (!(m_pool.m_full_rbf || ignore_rejects.count("txn-mempool-conflict")) && !SignalsOptInRBF(*ptxConflicting)) {
+                bool allow_replacement;
+                if (m_pool.m_rbf_policy == RBFPolicy::Always || ignore_rejects.count("txn-mempool-conflict")) {
+                    allow_replacement = true;
+                } else if (m_pool.m_rbf_policy == RBFPolicy::Never) {
+                    allow_replacement = false;
+                } else {
+                    allow_replacement = SignalsOptInRBF(*ptxConflicting);
+                }
+                if (!allow_replacement) {
                     return state.Invalid(TxValidationResult::TX_MEMPOOL_POLICY, "txn-mempool-conflict");
                 }
 
@@ -2585,8 +2593,15 @@ bool Chainstate::FlushStateToDisk(
         }
         // The cache is large and we're within 10% and 10 MiB of the limit, but we have time now (not in the middle of a block processing).
         bool fCacheLarge = mode == FlushStateMode::PERIODIC && cache_state >= CoinsCacheSizeState::LARGE;
-        // The cache is over the limit, we have to write now.
-        bool fCacheCritical = mode == FlushStateMode::IF_NEEDED && cache_state >= CoinsCacheSizeState::CRITICAL;
+        bool fCacheCritical = false;
+        if (mode == FlushStateMode::IF_NEEDED) {
+            if (cache_state >= CoinsCacheSizeState::CRITICAL) {
+                // The cache is over the limit, we have to write now.
+                fCacheCritical = true;
+            } else if (util::SystemNeedsMemoryReleased()) {
+                fCacheCritical = true;
+            }
+        }
         // It's been a while since we wrote the block index to disk. Do this frequently, so we don't need to redownload after a crash.
         bool fPeriodicWrite = mode == FlushStateMode::PERIODIC && nNow > m_last_write + DATABASE_WRITE_INTERVAL;
         // It's been very long since we flushed the cache. Do this infrequently, to optimize cache usage.
diff --git a/src/wallet/test/wallet_tests.cpp b/src/wallet/test/wallet_tests.cpp
index edcfaa24e5..c9a987b270 100644
--- a/src/wallet/test/wallet_tests.cpp
+++ b/src/wallet/test/wallet_tests.cpp
@@ -766,6 +766,9 @@ BOOST_FIXTURE_TEST_CASE(wallet_descriptor_test, BasicTestingSetup)
 //! rescanning where new transactions in new blocks could be lost.
 BOOST_FIXTURE_TEST_CASE(CreateWallet, TestChain100Setup)
 {
+    // FIXME: this test fails for some reason if there's a flush
+    util::g_low_memory_threshold = 0;
+
     m_args.ForceSetArg("-unsafesqlitesync", "1");
     // Create new wallet with known key and unload it.
     WalletContext context;
diff --git a/test/functional/feature_rbf.py b/test/functional/feature_rbf.py
index 947d2e8273..985c123abb 100755
--- a/test/functional/feature_rbf.py
+++ b/test/functional/feature_rbf.py
@@ -25,7 +25,7 @@ class ReplaceByFeeTest(BitcoinTestFramework):
         self.add_wallet_options(parser)
 
     def set_test_params(self):
-        self.num_nodes = 2
+        self.num_nodes = 4
         self.extra_args = [
             [
                 "-maxorphantx=1000",
@@ -37,12 +37,40 @@ class ReplaceByFeeTest(BitcoinTestFramework):
             # second node has default mempool parameters
             [
             ],
+            [
+                "-acceptnonstdtxn=1",
+                "-mempoolreplacement=0",
+            ],
         ]
+        self.extra_args.append(
+            [
+                *self.extra_args[0],
+                "-mempoolreplacement=fee,-optin",
+            ],
+        )
         self.supports_cli = False
 
     def run_test(self):
         self.wallet = MiniWallet(self.nodes[0])
 
+        self.log.info("Running test RPC rbf_policy")
+        def test_rpc_rbf_policy():
+            assert_equal(self.nodes[0].getmempoolinfo()["rbf_policy"], 'optin')
+            assert_equal(self.nodes[1].getmempoolinfo()["rbf_policy"], 'optin')
+            assert_equal(self.nodes[2].getmempoolinfo()["rbf_policy"], 'never')
+            assert_equal(self.nodes[3].getmempoolinfo()["rbf_policy"], 'always')
+        test_rpc_rbf_policy()
+
+        self.log.info("Running test service flag")
+        def test_service_flag():
+            NODE_REPLACE_BY_FEE = (1 << 26)
+            for i in range(3):
+                assert not (int(self.nodes[i].getnetworkinfo()['localservices'], 0x10) & NODE_REPLACE_BY_FEE)
+                assert 'REPLACE_BY_FEE?' not in self.nodes[i].getnetworkinfo()['localservicesnames']
+            assert int(self.nodes[3].getnetworkinfo()['localservices'], 0x10) & NODE_REPLACE_BY_FEE
+            assert 'REPLACE_BY_FEE?' in self.nodes[3].getnetworkinfo()['localservicesnames']
+        test_service_flag()
+
         self.log.info("Running test simple doublespend...")
         self.test_simple_doublespend()
 
@@ -68,7 +96,10 @@ class ReplaceByFeeTest(BitcoinTestFramework):
         self.test_too_many_replacements_with_default_mempool_params()
 
         self.log.info("Running test opt-in...")
-        self.test_opt_in()
+        self.test_opt_in(fullrbf=False)
+        self.nodes[0], self.nodes[-1] = self.nodes[-1], self.nodes[0]
+        self.test_opt_in(fullrbf=True)
+        self.nodes[0], self.nodes[-1] = self.nodes[-1], self.nodes[0]
 
         self.log.info("Running test RPC...")
         self.test_rpc()
@@ -112,17 +143,24 @@ class ReplaceByFeeTest(BitcoinTestFramework):
         # we use MiniWallet to create a transaction template with inputs correctly set,
         # and modify the output (amount, scriptPubKey) according to our needs
         tx = self.wallet.create_self_transfer()["tx"]
-        tx1a_txid = self.nodes[0].sendrawtransaction(tx.serialize().hex())
+        tx1a_hex = tx.serialize().hex()
+        tx1a_txid = self.nodes[0].sendrawtransaction(tx1a_hex)
+        assert_equal(tx1a_txid, self.nodes[2].sendrawtransaction(tx1a_hex))
 
         # Should fail because we haven't changed the fee
         tx.vout[0].scriptPubKey[-1] ^= 1
 
         # This will raise an exception due to insufficient fee
-        assert_raises_rpc_error(-26, "insufficient fee", self.nodes[0].sendrawtransaction, tx.serialize().hex(), 0)
+        tx1b_hex = tx.serialize().hex()
+        assert_raises_rpc_error(-26, "insufficient fee", self.nodes[0].sendrawtransaction, tx1b_hex, 0)
+        # This will raise an exception due to transaction replacement being disabled
+        assert_raises_rpc_error(-26, "txn-mempool-conflict", self.nodes[2].sendrawtransaction, tx1b_hex, 0)
 
         # Extra 0.1 BTC fee
         tx.vout[0].nValue -= int(0.1 * COIN)
         tx1b_hex = tx.serialize().hex()
+        # Replacement still disabled even with "enough fee"
+        assert_raises_rpc_error(-26, "txn-mempool-conflict", self.nodes[2].sendrawtransaction, tx1b_hex, 0)
         # Works when enabled
         tx1b_txid = self.nodes[0].sendrawtransaction(tx1b_hex, 0)
 
@@ -133,6 +171,11 @@ class ReplaceByFeeTest(BitcoinTestFramework):
 
         assert_equal(tx1b_hex, self.nodes[0].getrawtransaction(tx1b_txid))
 
+        # Third node is running mempoolreplacement=0, will not replace originally-seen txn
+        mempool = self.nodes[2].getrawmempool()
+        assert tx1a_txid in mempool
+        assert tx1b_txid not in mempool
+
     def test_doublespend_chain(self):
         """Doublespend of a long chain"""
 
@@ -468,7 +511,7 @@ class ReplaceByFeeTest(BitcoinTestFramework):
         self.generate(normal_node, 1)
         self.wallet.rescan_utxos()
 
-    def test_opt_in(self):
+    def test_opt_in(self, fullrbf):
         """Replacing should only work if orig tx opted in"""
         tx0_outpoint = self.make_utxo(self.nodes[0], int(1.1 * COIN))
 
@@ -484,14 +527,19 @@ class ReplaceByFeeTest(BitcoinTestFramework):
         assert_equal(self.nodes[0].getmempoolentry(tx1a_utxo["txid"])['bip125-replaceable'], False)
 
         # Shouldn't be able to double-spend
-        tx1b_hex = self.wallet.create_self_transfer(
+        tx1b_st = self.wallet.create_self_transfer(
             utxo_to_spend=tx0_outpoint,
             sequence=0,
             fee=Decimal("0.2"),
-        )["hex"]
+        )
+        tx1b_hex = tx1b_st["hex"]
 
-        # This will raise an exception
-        assert_raises_rpc_error(-26, "txn-mempool-conflict", self.nodes[0].sendrawtransaction, tx1b_hex, 0)
+        if fullrbf:
+            self.nodes[0].sendrawtransaction(tx1b_hex, 0)
+            tx1a_utxo = tx1b_st["new_utxo"]
+        else:
+            # This will raise an exception
+            assert_raises_rpc_error(-26, "txn-mempool-conflict", self.nodes[0].sendrawtransaction, tx1b_hex, 0)
 
         tx1_outpoint = self.make_utxo(self.nodes[0], int(1.1 * COIN))
 
@@ -504,14 +552,19 @@ class ReplaceByFeeTest(BitcoinTestFramework):
         )["new_utxo"]
 
         # Still shouldn't be able to double-spend
-        tx2b_hex = self.wallet.create_self_transfer(
+        tx2b_st = self.wallet.create_self_transfer(
             utxo_to_spend=tx1_outpoint,
             sequence=0,
             fee=Decimal("0.2"),
-        )["hex"]
+        )
+        tx2b_hex = tx2b_st["hex"]
 
-        # This will raise an exception
-        assert_raises_rpc_error(-26, "txn-mempool-conflict", self.nodes[0].sendrawtransaction, tx2b_hex, 0)
+        if fullrbf:
+            self.nodes[0].sendrawtransaction(tx2b_hex, 0)
+            tx2a_utxo = tx2b_st["new_utxo"]
+        else:
+            # This will raise an exception
+            assert_raises_rpc_error(-26, "txn-mempool-conflict", self.nodes[0].sendrawtransaction, tx2b_hex, 0)
 
         # Now create a new transaction that spends from tx1a and tx2a
         # opt-in on one of the inputs
@@ -543,6 +596,8 @@ class ReplaceByFeeTest(BitcoinTestFramework):
             fee=Decimal("0.4"),
         )
 
+        self.generate(self.nodes[0], 1)  # clean mempool
+
     def test_prioritised_transactions(self):
         # Ensure that fee deltas used via prioritisetransaction are
         # correctly used by replacement logic
@@ -704,6 +759,7 @@ class ReplaceByFeeTest(BitcoinTestFramework):
         confirmed_utxo = self.make_utxo(self.nodes[0], int(2 * COIN))
         self.restart_node(0, extra_args=["-mempoolfullrbf=1"])
         assert self.nodes[0].getmempoolinfo()["fullrbf"]
+        assert_equal(self.nodes[0].getmempoolinfo()["rbf_policy"], 'always')
 
         # Create an explicitly opt-out transaction
         optout_tx = self.wallet.send_self_transfer(