diff --git a/src/core/backbone_router/bbr_local.cpp b/src/core/backbone_router/bbr_local.cpp
index 9952583b2..862860fef 100644
--- a/src/core/backbone_router/bbr_local.cpp
+++ b/src/core/backbone_router/bbr_local.cpp
@@ -52,7 +52,7 @@ Local::Local(Instance &aInstance)
     , mState(OT_BACKBONE_ROUTER_STATE_DISABLED)
     , mMlrTimeout(Mle::kMlrTimeoutDefault)
     , mReregistrationDelay(Mle::kRegistrationDelayDefault)
-    , mSequenceNumber(Random::NonCrypto::GetUint8())
+    , mSequenceNumber(Random::NonCrypto::GetUint8() % 127)
     , mRegistrationJitter(Mle::kBackboneRouterRegistrationJitter)
     , mIsServiceAdded(false)
     , mDomainPrefixCallback(nullptr)
@@ -112,7 +112,7 @@ void Local::Reset(void)
     if (mState == OT_BACKBONE_ROUTER_STATE_PRIMARY)
     {
         // Increase sequence number when changing from Primary to Secondary.
-        mSequenceNumber++;
+        SequenceNumberIncrease();
         Get<Notifier>().Signal(kEventThreadBackboneRouterLocalChanged);
         SetState(OT_BACKBONE_ROUTER_STATE_SECONDARY);
     }
@@ -270,7 +270,7 @@ void Local::HandleBackboneRouterPrimaryUpdate(Leader::State aState, const Backbo
     {
         // Here original PBBR restores its Backbone Router Service from Thread Network,
         // Intentionally skips the state update as PBBR will refresh its service.
-        mSequenceNumber      = aConfig.mSequenceNumber + 1;
+        mSequenceNumber      = aConfig.mSequenceNumber;
         mReregistrationDelay = aConfig.mReregistrationDelay;
         mMlrTimeout          = aConfig.mMlrTimeout;
 
@@ -278,9 +278,13 @@ void Local::HandleBackboneRouterPrimaryUpdate(Leader::State aState, const Backbo
         if (mSkipSeqNumIncrease)
         {
             // BBR-TC-02 forces Sequence Number for the reference device with raw UDP API
-            mSequenceNumber = aConfig.mSequenceNumber;
+            // Do not increase Sequence Number in that case.
         }
+        else
 #endif
+        {
+            SequenceNumberIncrease();
+        }
 
         Get<Notifier>().Signal(kEventThreadBackboneRouterLocalChanged);
         if (AddService(true /* Force registration to refresh and restore Primary state */) == kErrorNone)
@@ -424,6 +428,24 @@ void Local::RemoveDomainPrefixFromNetworkData(void)
     LogDomainPrefix("Remove", error);
 }
 
+void Local::SequenceNumberIncrease(void)
+{
+    switch (mSequenceNumber)
+    {
+    case 126:
+    case 127:
+        mSequenceNumber = 0;
+        break;
+    case 254:
+    case 255:
+        mSequenceNumber = 128;
+        break;
+    default:
+        mSequenceNumber++;
+        break;
+    }
+}
+
 void Local::AddDomainPrefixToNetworkData(void)
 {
     Error error = kErrorNotFound; // only used for logging.
diff --git a/src/core/backbone_router/bbr_local.hpp b/src/core/backbone_router/bbr_local.hpp
index f4878eab1..23c337a35 100644
--- a/src/core/backbone_router/bbr_local.hpp
+++ b/src/core/backbone_router/bbr_local.hpp
@@ -273,6 +273,7 @@ private:
     void RemoveService(void);
     void AddDomainPrefixToNetworkData(void);
     void RemoveDomainPrefixFromNetworkData(void);
+    void SequenceNumberIncrease(void);
 #if (OPENTHREAD_CONFIG_LOG_LEVEL >= OT_LOG_LEVEL_INFO) && (OPENTHREAD_CONFIG_LOG_BBR == 1)
     void LogBackboneRouterService(const char *aAction, Error aError);
     void LogDomainPrefix(const char *aAction, Error aError);
diff --git a/tools/harness-thci/OpenThread.py b/tools/harness-thci/OpenThread.py
index a78eb6cef..95915ee43 100644
--- a/tools/harness-thci/OpenThread.py
+++ b/tools/harness-thci/OpenThread.py
@@ -1607,7 +1607,7 @@ class OpenThreadTHCI(object):
             self.stopListeningToAddrAll()
 
         # BBR dataset
-        self.bbrSeqNum = random.randint(0, 254)  # random seqnum except 255, so that BBR-TC-02 never need re-run
+        self.bbrSeqNum = random.randint(0, 126)  # 5.21.4.2
         self.bbrMlrTimeout = 3600
         self.bbrReRegDelay = 5
 
@@ -3182,7 +3182,12 @@ class OpenThreadTHCI(object):
         """
         assert not (SeqNumInc and SeqNum is not None), "Must not specify both SeqNumInc and SeqNum"
         if SeqNumInc:
-            SeqNum = (self.bbrSeqNum + 1) % 256
+            if self.bbrSeqNum in (126, 127):
+                self.bbrSeqNum = 0
+            elif self.bbrSeqNum in (254, 255):
+                self.bbrSeqNum = 128
+            else:
+                self.bbrSeqNum = (self.bbrSeqNum + 1) % 256
 
         return self.__configBbrDataset(SeqNum=SeqNum, MlrTimeout=MlrTimeout, ReRegDelay=ReRegDelay)