From f7fd7bd54b407fbde0ae6eaceab92a3af5e97e28 Mon Sep 17 00:00:00 2001
From: Heshi Wu <heshiw@celestica.com>
Date: Mon, 17 Aug 2026 02:02:28 +0000
Subject: [PATCH] Fix TrunkUtils.cpp A VLAN left with no ports must be dropped;
 otherwise it diverges between cold boot and warmboot

---
 fboss/agent/test/TrunkUtils.cpp | 67 ++++++++++++++++++++++++++++++++-
 1 file changed, 66 insertions(+), 1 deletion(-)

diff --git a/fboss/agent/test/TrunkUtils.cpp b/fboss/agent/test/TrunkUtils.cpp
index 0d2524d6a0811..e71edc78b1a87 100644
--- a/fboss/agent/test/TrunkUtils.cpp
+++ b/fboss/agent/test/TrunkUtils.cpp
@@ -26,6 +26,64 @@ cfg::AggregatePortMember makePortMember(int32_t port, cfg::LacpPortRate rate) {
   return aggMember;
 }
 
+namespace {
+// A VLAN left with no ports must be dropped; otherwise it diverges between cold
+// boot and warmboot.
+void removeUnreferencedVlans(
+    cfg::SwitchConfig* config,
+    const std::set<int32_t>& memberOldVlans,
+    int32_t aggVlan) {
+  // TODO:remove this once cisco fixes the LAG vlan removal issue
+  bool isCiscoAsic = false;
+  for (const auto& [_, switchInfo] :
+       *config->switchSettings()->switchIdToSwitchInfo()) {
+    auto asicType = *switchInfo.asicType();
+    if (asicType == cfg::AsicType::ASIC_TYPE_EBRO ||
+        asicType == cfg::AsicType::ASIC_TYPE_YUBA ||
+        asicType == cfg::AsicType::ASIC_TYPE_P200 ||
+        asicType == cfg::AsicType::ASIC_TYPE_GARONNE ||
+        asicType == cfg::AsicType::ASIC_TYPE_G202X) {
+      isCiscoAsic = true;
+      break;
+    }
+  }
+  if (isCiscoAsic) {
+    return;
+  }
+
+  std::set<int32_t> stillReferenced;
+  for (const auto& vlanPort : *config->vlanPorts()) {
+    stillReferenced.insert(vlanPort.vlanID().value());
+  }
+
+  for (auto oldVlan : memberOldVlans) {
+    if (oldVlan == aggVlan || stillReferenced.contains(oldVlan) ||
+        (oldVlan == *config->defaultVlan())) {
+      continue;
+    }
+
+    auto& vlans = *config->vlans();
+    vlans.erase(
+        std::remove_if(
+            vlans.begin(),
+            vlans.end(),
+            [&](const auto& v) { return v.id().value() == oldVlan; }),
+        vlans.end());
+
+    auto& intfs = *config->interfaces();
+    intfs.erase(
+        std::remove_if(
+            intfs.begin(),
+            intfs.end(),
+            [&](const auto& i) {
+              return i.type().value() == cfg::InterfaceType::VLAN &&
+                  i.vlanID().value() == oldVlan;
+            }),
+        intfs.end());
+  }
+}
+} // namespace
+
 void addAggPort(
     int key,
     const std::vector<int32_t>& ports,
@@ -54,15 +112,22 @@ void addAggPort(
   // Set VLAN for all members to be the same
   std::set<uint32_t> memberPorts(ports.begin(), ports.end());
   std::optional<int32_t> aggVlan;
+  std::set<int32_t> memberOldVlans;
   for (auto& vlanPort : *config->vlanPorts()) {
     if (memberPorts.contains(vlanPort.logicalPort().value())) {
+      int32_t oldVlan = vlanPort.vlanID().value();
+      memberOldVlans.insert(oldVlan);
       if (!aggVlan) {
-        aggVlan = vlanPort.vlanID().value();
+        aggVlan = oldVlan;
       }
       vlanPort.vlanID() = *aggVlan;
     }
   }
 
+  if (aggVlan.has_value()) {
+    removeUnreferencedVlans(config, memberOldVlans, *aggVlan);
+  }
+
   if (aggregatePortType == cfg::AggregatePortType::LAG_PORT) {
     // Set ingress VLAN for all members to be the same
     for (auto& port : *config->ports()) {