diff --git a/ep/README.md b/ep/README.md
index 1ddd5a08c..9bc07983e 100644
--- a/ep/README.md
+++ b/ep/README.md
@@ -138,6 +138,7 @@ Notes:
 | UCCL_IB_TC | Traffic class in RDMA network | 104/0 (IB/EFA) |
 | UCCL_EP_ENABLE_AGGRESSIVE_ATOMIC | Use relaxed atomics with manual `s_waitcnt vmcnt(0)` fences instead of acquire/release semantics. Required on AMD CDNA so the combine receiver actually sees the producer's tail-pointer updates over XGMI; without it the kernel deadlocks at scale. | 1 on AMD, 0 on CUDA |
 | UCCL_RDMA_ADAPTIVE_SLEEP | Enable adaptive sleeping on proxy threads, by putting the proxy threads into a sleeping state if there have been no new work requests / RDMA completion events after 120s. | null |
+| UCCL_UDP_SPORT_BASE | Base UDP source port for RoCEv2 QPs on mlx5 devices. Valid range: 1-65535. | 0 (driver decides) |
 
 ## Results
 
diff --git a/ep/include/proxy_ctx.hpp b/ep/include/proxy_ctx.hpp
index 85f12d4ef..916c2f5a3 100644
--- a/ep/include/proxy_ctx.hpp
+++ b/ep/include/proxy_ctx.hpp
@@ -74,6 +74,7 @@ struct ProxyCtx {
   // std::vector<ibv_cq*> extra_cqs;
   ibv_qp* ack_qp = nullptr;
   ibv_qp* recv_ack_qp = nullptr;
+  int udp_sport_base = 0;
   // EFA shared-QP model: when true, the QP and ack_recv_buf/mr fields above
   // alias another ProxyCtx (typically Proxy::ctx_) and must not be destroyed.
   bool qps_are_shared = false;
diff --git a/ep/include/rdma_util.hpp b/ep/include/rdma_util.hpp
index dd0383fa7..9913c85f4 100644
--- a/ep/include/rdma_util.hpp
+++ b/ep/include/rdma_util.hpp
@@ -13,12 +13,13 @@
 #include <mutex>
 #include <unordered_set>
 #include <vector>
+#include <dlfcn.h>
 #include <fcntl.h>
 #include <limits.h>
 #include <sys/socket.h>
 #include <unistd.h>
 
-void fill_local_gid(ProxyCtx& S, RDMAConnectionInfo* local_info) {
+static inline void fill_local_gid(ProxyCtx& S, RDMAConnectionInfo* local_info) {
   if (!S.context) {
     fprintf(stderr, "Error: context not initialized when filling GID\n");
     exit(1);
@@ -103,6 +104,48 @@ static inline int get_roce_version_from_env() {
   return roce_version;
 }
 
+static inline int get_udp_sport_base_from_env() {
+  static int udp_sport_base = 0;
+  char const* env = getenv("UCCL_UDP_SPORT_BASE");
+  if (env) {
+    int val = atoi(env);
+    if (val < 1 || val > 65535) {
+      fprintf(stderr,
+              "UCCL_UDP_SPORT_BASE=%d out of the valid range [1, 65535], "
+              "ignoring\n",
+              val);
+    } else {
+      udp_sport_base = val;
+    }
+  }
+  return udp_sport_base;
+}
+
+typedef int (*mlx5dv_modify_qp_udp_sport_fn)(struct ibv_qp*, uint16_t);
+
+static inline mlx5dv_modify_qp_udp_sport_fn load_mlx5dv_udp_sport_fn() {
+  static mlx5dv_modify_qp_udp_sport_fn fn =
+      []() -> mlx5dv_modify_qp_udp_sport_fn {
+    void* handle = dlopen("libmlx5.so", RTLD_NOW);
+    if (!handle) handle = dlopen("libmlx5.so.1", RTLD_NOW);
+    if (!handle) {
+      fprintf(stderr, "Failed to open libmlx5.so[.1]: %s\n", dlerror());
+      return nullptr;
+    }
+    auto f = (mlx5dv_modify_qp_udp_sport_fn)dlsym(handle,
+                                                  "mlx5dv_modify_qp_udp_sport");
+    if (!f) {
+      fprintf(stderr,
+              "dlsym failed on mlx5dv_modify_qp_udp_sport: %s, "
+              "skipping UDP sport configuration\n",
+              dlerror());
+      dlclose(handle);
+    }
+    return f;
+  }();
+  return fn;
+}
+
 static sa_family_t envIbAddrFamily(void) {
   sa_family_t family = AF_INET;
   char const* env = getenv("UCCL_IB_ADDR_FAMILY");
diff --git a/ep/src/proxy.cpp b/ep/src/proxy.cpp
index ff630403f..cbc3f6777 100644
--- a/ep/src/proxy.cpp
+++ b/ep/src/proxy.cpp
@@ -3,6 +3,7 @@
 #include "d2h_queue_host.hpp"
 #include "ep_util.hpp"
 #include "rdma.hpp"
+#include "rdma_util.hpp"
 #include "util/util.h"
 #include <arpa/inet.h>  // for htonl, ntohl
 #include <chrono>
@@ -162,6 +163,7 @@ void Proxy::set_peers_meta(std::vector<PeerMeta> const& peers) {
   ctxs_for_all_ranks_.resize(peers.size());
   for (size_t i = 0; i < peers.size(); ++i) {
     ctxs_for_all_ranks_[i] = std::make_unique<ProxyCtx>();
+    ctxs_for_all_ranks_[i]->udp_sport_base = get_udp_sport_base_from_env();
   }
 }
 
diff --git a/ep/src/rdma.cpp b/ep/src/rdma.cpp
index 9d84df5db..e5f304620 100644
--- a/ep/src/rdma.cpp
+++ b/ep/src/rdma.cpp
@@ -7,7 +7,6 @@
 #ifdef USE_DMABUF
 #include <condition_variable>
 #include <map>
-#include <dlfcn.h>
 #endif
 #include <arpa/inet.h>
 #include <netinet/in.h>
@@ -1338,6 +1337,26 @@ void modify_qp_to_rts(ProxyCtx& S, RDMAConnectionInfo* local_info) {
   }
 
   printf("ACK-QP modified to RTS state\n");
+
+  if (S.udp_sport_base != 0) {
+    auto fn = load_mlx5dv_udp_sport_fn();
+    if (fn) {
+      auto set_sport = [&](ibv_qp* qp, int sport, char const* name) {
+        if (fn(qp, (uint16_t)sport))
+          fprintf(stderr, "Warning: failed to set UDP sport %d on %s QP\n",
+                  sport, name);
+      };
+      set_sport(S.qp, S.udp_sport_base, "main");
+      for (size_t r = 0; r < S.data_qps_by_channel.size(); ++r)
+        set_sport(S.data_qps_by_channel[r], S.udp_sport_base + r + 1, "data");
+      set_sport(S.ack_qp, S.udp_sport_base + S.data_qps_by_channel.size() + 1,
+                "ACK");
+      set_sport(S.recv_ack_qp,
+                S.udp_sport_base + S.data_qps_by_channel.size() + 2,
+                "recv ACK");
+      printf("UDP source ports set starting from %d\n", S.udp_sport_base);
+    }
+  }
 }
 
 void post_receive_buffer_for_imm_on_qp(ProxyCtx& S, ibv_qp* qp) {