diff --git a/sys/net/gnrc/network_layer/sixlowpan/frag/rbuf.c b/sys/net/gnrc/network_layer/sixlowpan/frag/rbuf.c
index daa2beb61d..832d59acc2 100644
--- a/sys/net/gnrc/network_layer/sixlowpan/frag/rbuf.c
+++ b/sys/net/gnrc/network_layer/sixlowpan/frag/rbuf.c
@@ -240,7 +240,7 @@ static bool _rbuf_update_ints(rbuf_t *entry, uint16_t offset, size_t frag_size)
 static void _rbuf_gc(void)
 {
     rbuf_t *oldest = NULL;
-    uint32_t now_sec = xtimer_now() / SEC_IN_USEC;
+    uint32_t now_usec = xtimer_now();
     unsigned int i;
 
     for (i = 0; i < RBUF_SIZE; i++) {
@@ -248,7 +248,7 @@ static void _rbuf_gc(void)
             return;
         }
         else if ((rbuf[i].pkt != NULL) &&
-                 ((now_sec - rbuf[i].arrival) > RBUF_TIMEOUT)) {
+                 ((now_usec - rbuf[i].arrival) > RBUF_TIMEOUT)) {
             DEBUG("6lo rfrag: entry (%s, ", gnrc_netif_addr_to_str(l2addr_str,
                     sizeof(l2addr_str), rbuf[i].src, rbuf[i].src_len));
             DEBUG("%s, %u, %u) timed out\n",
@@ -259,7 +259,7 @@ static void _rbuf_gc(void)
             gnrc_pktbuf_release(rbuf[i].pkt);
             _rbuf_rem(&(rbuf[i]));
         }
-        else if ((oldest == NULL) || (rbuf[i].arrival < oldest->arrival)) {
+        else if ((oldest == NULL) || ((oldest->arrival - rbuf[i].arrival) < (UINT32_MAX / 2))) {
             oldest = &(rbuf[i]);
         }
     }
@@ -276,7 +276,7 @@ static rbuf_t *_rbuf_get(const void *src, size_t src_len,
                          size_t size, uint16_t tag)
 {
     rbuf_t *res = NULL;
-    uint32_t now_sec = xtimer_now() / SEC_IN_USEC;
+    uint32_t now_usec = xtimer_now();
 
     for (unsigned int i = 0; i < RBUF_SIZE; i++) {
         /* check first if entry already available */
@@ -292,7 +292,7 @@ static rbuf_t *_rbuf_get(const void *src, size_t src_len,
                   gnrc_netif_addr_to_str(l2addr_str, sizeof(l2addr_str),
                                          rbuf[i].dst, rbuf[i].dst_len),
                   (unsigned)rbuf[i].pkt->size, rbuf[i].tag);
-            rbuf[i].arrival = now_sec;
+            rbuf[i].arrival = now_usec;
             return &(rbuf[i]);
         }
 
@@ -311,7 +311,7 @@ static rbuf_t *_rbuf_get(const void *src, size_t src_len,
 
         *((uint64_t *)res->pkt->data) = 0;  /* clean first few bytes for later
                                              * look-ups */
-        res->arrival = now_sec;
+        res->arrival = now_usec;
         memcpy(res->src, src, src_len);
         memcpy(res->dst, dst, dst_len);
         res->src_len = src_len;
diff --git a/sys/net/gnrc/network_layer/sixlowpan/frag/rbuf.h b/sys/net/gnrc/network_layer/sixlowpan/frag/rbuf.h
index d65da43960..1b69764eb1 100644
--- a/sys/net/gnrc/network_layer/sixlowpan/frag/rbuf.h
+++ b/sys/net/gnrc/network_layer/sixlowpan/frag/rbuf.h
@@ -30,9 +30,9 @@
 extern "C" {
 #endif
 
-#define RBUF_L2ADDR_MAX_LEN (8U)    /**< maximum length for link-layer addresses */
-#define RBUF_SIZE           (4U)    /**< size of the reassembly buffer */
-#define RBUF_TIMEOUT        (3U)    /**< timeout for reassembly in seconds */
+#define RBUF_L2ADDR_MAX_LEN (8U)               /**< maximum length for link-layer addresses */
+#define RBUF_SIZE           (4U)               /**< size of the reassembly buffer */
+#define RBUF_TIMEOUT        (3U * SEC_IN_USEC) /**< timeout for reassembly in microseconds */
 
 /**
  * @brief   Fragment intervals to identify limits of fragments.
@@ -73,8 +73,8 @@ typedef struct rbuf_int {
 typedef struct {
     rbuf_int_t *ints;                   /**< intervals of the fragment */
     gnrc_pktsnip_t *pkt;                /**< the reassembled packet in packet buffer */
-    uint32_t arrival;                   /**< time in seconds of arrival of last
-                                         *   received fragment */
+    uint32_t arrival;                   /**< time in microseconds of arrival of
+                                         *   last received fragment */
     uint8_t src[RBUF_L2ADDR_MAX_LEN];   /**< source address */
     uint8_t dst[RBUF_L2ADDR_MAX_LEN];   /**< destination address */
     uint8_t src_len;                    /**< length of source address */