Refactor route add

Signed-off-by: Wenda Ni <wonda.ni@gmail.com>

tests/test_mirror.py

@@ -145,12 +145,12 @@ def test_MirrorAddRemove(self, dvs, testlog):
         self.add_neighbor("Ethernet16", nhop_ip, "02:04:06:08:10:12")
         assert self.get_mirror_session_state(session)["status"] == "inactive"
 
-        # add route to mirror destination via 10.0.0.1
-        self.add_route(dvs, "2.2.2.2", "10.0.0.1")
+        # add route to mirror destination via next hop ip
+        self.add_route(dvs, dst_ip, nhop_ip)
         assert self.get_mirror_session_state(session)["status"] == "active"
         assert self.get_mirror_session_state(session)["monitor_port"] == "Ethernet16"
         assert self.get_mirror_session_state(session)["dst_mac"] == "02:04:06:08:10:12"
-        assert self.get_mirror_session_state(session)["route_prefix"] == "2.2.2.2/32"
+        assert self.get_mirror_session_state(session)["route_prefix"] == "{}/{}".format(dst_ip, 32)
 
         # check asic database
         tbl = swsscommon.Table(self.adb, "ASIC_STATE:SAI_OBJECT_TYPE_MIRROR_SESSION")