Align default MTU value as SAI default (sonic-net#705)

* Align default MTU value as SAI default
* Added comments, updated VS test
* Corrected the values for physical interfaces, set by portmgr
* Fixed vnet VS test for RIF mtu

Author: prsunny

orchagent/port.h

@@ -11,7 +11,12 @@ extern "C" {
 #include <map>
 
 #define DEFAULT_PORT_VLAN_ID    1
-#define DEFAULT_MTU              9100
+/*
+ * Default MTU is derived from SAI_PORT_ATTR_MTU (1514)
+ * Orchagent adds extra 22 bytes for Ethernet header and FCS,
+ * hence setting to 1492 (1514 - 22)
+ */
+#define DEFAULT_MTU             1492
 
 namespace swss {
 

tests/test_interface.py

@@ -74,7 +74,7 @@ def test_InterfaceAddRemoveIpv6Address(self, dvs, testlog):
                 for fv in fvs:
                     if fv[0] == "SAI_ROUTER_INTERFACE_ATTR_TYPE":
                         assert fv[1] == "SAI_ROUTER_INTERFACE_TYPE_PORT"
-                    # the default MTU without any configuration is 9100
+                    # the default MTU without any configuration is 9100, set by portmgr
                     if fv[0] == "SAI_ROUTER_INTERFACE_ATTR_MTU":
                         assert fv[1] == "9100"
 
@@ -146,7 +146,7 @@ def test_InterfaceAddRemoveIpv4Address(self, dvs, testlog):
                 for fv in fvs:
                     if fv[0] == "SAI_ROUTER_INTERFACE_ATTR_TYPE":
                         assert fv[1] == "SAI_ROUTER_INTERFACE_TYPE_PORT"
-                    # the default MTU without any configuration is 9100
+                    # the default MTU without any configuration is 9100, set by portmgr
                     if fv[0] == "SAI_ROUTER_INTERFACE_ATTR_MTU":
                         assert fv[1] == "9100"
 
@@ -296,9 +296,9 @@ def test_InterfaceAddRemoveIpv4Address(self, dvs, testlog):
                 for fv in fvs:
                     if fv[0] == "SAI_ROUTER_INTERFACE_ATTR_TYPE":
                         assert fv[1] == "SAI_ROUTER_INTERFACE_TYPE_PORT"
-                    # the default MTU without any configuration is 9100
+                    # the default MTU without any configuration is 1492
                     if fv[0] == "SAI_ROUTER_INTERFACE_ATTR_MTU":
-                        assert fv[1] == "9100"
+                        assert fv[1] == "1492"
 
         # check ASIC route database
         tbl = swsscommon.Table(self.adb, "ASIC_STATE:SAI_OBJECT_TYPE_ROUTE_ENTRY")

tests/test_vnet.py

@@ -477,14 +477,15 @@ def check_router_interface(self, dvs, name, vlan_oid=0):
         expected_attr = {
                         "SAI_ROUTER_INTERFACE_ATTR_VIRTUAL_ROUTER_ID": self.vr_map[name].get('ing'),
                         "SAI_ROUTER_INTERFACE_ATTR_SRC_MAC_ADDRESS": switch_mac,
-                        "SAI_ROUTER_INTERFACE_ATTR_MTU": "9100",
+                        "SAI_ROUTER_INTERFACE_ATTR_MTU": "1492",
                     }
 
         if vlan_oid:
             expected_attr.update({'SAI_ROUTER_INTERFACE_ATTR_TYPE': 'SAI_ROUTER_INTERFACE_TYPE_VLAN'})
             expected_attr.update({'SAI_ROUTER_INTERFACE_ATTR_VLAN_ID': vlan_oid})
         else:
             expected_attr.update({'SAI_ROUTER_INTERFACE_ATTR_TYPE': 'SAI_ROUTER_INTERFACE_TYPE_PORT'})
+            expected_attr.update({'SAI_ROUTER_INTERFACE_ATTR_MTU': '9100'})
 
         new_rif = get_created_entry(asic_db, self.ASIC_RIF_TABLE, self.rifs)
         check_object(asic_db, self.ASIC_RIF_TABLE, new_rif, expected_attr)