.0.2.1/24")]},
-        "if2": {"prefixes4": [("198.51.100.1/24", "198.51.100.2/24")]},
+        "if2": {"prefixes4": [("198.51.100.1/24", "198.51.100.2/24")], "mtu": 1492},
     }
 
     def vnet2_handler(self, vnet):
-        ifname = vnet.iface_alias_map["if1"].name
-        if2name = vnet.iface_alias_map["if2"].name
-
         ToolsHelper.print_output("/sbin/pfctl -e")
         ToolsHelper.pf_rules([
             "set reassemble yes",
@@ -61,8 +58,6 @@ class TestICMP(VnetTestTemplate):
         ToolsHelper.print_output("/sbin/sysctl net.inet.ip.forwarding=1")
         ToolsHelper.print_output("/sbin/pfctl -x loud")
 
-        ToolsHelper.print_output("/sbin/ifconfig %s mtu 1492" % if2name)
-
     def vnet3_handler(self, vnet):
         # Import in the correct vnet, so at to not confuse Scapy
         import scapy.all as sp
@@ -70,7 +65,6 @@ class TestICMP(VnetTestTemplate):
         ifname = vnet.iface_alias_map["if2"].name
         ToolsHelper.print_output("/sbin/route add default 198.51.100.1")
         ToolsHelper.print_output("/sbin/ifconfig %s inet alias 198.51.100.3/24" % ifname)
-        ToolsHelper.print_output("/sbin/ifconfig %s mtu 1492" % ifname)
 
         def checkfn(packet):
             icmp = packet.getlayer(sp.ICMP)
diff --git a/tests/sys/netpfil/pf/nat66.py b/tests/sys/netpfil/pf/nat66.py
index 16b4ef3dd02b..339895b253ae 100644
--- a/tests/sys/netpfil/pf/nat66.py
+++ b/tests/sys/netpfil/pf/nat66.py
@@ -39,13 +39,11 @@ class TestNAT66(VnetTestTemplate):
         "vnet1": {"ifaces": ["if1"]},
         "vnet2": {"ifaces": ["if1", "if2"]},
         "vnet3": {"ifaces": ["if2"]},
-        "if1": {"prefixes6": [("2001:db8::2/64", "2001:db8::1/64")]},
+        "if1": {"prefixes6": [("2001:db8::2/64", "2001:db8::1/64")], "mtu": 9000},
         "if2": {"prefixes6": [("2001:db8:1::1/64", "2001:db8:1::2/64")]},
     }
 
     def vnet2_handler(self, vnet):
-        ifname = vnet.iface_alias_map["if1"].name
-        ToolsHelper.print_output("/sbin/ifconfig %s mtu 9000" % ifname)
         outifname = vnet.iface_alias_map["if2"].name
 
         ToolsHelper.print_output("/sbin/pfctl -e")
@@ -130,8 +128,6 @@ class TestNAT66(VnetTestTemplate):
     @pytest.mark.require_progs(["scapy"])
     def test_npt_icmp(self):
         cl_vnet = self.vnet_map["vnet1"]
-        ifname = cl_vnet.iface_alias_map["if1"].name
-        ToolsHelper.print_output("/sbin/ifconfig %s mtu 9000" % ifname)
 
         ToolsHelper.print_output("/sbin/route add -6 2001:db8:1::/64 2001:db8::1")
 
@@ -160,7 +156,6 @@ class TestNAT66(VnetTestTemplate):
     def test_npt_route_to_icmp(self):
         cl_vnet = self.vnet_map["vnet1"]
         ifname = cl_vnet.iface_alias_map["if1"].name
-        ToolsHelper.print_output("/sbin/ifconfig %s mtu 9000" % ifname)
         ToolsHelper.print_output("/sbin/ifconfig %s inet6 alias 2001:db8::3/64" % ifname)
 
         ToolsHelper.print_output("/sbin/route add -6 2001:db8:1::/64 2001:db8::1")