tests: Check if OSPFv3 loopback interface gets cost as 0

Signed-off-by: Donatas Abraitis <donatas@opensourcerouting.org>
(cherry picked from commit e8615867d0)

tests/topotests/ospf6_loopback_cost/r1/frr.conf

@@ -0,0 +1,16 @@
+!
+int lo
+ ipv6 address 2001:db8::1/128
+ ipv6 ospf6 area 0.0.0.0
+ ipv6 ospf6 passive
+!
+int r1-eth0
+ ipv6 address 2001:db8:1::1/64
+ ipv6 ospf6 area 0.0.0.0
+ ipv6 ospf6 hello-interval 1
+ ipv6 ospf6 dead-interval 4
+!
+router ospf6
+ ospf6 router-id 0.0.0.1
+exit
+!

tests/topotests/ospf6_loopback_cost/r2/frr.conf

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+!
+int lo
+ ipv6 address 2001:db8::2/128
+ ipv6 ospf6 area 0.0.0.0
+ ipv6 ospf6 passive
+!
+int r2-eth0
+ ipv6 address 2001:db8:1::2/64
+ ipv6 ospf6 area 0.0.0.0
+ ipv6 ospf6 hello-interval 1
+ ipv6 ospf6 dead-interval 4
+!
+router ospf6
+ ospf6 router-id 0.0.0.2
+exit
+!

tests/topotests/ospf6_loopback_cost/test_ospf6_loopback_cost.py

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+#!/usr/bin/env python
+# SPDX-License-Identifier: ISC
+
+# Copyright (c) 2023 by
+# Donatas Abraitis <donatas@opensourcerouting.org>
+#
+
+"""
+Test if OSPFv3 loopback interfaces get a cost of 0.
+
+https://www.rfc-editor.org/rfc/rfc5340.html#page-37:
+
+If the interface type is point-to-multipoint or the interface is
+in the state Loopback, the global scope IPv6 addresses associated
+with the interface (if any) are copied into the intra-area-prefix-LSA
+with the PrefixOptions LA-bit set, the PrefixLength set to 128, and
+the metric set to 0.
+"""
+
+import os
+import sys
+import json
+import pytest
+import functools
+
+pytestmark = pytest.mark.ospf6d
+
+CWD = os.path.dirname(os.path.realpath(__file__))
+sys.path.append(os.path.join(CWD, "../"))
+
+# pylint: disable=C0413
+from lib import topotest
+from lib.topogen import Topogen, TopoRouter, get_topogen
+
+
+def setup_module(mod):
+    topodef = {"s1": ("r1", "r2")}
+    tgen = Topogen(topodef, mod.__name__)
+    tgen.start_topology()
+
+    router_list = tgen.routers()
+
+    for _, (rname, router) in enumerate(router_list.items(), 1):
+        router.load_frr_config(os.path.join(CWD, "{}/frr.conf".format(rname)))
+
+    tgen.start_router()
+
+
+def teardown_module(mod):
+    tgen = get_topogen()
+    tgen.stop_topology()
+
+
+def test_ospf6_loopback_cost():
+    tgen = get_topogen()
+
+    if tgen.routers_have_failure():
+        pytest.skip(tgen.errors)
+
+    r1 = tgen.gears["r1"]
+
+    def _show_ipv6_route():
+        output = json.loads(r1.vtysh_cmd("show ipv6 route json"))
+        expected = {
+            "2001:db8::1/128": [
+                {
+                    "metric": 0,
+                    "distance": 110,
+                }
+            ],
+            "2001:db8::2/128": [
+                {
+                    "metric": 10,
+                    "distance": 110,
+                }
+            ],
+        }
+        return topotest.json_cmp(output, expected)
+
+    test_func = functools.partial(
+        _show_ipv6_route,
+    )
+    _, result = topotest.run_and_expect(test_func, None, count=60, wait=1)
+    assert result is None, "Loopback cost isn't 0"
+
+
+if __name__ == "__main__":
+    args = ["-s"] + sys.argv[1:]
+    sys.exit(pytest.main(args))