[test_fib] Enable storage backend topologies (sonic-net#3909)

Approach
What is the motivation for this PR?
Restore changes of sonic-net#3734 that are removed by sonic-net#3702 .

How did you do it?
How did you verify/test it?

tests/common/fixtures/fib_utils.py

@@ -115,6 +115,7 @@ def get_fib_info(duthost, dut_cfg_facts, duts_mg_facts):
 
         po = asic_cfg_facts.get('PORTCHANNEL', {})
         ports = asic_cfg_facts.get('PORT', {})
+        sub_interfaces = asic_cfg_facts.get('VLAN_SUB_INTERFACE', {})
 
         with open("/tmp/fib/{}/tmp/fib.{}.txt".format(duthost.hostname, timestamp)) as fp:
             fib = json.load(fp)
@@ -135,7 +136,9 @@ def get_fib_info(duthost, dut_cfg_facts, duts_mg_facts):
                             else:
                                 oports.append([str(duts_mg_facts['minigraph_ptf_indices'][x]) for x in po[ifname]['members']])
                     else:
-                        if ports.has_key(ifname):
+                        if sub_interfaces.has_key(ifname):
+                            oports.append([str(duts_mg_facts['minigraph_ptf_indices'][ifname.split('.')[0]])])
+                        elif ports.has_key(ifname):
                             if 'role' in ports[ifname] and ports[ifname]['role'] == 'Int':
                                 skip = True
                             else:
@@ -181,8 +184,8 @@ def gen_fib_info_file(ptfhost, fib_info, filename):
     ptfhost.copy(src=tmp_fib_info.name, dest=filename)
 
 
-@pytest.fixture(scope='module')
-def fib_info_files(duthosts, ptfhost, duts_running_config_facts, duts_minigraph_facts, tbinfo):
+@pytest.fixture(scope='function')
+def fib_info_files(duthosts, ptfhost, duts_running_config_facts, duts_minigraph_facts, tbinfo, request):
     """Get FIB info from database and store to text files on PTF host.
 
     For T2 topology, generate a single file to /root/fib_info_all_duts.txt to PTF host.
@@ -200,11 +203,17 @@ def fib_info_files(duthosts, ptfhost, duts_running_config_facts, duts_minigraph_
         list: List of FIB info file names on PTF host.
     """
     duts_config_facts = duts_running_config_facts
+    testname = request.node.name
     files = []
     if tbinfo['topo']['type'] != "t2":
         for dut_index, duthost in enumerate(duthosts):
             fib_info = get_fib_info(duthost, duts_config_facts[duthost.hostname], duts_minigraph_facts[duthost.hostname])
-            filename = '/root/fib_info_dut{}.txt'.format(dut_index)
+            if 'test_basic_fib' in testname and 'backend' in tbinfo['topo']['name']:
+                # if it is a storage backend topology(bt0 or bt1) and testcase is test_basic_fib
+                # add a default route as failover in the prefix matching
+                fib_info[u'0.0.0.0/0'] = []
+                fib_info[u'::/0'] = []
+            filename = '/root/fib_info_dut_{0}_{1}.txt'.format(testname, dut_index)
             gen_fib_info_file(ptfhost, fib_info, filename)
             files.append(filename)
     else:

tests/fib/test_fib.py

@@ -10,6 +10,7 @@
 from tests.common.fixtures.ptfhost_utils import change_mac_addresses        # lgtm[py/unused-import]
 from tests.common.fixtures.ptfhost_utils import remove_ip_addresses         # lgtm[py/unused-import]
 from tests.common.fixtures.ptfhost_utils import copy_ptftests_directory     # lgtm[py/unused-import]
+from tests.common.fixtures.ptfhost_utils import set_ptf_port_mapping_mode   # lgtm[py/unused-import]
 from tests.common.fixtures.ptfhost_utils import ptf_test_port_map
 from tests.ptf_runner import ptf_runner
 from tests.common.helpers.assertions import pytest_assert