Bug: test_vnet_decap does not support in v6 only topo

[weiguo-nvidia]

Is it platform specific

generic

Importance or Severity

Critical

Description of the bug

The case test_vnet_decap[inner_ipv4-outer_ipv4] and test_vnet_decap[inner_ipv6-outer_ipv4] do not support v6 only topo.
For v6 only topo, such as t1-isolated-v6-d56u1-lag, only has ipv6 address, no ipv4 address.
Need align this case to support v6 only topo

RuntimeError: There are not enough interfaces needed to perform the test. We need atleast 2 interfaces, but only 0 are available.
request = >
duthosts = []
inner_ip_version = 6, outer_ip_version = 4

    @pytest.fixture
    def setup(request, duthosts, rand_one_dut_hostname, tbinfo, inner_ip_version, outer_ip_version):
        """
        Creates a VXLAN tunnel, a VNET, and one VNET route (with a single endpoint). Also finds an appropriate
        Ethernet port for sending the IP-in-IP packet to the DUT.
        Yields test configuration and data.
        """
        duthost = duthosts[rand_one_dut_hostname]
        asic_type = duthost.facts["asic_type"]
        if asic_type not in ["cisco-8000", "mellanox"]:
            pytest.skip("The VNET decap test will only run on Cisco-8000 and Mellanox ASICs.")
        platform = duthost.facts["platform"]
        if platform in ['x86_64-mlnx_msn2700-r0', 'x86_64-mlnx_msn2700a1-r0']:
            pytest.skip("Mellanox msn2700 switches do not support VNET decapsulation.")
    
        # Should I keep the temporary files copied to DUT?
        ecmp_utils.Constants["KEEP_TEMP_FILES"] = request.config.option.keep_temp_files
        # Is debugging going on, or is it a production run? If it is a
        # production run, use time-stamped file names for temp files.
        ecmp_utils.Constants["DEBUG"] = request.config.option.debug_enabled
        # The host id in the ip addresses for DUT. It can be anything,
        # but helps to keep as a single number that is easy to identify
        # as DUT.
        ecmp_utils.Constants["DUT_HOSTID"] = request.config.option.dut_hostid
    
        # Setup
        router_mac = duthost.facts["router_mac"]
        ecmp_utils.configure_vxlan_switch(duthost, vxlan_port=VXLAN_DST_PORT, dutmac=router_mac)
        minigraph_facts = duthost.get_extended_minigraph_facts(tbinfo)
        outer_ip_version_str = f"v{outer_ip_version}"
>       vnet_interface = ecmp_utils.select_required_interfaces(duthost, 1, minigraph_facts, outer_ip_version_str)[0]

asic_type  = 'mellanox'
duthost    = 
duthosts   = []
inner_ip_version = 6
minigraph_facts = {'deployment_id': '1', 'dhcp_servers': [], 'dhcpv6_servers': [], 'forced_mgmt_routes': ['10.75.0.0/16', '10.213.0.0/16', '10.215.0.0/16', '10.9.0.0/16', '10.212.0.0/16', '10.245.0.0/16', ...], ...}
outer_ip_version = 4
outer_ip_version_str = 'v4'
platform   = 'x86_64-nvidia_sn5640-r0'
request    = >
router_mac = '38:25:f3:75:98:0a'

vxlan/test_vnet_decap.py:88: 
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 

self = 
duthost = , number_of_required_interfaces = 1
minigraph_data = {'deployment_id': '1', 'dhcp_servers': [], 'dhcpv6_servers': [], 'forced_mgmt_routes': ['10.75.0.0/16', '10.213.0.0/16', '10.215.0.0/16', '10.9.0.0/16', '10.212.0.0/16', '10.245.0.0/16', ...], ...}
af = 'v4'

    def select_required_interfaces(
            self, duthost, number_of_required_interfaces, minigraph_data, af):
        '''
        Pick the required number of interfaces to use for tests.
        These interfaces will be selected based on if they are currently
        running a established BGP.  The interfaces will be picked from the T0
        facing side.
        '''
        bgp_interfaces = self.get_all_interfaces_running_bgp(
            duthost,
            minigraph_data,
            "T0")
    
        # Randomly pick the interface from the above list
        list_of_bgp_ips = []
        for neigh_ip_address in bgp_interfaces:
            if isinstance(ip_address(neigh_ip_address), self.IP_TYPE[af]):
                list_of_bgp_ips.append(neigh_ip_address)
    
        ret_interface_list = []
        available_number = len(list_of_bgp_ips)
        # Confirm there are enough interfaces (basicaly more than or equal
        # to the number of vnets).
        if available_number <= number_of_required_interfaces+1:
>           raise RuntimeError(
                "There are not enough interfaces needed to perform the test. "
                "We need atleast {} interfaces, but only {} are "
                "available.".format(
                    number_of_required_interfaces+1, available_number))
E           RuntimeError: There are not enough interfaces needed to perform the test. We need atleast 2 interfaces, but only 0 are available.

af         = 'v4'
available_number = 0
bgp_interfaces = {'fc00::102': {'Ethernet64': 'fc00::101'}, 'fc00::122': {'Ethernet72': 'fc00::121'}, 'fc00::142': {'Ethernet80': 'fc00::141'}, 'fc00::162': {'Ethernet88': 'fc00::161'}, ...}
duthost    = 
list_of_bgp_ips = []
minigraph_data = {'deployment_id': '1', 'dhcp_servers': [], 'dhcpv6_servers': [], 'forced_mgmt_routes': ['10.75.0.0/16', '10.213.0.0/16', '10.215.0.0/16', '10.9.0.0/16', '10.212.0.0/16', '10.245.0.0/16', ...], ...}
neigh_ip_address = 'fc00::162'
number_of_required_interfaces = 1
ret_interface_list = []
self       = 

common/vxlan_ecmp_utils.py:141: RuntimeError

Steps to Reproduce

Run case tests/vxlan/test_vnet_decap.py

Actual Behavior and Expected Behavior

Case pass on v6 only topo

Relevant log output

Output of show version

Attach files (if any)

No response