[vstest]: add default vs test

Check sonic-net#6483

add test to make sure default route change in eth0 does not
affect the default route in the default vrf

Signed-off-by: Guohan Lu <lguohan@gmail.com>

Author: lguohan

platform/vs/tests/bgp/files/default_route/bgpd.conf

@@ -0,0 +1,4 @@
+router bgp 65501
+  bgp router-id 1.1.1.1
+  no bgp ebgp-requires-policy
+  neighbor 10.10.10.1 remote-as 65502

platform/vs/tests/bgp/files/default_route/default_route.conf

@@ -0,0 +1,17 @@
+neighbor 10.10.10.0 {
+    router-id 1.2.3.4;
+    local-address 10.10.10.1;
+    local-as 65502;
+    peer-as 65501;
+    group-updates false;
+
+    family {
+        ipv4 unicast;
+    }
+
+    static {
+        route 0.0.0.0/0 {
+            next-hop 10.10.10.1;
+        }
+    }
+}

platform/vs/tests/bgp/test_default_route.py

@@ -0,0 +1,48 @@
+from swsscommon import swsscommon
+import os
+import re
+import time
+import json
+import pytest
+
+def test_DefaultRoute(dvs, testlog):
+
+    dvs.copy_file("/etc/frr/", "bgp/files/default_route/bgpd.conf")
+    dvs.runcmd("supervisorctl start bgpd")
+    dvs.runcmd("ip addr add 10.10.10.0/31 dev Ethernet0")
+    dvs.runcmd("config interface startup Ethernet0")
+    dvs.runcmd("ip route del 0.0.0.0/0")
+    dvs.runcmd("vtysh -c \"confgure terminal\" -c \"ip route 0.0.0.0/0 via 172.17.0.1 200\"")
+
+    dvs.servers[0].runcmd("ip addr add 10.10.10.1/31 dev eth0")
+    dvs.servers[0].runcmd("ifconfig eth0 up")
+
+    time.sleep(5)
+
+    print(dvs.runcmd("supervisorctl status"))
+
+    p = dvs.servers[0].runcmd_async("exabgp -d bgp/files/default_route/default_route.conf")
+
+    time.sleep(10)
+
+    (exit_code, output) = dvs.runcmd(["redis-cli", "hgetall", "ROUTE_TABLE:0.0.0.0/0"])
+    print(exit_code, output)
+
+    # make sure 10.10.10.1 is the correct next hop for default route
+    assert "10.10.10.1" in output
+
+    # insert default route for table default
+    dvs.runcmd("ip route add default via 172.17.0.1 table default")
+
+    (exit_code, output) = dvs.runcmd(["redis-cli", "hgetall", "ROUTE_TABLE:0.0.0.0/0"])
+    print(exit_code, output)
+
+    time.sleep(10)
+    # make sure 10.10.10.1 is still the correct next hop for default route
+    assert "10.10.10.1" in output
+
+    p.terminate()
+    p = p.wait()
+
+
+