[tests/generic_config_updater] Replace IP testcase for BGP Listener (#…

…5092)

### Description of PR

Summary: Replace IP testcase for BGP Listener 
Fixes # (issue)

### Type of change
- [ ] Bug fix
- [ ] Testbed and Framework(new/improvement)
- [x] Test case(new/improvement)

### Approach
#### What is the motivation for this PR?
1. Add IP replacement case for BGPL;
2. Modify generators function to adapt to our case
#### How did you do it?
Add the testcase. 
#### How did you verify/test it?
Run test of sonic-mgmt/tests/generic_config_updater/test_bgpl.py on KVM

tests/common/helpers/generators.py

@@ -10,7 +10,7 @@ def generate_ips(num, prefix, exclude_ips):
     exclude_ips.append(prefix.network)
     available_ips = list(prefix)
 
-    if len(available_ips) - len(exclude_ips)< num:
+    if len(available_ips) - len(exclude_ips) < num:
         raise Exception("Not enough available IPs")
 
     generated_ips = []
@@ -41,14 +41,16 @@ def route_through_default_routes(host, ip_addr):
     return ret
 
 
-def generate_ip_through_default_route(host):
+def generate_ip_through_default_route(host, exclude_ips=None):
     """
     @summary: Generate a random IP address routed through default routes
     @param host: The duthost
+    @param exclude_ips: IPs to exclude, default None
     @return: A str, on None if non ip is found in given range
     """
+    exclude_ips = exclude_ips if exclude_ips is not None else []
     for leading in range(11, 255):
-        ip_addr = generate_ips(1, "{}.0.0.1/24".format(leading), [])[0]
+        ip_addr = generate_ips(1, "{}.0.0.1/24".format(leading), exclude_ips)[0]
         if route_through_default_routes(host, ip_addr):
             return ip_addr
     return None

tests/generic_config_updater/test_bgpl.py

@@ -135,7 +135,7 @@ def bgpmon_tc1_add_duplicate(duthost, bgpmon_setup_info):
     finally:
         delete_tmpfile(duthost, tmpfile)
 
-def bgpmon_tc1_admin_change(duthost):
+def bgpmon_tc1_admin_change(duthost, bgpmon_setup_info):
     """ Test to admin down bgpmon config
     """
     peer_addr, _, _ = bgpmon_setup_info
@@ -162,6 +162,44 @@ def bgpmon_tc1_admin_change(duthost):
     finally:
         delete_tmpfile(duthost, tmpfile)
 
+def bgpmon_tc1_ip_change(duthost, bgpmon_setup_info):
+    """ Test to replace bgpmon ip address
+    """
+    peer_addr, local_addr, bgp_asn = bgpmon_setup_info
+    peer_addr_replaced = generate_ip_through_default_route(duthost, [IPNetwork(peer_addr).ip])
+    peer_addr_replaced = str(IPNetwork(peer_addr_replaced).ip)
+    json_patch = [
+        {
+            "op": "remove",
+            "path": "/BGP_MONITORS/{}".format(peer_addr)
+        },
+        {
+            "op": "add",
+            "path": "/BGP_MONITORS/{}".format(peer_addr_replaced),
+            "value": {
+                "admin_status": "up",
+                "asn": bgp_asn,
+                "holdtime": "180",
+                "keepalive": "60",
+                "local_addr": local_addr,
+                "name": "BGPMonitor",
+                "nhopself": "0",
+                "rrclient": "0"
+            }
+        }
+    ]
+
+    tmpfile = generate_tmpfile(duthost)
+    logger.info("tmpfile {}".format(tmpfile))
+
+    try:
+        output = apply_patch(duthost, json_data=json_patch, dest_file=tmpfile)
+        expect_op_success(duthost, output)
+
+        check_bgpmon_with_addr(duthost, peer_addr_replaced)
+    finally:
+        delete_tmpfile(duthost, tmpfile)
+
 def bgpmon_tc1_remove(duthost):
     """ Test to remove bgpmon config
     """
@@ -194,4 +232,6 @@ def test_bgpmon_tc1_add_and_remove(duthost, bgpmon_setup_info):
     """
     bgpmon_tc1_add_init(duthost, bgpmon_setup_info)
     bgpmon_tc1_add_duplicate(duthost, bgpmon_setup_info)
+    bgpmon_tc1_admin_change(duthost, bgpmon_setup_info)
+    bgpmon_tc1_ip_change(duthost, bgpmon_setup_info)
     bgpmon_tc1_remove(duthost)