[YANG] DHCPv4 Relay Yang Model (#22338)

Why I did it
Yang model for new DHCPv4 Relay design. See following PR for details:

Signed-off-by: Ashutosh Agrawal <[email protected]>

Author: ashutosh-agrawal

src/sonic-yang-models/doc/Configuration.md

@@ -31,6 +31,7 @@
   * [Device Metadata](#device-metadata)
   * [Device neighbor metada](#device-neighbor-metada)
   * [DHCP_RELAY](#dhcp_relay)
+  * [DHCPV4_RELAY](#dhcpv4_relay)
   * [DHCP Server IPV4](#dhcp_server_ipv4)
   * [BMP](#bmp)
   * [DSCP_TO_TC_MAP](#dscp_to_tc_map)
@@ -1138,6 +1139,28 @@ instance is supported in SONiC.
 
 ```
 
+### DHCPV4_RELAY
+
+```
+{
+"DHCPV4_RELAY": {
+    "Vlan1000": {
+        "dhcpv4_servers": [
+            "192.168.0.1",
+            "192.168.0.2"
+        ],
+        "server_vrf": "Vrf_RED",
+        "source_interface": "Loopback0",
+        "link_selection": "enable",
+        "vrf_selection": "enable",
+        "server_id_override": "enable",
+        "agent_relay_mode": "append",
+        "max_hop_count": 10
+    }
+}
+
+```
+
 ### BMP
 BMP related configuration are defined in **bgp_neighbor_table**,**bgp_rib_in_table**, **bgp_rib_out_table** tables.
 

src/sonic-yang-models/setup.py

@@ -45,6 +45,7 @@
     'sonic-device_neighbor.yang',
     'sonic-device_neighbor_metadata.yang',
     'sonic-dhcp-server.yang',
+    'sonic-dhcpv4-relay.yang',
     'sonic-dhcpv6-relay.yang',
     'sonic-dns.yang',
     'sonic-events-bgp.yang',

src/sonic-yang-models/tests/files/sample_config_db.json

@@ -2142,6 +2142,36 @@
                 "interface_id": "true"
             }
         },
+        "DHCPV4_RELAY": {
+            "Vlan111": {
+                "dhcpv4_servers": [
+                    "10.222.72.116"
+                ],
+                "server_vrf": "Vrf_blue",
+                "source_interface": "Loopback0",
+                "link_selection": "enable",
+                "vrf_selection": "enable",
+                "server_id_override": "enable",
+                "agent_relay_mode": "forward_untouched",
+                "max_hop_count": "4"
+            },
+            "Vlan222": {
+                "dhcpv4_servers": [
+                    "10.222.72.116",
+                    "10.222.82.226"
+                ]
+            },
+            "Vlan777": {
+                "dhcpv4_servers": [
+                    "10.222.82.226"
+                ],
+                "source_interface": "Loopback0",
+                "link_selection": "enable",
+                "vrf_selection": "disable",
+                "server_id_override": "disable",
+                "agent_relay_mode": "discard"
+            }
+        },
         "DHCP_SERVER_IPV4": {
             "Vlan100": {
                 "gateway": "100.1.1.1",

src/sonic-yang-models/tests/yang_model_tests/tests/dhcpv4_relay.json

@@ -0,0 +1,40 @@
+{
+    "DHCPv4_RELAY_VALID_FORMAT_BASE": {
+        "desc": "Add dhcpv4 relay with minimum config."
+    },
+    "DHCPv4_RELAY_WITH_MULTIPLE_SERVERS": {
+        "desc": "Add dhcpv4 relay with multiple servers."
+    },
+    "DHCPv4_RELAY_WITH_SERVER_VRF": {
+        "desc": "Add dhcpv4 relay with server VRF and required options."
+    },
+    "DHCPv4_RELAY_WITH_LINK_SELECT": {
+        "desc": "Add dhcpv4 relay with link-select option enabled."
+    },
+    "DHCPv4_RELAY_WITH_AGENT_RELAY_MODE_DISCARD": {
+        "desc": "Add dhcpv4 relay with minimum config."
+    },
+    "DHCPv4_RELAY_WITH_VALID_MAX_HOP_CNT": {
+        "desc": "Add dhcpv4 relay with valid max_hop_cnt."
+    },
+    "DHCPv4_RELAY_WITH_MISSING_SERVER_IP": {
+        "desc": "Add dhcpv4 relay with missing server_ip.",
+        "eStrKey": "MinElements"
+    },
+    "DHCPv4_RELAY_WITH_SERVER_VRF_AND_MISSING_LINK_SELECT_OPTION": {
+        "desc": "Add dhcpv4 relay with server_vrf and missing link-selection option.",
+        "eStrKey": "Must"
+    },
+    "DHCPv4_RELAY_WITH_SERVER_VRF_AND_MISSING_VRF_SELECT_OPTION": {
+        "desc": "Add dhcpv4 relay with server_vrf and missing vrf-selection option.",
+        "eStrKey": "Must"
+    },
+    "DHCPv4_RELAY_WITH_SERVER_VRF_AND_MISSING_SERVER_OVERRIDE_OPTION": {
+        "desc": "Add dhcpv4 relay with server_vrf and missing lserver-override option.",
+        "eStrKey": "Must"
+    },
+    "DHCPv4_RELAY_WITH_SERVER_VRF_AND_MISSING_SOURCE_INTERFACE_OPTION": {
+        "desc": "Add dhcpv4 relay with server_vrf and missing source-interface option.",
+        "eStrKey": "Must"
+    }
+}