Update arp_update to refresh neighbor entries from APP_DB (#4125)

Author: prsunny

files/scripts/arp_update

@@ -46,5 +46,26 @@ while /bin/true; do
       ip6cmd="ip -6 neigh show | grep -v fe80 | grep $vlan | cut -d ' ' -f 1,3 | $ndisc6cmd"
       eval `eval $ip6cmd`
   done
+
+  # refresh neighbor entries from APP_DB in case of mismatch with kernel
+  DBNEIGH=$(sonic-db-cli APPL_DB keys NEIGH_TABLE*)
+  KERNEIGH4=$(ip -4 neigh show | grep Vlan | cut -d ' ' -f 1,3  --output-delimiter=',')
+  KERNEIGH6=$(ip -6 neigh show | grep -v fe80 | grep Vlan | cut -d ' ' -f 1,3  --output-delimiter=',')
+  for neigh in $DBNEIGH; do
+      intf="$( cut -d ':' -f 2 <<< "$neigh" )"
+      ip="$( cut -d ':' -f 3- <<< "$neigh" )"
+      if [[ $intf == *"Vlan"* ]]; then
+          if [[ $ip == *"."* ]] && [[ ! $KERNEIGH4 =~ "${ip},${intf}" ]]; then
+              pingcmd="timeout 0.2 ping -I $intf -n -q -i 0 -c 1 -W 1 $ip >/dev/null"
+              eval $pingcmd
+              logger "arp_update: mismatch arp entry, pinging ${ip} on ${intf}"
+          elif [[ $ip == *":"* ]] && [[ ! $KERNEIGH6 =~ "${ip},${intf}" ]]; then
+              ping6cmd="timeout 0.2 ping6 -I $intf -n -q -i 0 -c 1 -W 1 $ip >/dev/null"
+              eval $ping6cmd
+              logger "arp_update: mismatch v6 nbr entry, pinging ${ip} on ${intf}"
+          fi
+      fi
+  done
+
   sleep 300
 done