[arp_update]: Set failed IPv6 neighbors to incomplete (#11919)

After pinging any failed IPv6 neighbor entries, set the remaining failed/incomplete entries to a permanent INCOMPLETE state. This manual setting to INCOMPLETE prevents these entries from automatically transitioning to FAILED state, and since they are now incomplete any subsequent NA messages for these neighbors is able to resolve the entry in the cache.

Signed-off-by: Lawrence Lee <lawlee@microsoft.com>

files/scripts/arp_update

@@ -50,6 +50,17 @@ while /bin/true; do
       eval `eval $ip6cmd`
 
       if [[ $SUBTYPE == "dualtor" ]]; then
+        # manually set any remaining FAILED/INCOMPLETE entries to permanently INCOMPLETE
+        # this prevents any remaining INCOMPLETE entries from automatically transitioning to FAILED
+        # once these entries are incomplete, any subsequent neighbor advertisement messages
+        # are able to resolve the entry
+
+        # generates the following command for each failed or incomplete IPv6 neighbor
+        # ip neigh replace <neighbor IPv6> dev <VLAN name> nud incomplete
+        neigh_replace_template="sed -e 's/^/ip neigh replace /' -e 's/,/ dev /' -e 's/$/ nud incomplete;/'" 
+        ip_neigh_replace_cmd="ip -6 neigh show | grep -v fe80 | grep Vlan1000 | grep -E 'FAILED|INCOMPLETE' | cut -d ' ' -f 1,3 --output-delimiter=',' | $neigh_replace_template"
+        eval `eval $ip_neigh_replace_cmd`
+
         # on dual ToR devices, try to resolve failed neighbor entries since
         # these entries will have tunnel routes installed, preventing normal 
         # neighbor resolution (SWSS PR #2137)