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VyOS CLI command
  set service ndp-proxy interface eth0 prefix 2001:db8::/64 mode 'static'
Will generate the following NDP proxy configuration
  $ cat /run/ndppd/ndppd.conf
  # autogenerated by service_ndp-proxy.py
  # This tells 'ndppd' how often to reload the route file /proc/net/ipv6_route
  route-ttl 30000
  # This sets up a listener, that will listen for any Neighbor Solicitation
  # messages, and respond to them according to a set of rules
  proxy eth0 {
      # Turn on or off the router flag for Neighbor Advertisements
      router no
      # Control how long to wait for a Neighbor Advertisment message before invalidating the entry (milliseconds)
      timeout 500
      # Control how long a valid or invalid entry remains in the cache (milliseconds)
      ttl 30000
      # This is a rule that the target address is to match against. If no netmask
      # is provided, /128 is assumed. You may have several rule sections, and the
      # addresses may or may not overlap.
      rule 2001:db8::/64 {
          static
      }
  }
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