Rashmi Bhardwaj | Blog,Protocol

NDP is abbreviation for Neighbour Discovery Protocol and can be considered as a methodology used by IPv6 hosts to carry out various link-local operations.

With IPv4, we have no means to detect whether or not a neighbor is reachable. With the neighbor discovery protocol, a neighbor unreachability detection mechanism has been defined including Duplicate IP address detection.

NDP operates in the Link Layer of the Internet model (RFC 1122), and is responsible for –


  • Replacement of ARP, ICMP (redirects, router discovery)
  • Address auto configuration of nodes
  • Discovery of other nodes on the link
  • Determining the addresses of other nodes
  • Duplicate address detection
  • Discovering available routers and Domain Name System (DNS) servers
  • Discovery of address prefix and
  • Maintaining reachability information of other active neighbour nodes

Five ICMPv6 neighbour discovery messages

  • Router solicitation
  • Router advertisement
  • Neighbor solicitation
  • Neighbor advertisement
  • Redirect
  1. Router Advertisement (RA) messages are –
  • ICMPv6 type 134
  • Always originated by routers.
  • Used to indicate the presence of the Router on a link.
  • Carry link-specific parameters which the hosts on the link should use for their network parameters configuration.
  • Sent periodically on a link and also sent in response to a Router Solicitation message from a host.
  • Routers send periodic Router Advertisements (RA) to the all-nodes multicast address
  1. Router Solicitation (RS) messages are –
  • ICMPv6 type 133
  • Are sent by booting nodes to request RAs for configuring the interfaces
  • RAs for configuring the interfaces
  • Originated only by the hosts.
  • Originated by hosts to find the Routers on the link.
  • Routers respond to RS messages by sending an RA.
  1. Neighbor Solicitation (NS) messages are –
  • ICMPv6 type 135
  • Originated by the nodes.
  • Originated by nodes to request the link layer address of another node.
  • Used for duplicate address detection and neighbor unreachability detection.
  1. Neighbor Advertisements (NA) messages are –
  • ICMPv6 type 136
  • Almost Always sent in response to an NS message from a node.
  • Can be sent by a node when its link-layer address is changed. This NA message is sent as an unsolicited NA to advertise its new address.
  1. Redirect messages are –
  • ICMPV6 type 137
  • Used in the same way as IPv4 ICMP redirect messages.
  • Always sent by the router to a host asking the host to update its routing information.
  • Redirect is used by a router to signal the reroute of a packet to a better router ie on receiving a packet from a host, the router can send Redirect message back to the host only when a router knows that the best path for that host to reach the destination is another router and not itself.  On receiving the Redirect message the host can update its routing information, and send next packets directly to the other router.


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