Neighbour Discovery protocol - IPv6

IPv6 NDP

Neighbor Discovery protocol in IPv6 is useful for 

1. Router Discovery
2. SLAAC
3. Duplicate Address Detection (DAD)
4. Neighbour MAC Discovery

Router Discovery

• Hosts learn the IPv6 addresses of the available IPv6 routers in the same subnet using NDP messages.
• Two messages RS and RA help routers or hosts to learn addressing and subnetting information.

• Router Solicitation (RS):

Hosts send RS message to the "all IPv6 routers" using local- scope multicast address - FF02::2. This message asks all the IPv6 routers on the local-link only to identify themselves.

• Router Advertisement (RA):

RA messages sent by the router lists many details including the link-local IPv6 address of the router. Routers reply to the RS messages by sending the RA message to the unicast address of the requested host. Also, routers send unsolicited RA messages periodically to "all the IPv6 hosts" using the Local-scope multicast address - FF02::1.

Neighbour MAC Discovery

Hosts discover neighbour's link address (MAC address) using NS and NA messages.

• Neighbour Solicitation (NS)

Hosts check its NDP neighbour table (Same like IPv4 ARP cache) and if it does not find the link-layer address of the other host, it sends NDP NS message to the solicited-node multicast address FF02::1:FFxx.xxxx. Only IPv6 hosts that match the last 6 hex digits of the address will listen for this solicited-node multicast address. As a result, only a small subset of hosts on this link will process the received NDP NS message.

• Neighbour Advertisement (NA)

This message is a reply message from the host to the NDP NS message which lists the matching link layer (MAC) address. NA message is sent back to the unicast address of the host that sent the original NS message. Finally, the host that sent NS message records MAc address in its NDP neighbour table.