Hello Chandrasekhar
Both the RA and the NA are part of what is known as Neighbor Discovery Protocol (NDP). They are similar, but are each used in different situations and for different purposes.
Both of these messages come in pairs.
Specifically, the Neighbor Advertisement (NA) is a response to the Neighor Solicitation (NS) message. These are used primarily to find the Layer 2 address of another IPv6 address on the local link. This essentially does the same thing as ARP does for IPv4, but it works completely differently, using what is known as a solicited node multicast address. Specifically, every IPV6 device will compute a solicited node multicast address by taking the multicast group address (FF02::1:FF /104) and adding the last 6 hexadecimal characters from its own IPv6 address. It will then join this multicast group address and will “listen” to it. NS and NA are also used for additional features such as Duplicate Address Detection (DAD) for example. More info about this can be found here:
The Router Advertisement (RA) and Router Solicitation (RS) messages on the other hand are primarily used for SLAAC, autoconfiguration parameters, and DHCPv6. Devices that are configured as gateways on the local link will periodically send out RAs telling all the hosts that they are the gateway. A host may also send an RS message asking to find the local gateway. Unlike NS’es which are sent to all neighbors on a local link, RS’es are sent only to gateways on the local link using the FF02::2 all routers multicast address. RAs include information used by hosts to configure their IPv6 parameters. These include the prefix used, the IPv6 address of the local gateway, and optionally, the IPv6 address of a DHCPv6 server for additional configuration paramters.
More about this procedure can be found in the following lesson:
I hope this has been helpful!
Laz