What is RA?

An RA (Router Advertisement) is an ICMPv6 message a router sends on a LAN to announce its prefixes, offer itself as default gateway and share settings.

Router Advertisements are part of the Neighbour Discovery Protocol (NDP) and are ICMPv6 type 134. A router sends them periodically to the all-nodes multicast address ff02::1, and immediately in reply to a Router Solicitation (type 133) from a host that has just joined. They always come from the router's link-local address. An RA carries:

• a router lifetime: how long hosts may use this router as their default gateway; 0 means "don't use me as a default router";
• one or more prefix information options, such as 2001:db8:1000:1::/64, with flags saying whether hosts may build addresses from it with SLAAC;
• the M (managed) and O (other configuration) flags, which tell hosts to use DHCPv6 for addresses or for extra settings;
• optionally, DNS servers in an RDNSS option, plus details such as the link MTU.

This is a real difference from IPv4, where DHCP hands out the default gateway. In IPv6, hosts learn their default route only from RAs, even when DHCPv6 gives them an address. Common problems follow from that: a host with an address but no default route usually isn't receiving RAs, or is ignoring them (Linux ignores RAs by default when forwarding is enabled), or the RA has a router lifetime of 0. Firewalls must allow ICMPv6 to and from the router, or neighbour discovery and SLAAC break. Because any device can send RAs, a misconfigured or malicious one can hijack a LAN's traffic, which is why switches offer RA Guard. The IPv6 on VyOS lesson configures Router Advertisements with a DNS server and shows hosts picking them up.