IPv6 prefixes and link-local next hops
RIPng advertises an IPv6 prefix and prefix length, along with a metric and route tag. Updates between routers on a link normally come from a link-local address and use UDP port 521 with destination FF02::9, the link-local multicast group for RIPng routers.
A receiving router normally uses the advertising neighbor’s link-local address as next hop. A special next-hop entry can identify another next hop on the same link. The interface matters when forwarding to a link-local next hop.
Periodic updates and unreachable metrics
Like IPv4 RIP, RIPng normally adds one hop to an advertised metric and treats 16 as unreachable. Its regular updates are roughly 30 seconds apart. A changed route can also produce a triggered update carrying changed entries.
If an advertising neighbor disappears without sending a withdrawal, its learned routes eventually expire. A router that can still reach its neighbors can advertise an unreachable metric for a destination it lost.
Try it in a lab
- Run RIPng on the transit links in a three-router IPv6 lab. Advertise a /64 LAN at the far end and verify that the first router learns the /64 with a link-local next hop on the expected interface.
- Inspect an update and locate UDP port 521, the FF02::9 destination, the prefix length and the hop metric.
- Remove the advertised LAN while leaving the transit interfaces up. Check that the learned route is withdrawn and traffic to the LAN ceases.