IPv6 Routing and Default Gateways

IPv6 routing looks a lot like IPv4 routing — static routes, dynamic protocols, longest prefix match. The differences are in the details: hosts learn their default gateway from RAs instead of DHCP, and next-hop addresses are usually link-local.

#Static IPv6 Routes#link

`::/0` is the IPv6 default route — equivalent to `0.0.0.0/0` in IPv4. When both a next-hop address and an egress interface are specified, and the next-hop is link-local, you must specify the interface (because link-local addresses are only unique on the link).

#How Hosts Learn Their Default Gateway#link

IPv4 hosts learn their gateway from DHCP (or static config). IPv6 hosts learn it from Router Advertisements — the router includes its own link-local address in the RA, and the host installs a default route pointing to that link-local address. No DHCP needed for basic connectivity.

`proto ra` means this route was learned from a Router Advertisement. The gateway is a link-local address (fe80::1). The `expires` counter counts down toward the next expected RA — if the router stops sending RAs, the host will remove the default route after expiry.

#Common IPv6 Routing Gotchas#link

• ▪`ipv6 unicast-routing` must be enabled globally, or routers won't forward
• ▪Link-local next-hop routes need an explicit egress interface
• ▪RA prefix must be on-link with the L flag or hosts won't use it for direct reachability
• ▪Multiple RAs from multiple routers = multiple default routes; host picks one
• ▪Anycast and P2P links commonly use link-local for next-hop, so plan link-local addresses consistently

#Verifying IPv6 Routes#link

C is connected, L is the interface's own address, S is static. The `::/0` static route is the default gateway — same concept as IPv4's `S* 0.0.0.0/0`. Both `ND` (from RA) and `NDp` (ND prefix) codes exist for dynamically learned routes.

#Quick Check#link