How do you enable SLAAC (Stateless Address Autoconfiguration) on a Cisco router, and what happens when you do?

Configure an IPv6 GUA (Global Unicast Address) on the interface and enable ipv6 unicast-routing — the router automatically starts sending RA (Router Advertisement) messages.

Configuration:

R1(config)# ipv6 unicast-routing
R1(config)# interface g0/0/1
R1(config-if)# ipv6 address 2001:db8:acad:1::1/64
R1(config-if)# no shutdown

What happens behind the scenes:

1. The router joins the IPv6 all-routers multicast group (ff02::2)
2. It begins sending RA messages every 200 seconds to the all-nodes multicast address (ff02::1)
3. The RA contains the network prefix (2001:db8:acad:1::/64), prefix length, and the router's link-local address as the default gateway

Verification:

R1# show ipv6 interface g0/0/1

Look for:

• The GUA and link-local address
• Joined group addresses including ff02::2 (all-routers)

Important: The ipv6 unicast-routing command is the trigger — without it, the router won't send RAs even if it has an IPv6 address configured.

Go deeper:

• RFC 4861 §4.2 — Router Advertisement message — the unsolicited RAs to ff02::1 carrying prefix and gateway info.
• IPv6 — SLAAC (Wikipedia) — how router advertisements feed host self-configuration.