IPv6 SLAAC Deep Dive

What is SLAAC

SLAAC allows a host to:

• Automatically generate its IPv6 address
• Learn the default gateway
• Operate without DHCP

This is how it works step by step:

1. Interface comes up

When the device connects to a network, its network interface initialises and enables IPv6.

2. Link-local address creation

The device first generates a link-local address (always starts with fe80::). It generally builds this using its MAC address (via modified EUI-64).

3. Duplicate Address Detection (DAD)

Before using the address, the device checks if it’s already in use:

• It sends a Neighbour Solicitation (NS) message.
• If no one responds, the address is considered unique and usable.

4. Router Solicitation (RS)

The device asks: “Any routers out there?”

• It sends a Router Solicitation message to the network.

5. Router Advertisement (RA)

A router responds with a Router Advertisement, which contains:

• Network prefix (e.g., 2001:db8:abcd::/64)
• Default gateway info
• Flags indicating whether SLAAC and/or DHCPv6 should be used

6. Global address generation

Using the prefix from the RA:

• The device combines:
• The network prefix (from router)
• Its interface ID (from earlier)
• This forms a global unicast IPv6 address

7. Duplicate Address Detection (again)

The device repeats DAD for this new global address to ensure no conflict.

8. Address becomes usable

Once verified:

• The device can now communicate on the network and the internet using IPv6.
• It also installs the router as its default gateway.

📊 Packet-by-Packet Breakdown

Let’s walk through the actual flow from your capture.

Let’s try it in our Lab,

I am configuring this on my router:

conf t
ipv6 unicast-routing

interface fa0/0
ipv6 address 2001:db8:1::1/64
no shutdown

Let us analyse what happens packet by packet:

🔹 Packet 22 — Neighbor Solicitation (DAD)

• Src :: → Dst ff02::1:ffc1:0
• Checking if fe80::c803:11ff:fec1:0 is already in use.
• No reply → address is considered unique.

🔹 Packet 23 , 24, 25 — MLD Report v2

• Host joins multicast groups (ff02::1, solicited-node group).
• Ensures it can receive ND / RA messages.

🔹 Packet 26 — Neighbor Advertisement

• Router replies: fe80::c803:11ff:fec1:0 is at ca:03:11:c1:00:00
• Confirms ownership of link-local address.

🔹 Packet 27 — Neighbor Solicitation (for global)

• Src :: → Dst ff02::1:ff00:1
• DAD for global IPv6 2001:db8:1::1.

🔹 Packet 28 — Router Advertisement (KEY SLAAC PACKET)

• Router → ff02::1 (all nodes)
• Contains:
• Prefix 2001:db8:1::/64
• Flags (A=1 for SLAAC)
• Host uses this to generate global IPv6 address.

🔹 Packet 33 — Neighbor Advertisement (global)

• Router advertises: 2001:db8:1::1 is at MAC
• Confirms global IPv6 ownership.

In Nutshell:

• DAD (link-local) → Packet 22
• Multicast join → 23–25
• Router presence → 26
• DAD (global) → 27
• RA → SLAAC happens → 28 (critical)
• Address confirmed → 33

Router Advertisement Fields

From this packet, which is packet no 28:

🔹 Global RA Flags

Managed (M): 0
Other (O): 0

Meaning:

This is a pure SLAAC environment.

🔹 Router Lifetime

1800 seconds

Host installs:

Default route valid for 30 minutes

🔹 Prefix Information

Prefix: 2001:db8:1::/64

Critical Flags inside Prefix

On-link (L): 1
Autonomous (A): 1

Interpretation

L flag = 1

• Network is directly reachable (same subnet)

A flag = 1

• SLAAC is enabled

This is the actual trigger for address creation

🔹 Lifetime Values

Valid Lifetime: 30 days
Preferred Lifetime: 7 days

Behavior:

• Preferred → actively used
• Valid → still usable but deprecated

🔹 MTU Option

MTU: 1500

Host adopts MTU from RA