IPv6 Neighbor Discovery Protocol (NDP)

Engineers designed IPv6 to avoid the limitations of IPv4. In IPv4, different methods were needed to perform basic network functions.

• We need ARP to resolve MAC addresses.

• They used ICMP Router Discovery to find routers.

Figure 1 – IPv4 ARP and ICMP vs IPv6 NDP

This created complexity and depended on broadcast, which is inefficient.

IPv6 simplifies things with one protocol: the Neighbor Discovery Protocol (NDP). RFC 4861 details this protocol.

NDP uses ICMPv6 messages with multicast instead of broadcast. Thanks to this, IPv6 devices can:

• Discover routers.

• Learn prefixes and configuration parameters.

• Resolve the MAC address of neighbors.

At the CCNA level, you need to focus on the four essential NDP message types:

• Router Solicitation (RS)

• Router Advertisement (RA)

• Neighbor Solicitation (NS)

• Neighbor Advertisement (NA)

Now that we understand why IPv6 needs NDP, let’s start with the first step: how a host discovers a router on the network.

This is done using the Router Solicitation (RS).

Like in IPv4, where hosts could use ICMP Router Discovery, RS is the IPv6 way to ask routers for information.

When a host connects to an IPv6 network, it doesn’t know the prefix or the default gateway yet. To learn this quickly, it sends a Router Solicitation (RS).

In our example, PC1 asks, “Who is the router here?”
The message is sent to the special multicast address FF02::2 (all routers) so that only routers will receive it.

Figure 2 – IPv6 NDP Router Solicitation from PC to router

At Layer 2, this IPv6 multicast maps to the Ethernet address 33:33:00:00:00:02.
The source IPv6 and MAC belong to PC1, so the router knows who is asking the question.

In short, PC1 sends an RS (ICMPv6 type 133) to FF02::2. Only routers listen to this address and will reply with a Router Advertisement (RA).

When a router receives an RS, it must reply. This reply is called a Router Advertisement (RA).
In IPv4, this was done with ICMP Router Discovery, but IPv6 makes it simpler and more complete with RA.

In our example, R1 replies: “I’m the router, here is the prefix and my MAC address.”
An RA can also be sent periodically, even without an RS, so that hosts on the network always know who the router is.

Figure 3 – IPv6 NDP Router Advertisement from Router to Hosts