Router Advertisement (RA) Messages

Router Advertisement (RA) messages are a cornerstone of IPv6 networking, allowing routers to broadcast their presence and share critical configuration details with devices on the network. This guide provides an in-depth exploration of RA messages, their structure, and their role in IPv6 networks—perfect for network professionals and enthusiasts aiming to master IPv6 configuration.

What Are Router Advertisement Messages?

RA messages belong to the Internet Control Message Protocol for IPv6 (ICMPv6). Routers send these messages either periodically to the all-nodes multicast address (FF02::1) or in response to Router Solicitation (RS) messages from hosts. Their primary purpose is to:

• Announce the router’s presence.
• Deliver configuration parameters like network prefixes, default gateways, and MTU settings.

Key information provided includes:

• Network Prefixes: Used for Stateless Address Autoconfiguration (SLAAC).
• Default Gateway: The router’s address for external traffic routing.
• MTU: The maximum packet size allowed without fragmentation.
• Hop Limit: The maximum hops a packet can travel.

Structure of an RA Message

An RA message comprises an ICMPv6 header and optional fields. Key components include:

• Type (134): Marks the message as an RA.
• Current Hop Limit: Recommended hop limit for hosts.
• Managed Address Configuration Flag (M): Signals whether DHCPv6 should assign addresses.
• Other Configuration Flag (O): Indicates if DHCPv6 provides additional settings.
• Router Lifetime: Duration (in seconds) the router serves as the default gateway.
• Reachable Time: Time a neighbor is considered reachable after confirmation.
• Retransmit Timer: Interval between Neighbor Solicitation retransmissions.

Options in RA Messages

The ICMPv6 Router Advertisement (RA) message suggests that a device is getting an IPv6 global unicast address. The device operating system is also the final authority to get the IPv6 address. Furthermore, the ICMPv6 RA message consists of the following:

• Network prefix along with the prefix length
• Default gateway address
• DNS addresses, along with the domain name

There are three options for Router Advertisement (RA) messages, which are used to get an IPv6 address automatically. The RA message option 1, SLAAC, is the default option for the router. We can configure the router interface for the other option manually:

SLAAC

There are two types of IPv6 address auto-configuration. One is the old type that automatically configures IP addresses from IPv4 DHCP. The other type is to make the auto-configuration in IPv6, which empowers the hosts to make the auto-configuration by themselves without the need to communicate with anybody else on the network. IPv6 makes the life of network administrators easier, especially when dealing with the vast address space provided by IPv6. The IPv6 address is much more significant than IPv4.

SLAAC is the default RA option, which says I’m all you need (Prefix, Prefix-length, Default Gateway). As a result, an IPv6 host can configure itself with complete address settings automatically. Using SLAAC, a router interface is assigned a 64-bit prefix, and then the last 64 bits of its address are derived by the host or router with the help of EUI-64 process. The figure below illustrates the SLAAC Process.

The host computer sends a Router Solicitation (RS) message to the Router, and the router replies with an RA message, including the IPv6 Prefix, Prefix length, and all other related information.

What it includes:

• The prefix (e.g., 2001:db1:1:3::/64), defines the network portion of the IPv6 address.
• The prefix length (e.g., 64 bits), indicates how many bits of the prefix are significant.
• Flags:
  • On-link flag (L-bit): If set, hosts consider the prefix to be on-link, meaning they can communicate directly with other devices using this prefix without involving a router.
  • Autonomous address configuration flag (A-bit): If set, hosts can use the prefix to autonomously generate their IPv6 addresses via SLAAC.

Purpose: This option enables hosts to automatically configure their IPv6 addresses and determine whether they need a router to reach other devices on the same prefix. It’s the cornerstone of SLAAC, a key feature of IPv6.

SLAAC and DHCPv6 stateless

We can configure the router interface to send a router advertisement (RA) message using SLAAC and stateless DHCPv6. A stateless DHCPv6 server distributes DNS server addresses and domain names only. It does not allocate global unicast addresses. SLAAC and Stateless DHCPv6 is RA option 2, which says my information is here, but you also need to get other information like DNS addresses from a DHCPv6 server.

SLAAC creates its own IPv6 global unicast address, the router’s link-local address, and the RA’s source IPv6 address for the default gateway address, and a stateless DHCPv6 server obtains other information like DNS server address and a domain name. The figure below illustrates the SLAAC and DHCPv6 process.

1. The clients send RS messages to the router for IPv6 address prefix, prefix length and other related information.
2. The Router replies with a Router Advertisement (RA) message, including prefix, prefix length and the DHCPv6 server.
3. The client starts the DHCPv6 process with a DHCPv6 server.

Stateful DHCPv6

It works like DHCP for IPv4 addresses. A device can get its addressing plan and information, including a global unicast address, prefix length, and the addresses of DNS servers, automatically using the services of a stateful DHCPv6 server. The RA message in this option says I can’t give you any information you need. Send a request to the DHCPv6 server for all your required information. This option suggests devices:

• The router’s link-local address is the RA’s source IPv6 address for the default gateway address.
• A stateful DHCPv6 server to obtain a global unicast address, DNS server address, domain name, and all other information.

A stateful DHCPv6 server allocates and maintains a list of devices that receive IPv6 addresses. The default gateway address can only be obtained from the RA message. The stateless or stateful DHCPv6 server does not provide the default gateway address. The figure below illustrates the DHCPv6 process.

1. The host requests an IPv6 address assignment, including other related information.
2. The server replies with the assigned IPv6 address, including other related information like lease time, default gateway, and DNS server address.

Summary of RA Message Options

Here’s a concise overview of the key options in RA messages and their purposes:

Conclusion

The options in Router Advertisement (RA) messages are essential for configuring hosts on an IPv6 network efficiently and automatically. By including these options, routers provide hosts with the information needed to:

• Generate their own IPv6 addresses using SLAAC.
• Route traffic to other networks effectively.
• Resolve domain names using DNS servers and search lists.
• Optimize packet sizes to avoid fragmentation.
• Communicate directly with the router at the link layer.

This automation and flexibility make RA messages a vital component of IPv6, enabling network administrators to manage host configurations seamlessly without manual intervention on each device.