Router Advertisement (RA) Messages

The RA message option 1, SLAAC is the default option for the router. The router interface can be configured for three options:

  1. SLAAC – Which says I’m all you need (Prefix, Prefix-length, Default Gateway)”
  2. SLAAC and DHCPv6 stateless–My information are here but you also need to get other information like DNS addresses from a DHCPv6 server.
  3. DHCPv6 Only – I can’t give you any information. Send a request to DHCPv6 server for all your required information.

RA Option 1- SLAAC

The ICMPv6 RA message is a suggestion to a device on how to get an IPv6 global unicast address. The device operating system is final authority to get an IPv6 address.This message suggests that the receiving device use the information in the RA message to create its own IPv6 global unicast address. The DHCPv6 services are not required for SLAAC.

Basically, SLAAC is no central server required to allocate global unicast addresses. The SLAAC is not keeping a list of devices and their addresses. The client device uses the information in the RA message to create its own global unicast address. The host device sends a request for addressing information to the local router. The local router advertises its addressing information (Prefix, Prefix length and Default Gateway) through RA message towards host computer. The two parts of the address are created as follows:

  • Prefix– Received in the RA message
  • Interface ID– Uses the EUI-64 process or by generating a random 64-bit number

RA Option 2 – SLAAC and DHCPv6 stateless

The router’s interface can be configured to send a router advertisement using SLAAC and stateless DHCPv6. A stateless DHCPv6 server distributes DNS server addresses and domain names only. It does not allocate global unicast addresses.The RA Option 2 (SLAAC and Stateless DHCPv6) functions are here:

  • SLAAC to create its own IPv6 global unicast address, router’s link-local address and the RA’s source IPv6 address for the default gateway address.
  • A stateless DHCPv6 server to obtain other information like DNS server address and a domain name.

RA Option 3 – Stateful DHCPv6

Stateful DHCPv6 is work just like DHCP for IPv4 addresses. A device can get its addressing information including a global unicast address, prefix length, and the addresses of DNS servers automatically using the services of a stateful DHCPv6 server. This option suggests devices:

  • The link-local address of the router, 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 which receive IPv6 address.The default gateway address can only be obtained from the RA message. The stateless or stateful DHCPv6 server does not afford the default gateway address.


Configuration of Global IPv6 Unicast Address

Static Configuration – Router

Most of the configuration commands in the Cisco routers are similar for both IPv4 and IPv6. The only difference is the use of ipv6 in place of IP within the commands. The command to configure an IPv6 global unicast address on any interface is “ipv6 address ipv6-address/prefix-length “

Example Configuration on Router fa0/0 and fa0/1 interfaces

Router>enable

Router#configure terminal

Router(config)interface fa/0/0

Router(config-if)#ipv6 address 2001:DB8A:AACA:A::1/64

Router(config-if)no shutdown

Router(config-if)exit

Router(config)interface fa/0/1

Router(config-if)#ipv6 address 2001:DB8A:AACA:B::1/64

Router(config-if)no shutdown

Router(config-if)exit

Router(config) do wr

Building configuration…

[OK]

Static Configuration- Host

Manual configuration of the IPv6 address is similar to configure an IPv4 address. As shown in Figure, the default gateway address configured for PC1 is 2001:DB8A:AACA:A::1. This is the global unicast address of the Router Fast Ethernet interface 0/0 on the same network. Link-local address of the router can also be configured as the gateway for the host. Both configurations will work.

configuration

Configuring static addresses on clients is best for a small network. For larger network dynamic assignment of IPv6 addresses is best practice.

There are two ways in which a device can get an IPv6 global unicast address automatically:

  • Stateless Address Auto Configuration (SLAAC)
  • Dynamic Host Configuration Protocol version 6 (DHCPv6)

Using DHCPv6 or SLAAC, the local router’s link-local address will automatically be specified as the default gateway address for the host.

Dynamic Configuration – SLAAC

The SLAAC is a unique feature for IPv6 addresses. Stateless address configuration means that a device to obtain its prefix, prefix length, default gateway address, and other information from an IPv6 router without the use of a DHCPv6 server. All Cisco devices have the capability of the SLAAC. By default, SLAAC does not provide anything to the client outside of an IPv6 address and a default gateway. Using SLAAC, devices rely on the local router’s ICMPv6 Router Advertisement (RA) messages to obtain the necessary information.

IPv6 enabled routers to send out ICMPv6 RA messages after every 200 seconds, to all IPv6-enabled devices on the network. An RA message will also be sent in response to a host sending an ICMPv6 Router Solicitation (RS) message.

IPv6 routing is not enabled by default. To enable a router IPv6 following command will be used.

Router>enable

Router#configure terminal

Enter configuration commands, one per line. End with CNTL/Z.

Router(config)#ipv6 unicast-routing

Router(config)#

The ICMPv6 RA Message

The ICMPv6 RA message is a suggestion to a device on how to get an IPv6 global unicast address. The device operating system is final authority to get the IPv6 address. The ICMPv6 RA message consists of:

  • Network prefix and prefix length
  • Default gateway address
  • DNS addresses and domain name

There are three options for RA messages which used to get an IPv6 address automatically:

1. SLAAC

2. SLAAC with a stateless DHCPv6 server

3. DHCPv6 (no SLAAC)

All Three option will be discussed in next lesson.