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I have already written about IPv6 multicast addresses in my previous article. IPv6 multicast addresses work similarly to IPv4 multicast addresses. IPv6 enabled devices can join and listen for multicast traffic on an IPv6 multicast address.

The multicast address comprises an 8-bit address, 4-bit flag, 4-bit scope, and 112-bit group ID fields. An IPv6 multicast address can identify multiple network interfaces. In IPv6 multicasting, IPv6 datagram packets addressed to a multicast address are delivered to all interfaces identified by the address. The figure below illustrates the multicast address.

Format Prefix (FP)

The first eight bits of an IPv6 multicast address are always “1111 1111”. They are called the “Format Prefix”. The prefix of the multicast group is similar to a link-local address.

Flags

The Flags field has four bits. The four reserved bits can be used to indicate the nature of certain multicast addresses. Currently, the first three bits are not in use; all the first three bits are set to zero. The fourth bit is the “T” (Transit) bit.

If the value of the T field is zero, this marks the multicast address as a permanently assigned and “well-known” multicast address. If set to one, this means it is not permanently assigned a multicast address.

Scope ID

Scope ID is four fields defining the multicast address’s scope. There are 16 different possible values from 0 to 15. The value is illustrated in the following image.

Group ID

It is 112 bits long and used to identify the multicast group within the given scope.

Assigned IPv6 Multicast Address

We can send a single packet to one or more destinations using a multicast address. The multicast IPv6 address Prefix is FF00::/8. Multicast addresses can only be destination addresses. There are two types of IPv6 multicast addresses:

• Assigned multicast
• Solicited-node multicast

An assigned multicast address is a single address to reach a group of devices running a standard service. It is used in situations with specific protocols such as DHCPv6.Two common IPv6-assigned multicast groups are the following:

All-nodes multicast group

All-nodes multicast groups can join all IPv6-enabled devices. The ff002::1 IPv6 address is reserved for this group. A packet sent to this group should be received and processed by all IPv6 interfaces. RA message to the all-nodes multicast group is an example of an All-nodes multicast group.

When an IPv6 router sends an Internet Control Message Protocol version 6 (ICMPv6) RA message to the all-nodes multicast group, it informs all IPv6-enabled devices on the network about the IPv6 prefix, prefix length, default gateway, and all other related information.

All-routers multicast group

All router multicast groups can join all routers in the local network segment. The IPv6 address FF02::2 is reserved for the all-routers multicast group.  A local router can join and become a member of the all-routers multicast group when it is enabled as an IPv6 router with the “ipv6 unicast-routing” command. The “ipv6 unicast-routing” is the command of Global Configuration Mode.

All IPv6-enabled routers on a local network can receive and process a packet sent to this group. IPv6-enabled devices send ICMPv6 Router Solicitation (RS) messages to an all-routers multicast address. The Router Solicitation (RS) message requests a Router Advertisement (RA) message from the IPv6 router to assist the device in its address configuration.

Solicited-Node IPv6 Multicast Addresses

A solicited-node multicast address is like an all-node multicast address. We can map the solicited-node multicast address to a particular Ethernet multicast address. This allows the Ethernet NIC to filter the frame by examining the destination MAC address without sending it to the IPv6 process to see if the device is the deliberate target of the IPv6 packet.

The Solicited-node multicast is a flooding optimization.  If sufficient information were already known to support unicast operation, then there would be no point. The solicited-node multicast is used when there is no information to support unicast operation. The solicited node allows the flooded traffic to reach all nodes like broadcast.

Solicited-node multicast addresses can be created automatically using a special mapping of the device’s unicast address with the solicited-node multicast prefix, which is ff02:0:0:0:0:1:ff00::/104. It can be created automatically for every unicast address on a device.

Reserved IPv6 Multicast Addresses

Address	Description
ff02::1	All nodes on the local network
ff02::2	All routers on the local network
Ff02::4	The all-Distance Vector Multicast Routing Protocol address.
ff02::9	Routing Information Protocol (RIP) routers
ff02::a	EIGRP routers
ff02::d	Protocol Independent Multicast routers
ff02::e	Resource Reservation Protocol (RSVP)
ff02::1:2	All Dynamic Host Configuration Protocol servers and relay agents on the local network site
ff02::1:3	Link-local multicast name resolution