IPv6 Multicast Addresses Explained

I have already written about IPv6 multicast addresses in my previous article that IPv6 multicast addresses work similarly to IPv4 multicast addresses. The IPv6 enabled devices can join and listen for multicast traffic on an IPv6 multicast address. The multicast address is composed of 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 an IPv6 multicast address are delivered to all interfaces that are identified by the address. The figure below illustrates the multicast address.

IPv6 Multicast

Format Prefix (FP)

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

Flags

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, there all first three bits are set to zero. The fourth bits is T” (Transit) bit.

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

Scope ID

Scope ID is four bits of fields defining the scope of the multicast address. The value of scope ID is 16 different possible values from 0 to 15. According to the value is illustrates 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

Using IPv6 multicast address we can send a single packet to one or more destinations. 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 IPv6 multicast address is a single address to reach a group of devices running a common service. It is used in a situation with specific protocols such as DHCPv6.Two common IPv6 assigned multicast groups are the following:

All-nodes multicast group

All-nodes multicast group can join all IPv6-enabled devices. The ff002::1 IPv6 address is reserved for this group. A packet sent to this group should receive and processed by all IPv6 interfaces in the group. RA message to the all-nodes multicast group is the example of All-nodes multicast group. When an IPv6 router sends 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 IPv6 prefix, prefix length, default gateway and all other related information.

All-routers multicast group

All-routers multicast group can join all routers on the local network segment. The IPv6 address FF02::2 is reserved for the all-routers multicast group.  A local router can join and becomes a member of the all-routers multicast group when it 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 to the all-nodes multicast address. We can map the solicited-node multicast address to a special 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 was already known to support unicast operation then there would be no point. When there is no information to support unicast operation, the solicited-node multicast is used. The solicited-node allows for the flooded traffic to reach all nodes like broadcast.

Solicited-node multicast addresses can create 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

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