IPv6 Multicast - Addressing

IPv6 multicast is an important new blueprint topic for the Version 4.X CCIE R&S Lab Exam as well as the Written Qualification Exam. In this post, we will start at the most logical starting point for this topic - the IPv6 multicast addressing in use.

Like in IP version 4, multicast refers to addressing nodes so that a copy of data will be sent to all nodes that possess the address. Multicast allows for the elimination of broadcasts in IPv6. Broadcasts in IP version 4 were problematic, since the copy of data is delivered to all nodes in the network, whether the node cares to receive the information or not.

Multicast addresses are quickly detected by the initial bit settings. A multicast address begins with the first 8 bits set to 1 (11111111). The corresponding IPv6 prefix notation is FF00::/8.

Following the initial 8 bits, there are 4 bits (labeled 0RPT) which are flag fields. The high-order flag is reserved, and must be initialized to 0. If the R bit is set to 1, then the P and T bits must also be set to 1. This indicates there is an embedded Rendezvous Point (RP) address in the multicast address.

The next four bits are scope. The possible scope values are:

0  reserved
1  Interface-Local scope
2  Link-Local scope
3  reserved
4  Admin-Local scope
5  Site-Local scope
6  (unassigned)
7  (unassigned)
8  Organization-Local scope
9  (unassigned)
A  (unassigned)
B  (unassigned)
C  (unassigned)
D  (unassigned)
E  Global scope
F  reserved

The remaining 112 bits of the address make up the multicast Group ID. An example of an IPv6 multicast address would be all of the NTP servers on the Internet - FF0E:0:0:0:0:0:0:101.

Notice, like in IPv4 multicast, there are many reserved addresses of link-local scope. Here are some examples:

FF02:0:0:0:0:0:0:1 - all nodes
FF02:0:0:0:0:0:0:2 - all routers
FF02:0:0:0:0:0:0:9 - all RIP

A special, reserved IPv6 multicast address that you should be aware of is the Solicited-Node multicast address:

FF02:0:0:0:0:1:FFXX:XXXX

A Solicited-Node multicast address is created automatically for you by the router. It takes the low-order 24 bits of the IPv6 address (unicast or anycast) and appends those bits to the prefix FF02:0:0:0:0:1:FF00::/104. This results in a multicast address within the range FF02:0:0:0:0:1:FF00:0000 to FF02:0:0:0:0:1:FFFF:FFFF. These addresses are used by the IPv6 Neighbor Discovery (ND) protocol in order to provide a much more efficient address resolution protocol than Address Resolution Protocol (ARP) of IPv4.

Now that we understand the addressing, let us see it in action on a Cisco router.

R1#configure terminal

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

R1(config)#ipv6 unicast-routing

R1(config)#interface fa0/0

R1(config-if)#ipv6 address 2001:1::/64 eui-64

R1(config-if)#no shutdown

R1(config-if)#

*Mar  1 00:03:32.627: %LINK-3-UPDOWN: Interface FastEthernet0/0, changed state to up

*Mar  1 00:03:33.627: %LINEPROTO-5-UPDOWN: Line protocol on Interface FastEthernet0/0, changed state to up

R1(config-if)#do show ipv6 interface fa0/0

FastEthernet0/0 is up, line protocol is up

 IPv6 is enabled, link-local address is FE80::C001:1FF:FE47:0

 No Virtual link-local address(es):

 Global unicast address(es):

 2001:1::C001:1FF:FE47:0, subnet is 2001:1::/64 [EUI]

 Joined group address(es):

 FF02::1

 FF02::2

 FF02::1:FF47:0

 MTU is 1500 bytes

 ICMP error messages limited to one every 100 milliseconds

 ICMP redirects are enabled

 ICMP unreachables are sent

 ND DAD is enabled, number of DAD attempts: 1

 ND reachable time is 30000 milliseconds

 ND advertised reachable time is 0 milliseconds

 ND advertised retransmit interval is 0 milliseconds

 ND router advertisements are sent every 200 seconds

 ND router advertisements live for 1800 seconds

 ND advertised default router preference is Medium

 Hosts use stateless autoconfig for addresses.

Notice that because I enabled the IPv6 routing capabilities for this device, one of the multicast groups joined is ALL ROUTERS for the local-link (FF02::2). Also note the Solicited-Node multicast address of FF02::1:FF47:0.

I hope you have enjoyed this presentation on IPv6 multicast and will be joining us for more. If you want practice right away with these topics, check out any of our CCIE R&S products.