IPv6 Solicited Node Multicast Address

Every device that uses an IPv6 address will also compute and join a solicited node multicast group address. This address is required for IPv6 Neighbor Discovery which we use for layer two address discovery.

All solicited node multicast group addresses start with FF02::1:FF /104:

FF /8 is the IPv6 multicast range.
FF02 /16 is the multicast link local scope.

Let’s take a look on a Cisco IOS router to see what these solicited node multicast group addresses look like:

R1(config)#interface FastEthernet 0/0
R1(config-if)#ipv6 enable

I just enabled IPv6 on an interface, this causes the router to create a link-local IPv6 address. It will also compute and join the solicited node multicast group address:

R1#show ipv6 interface FastEthernet 0/0
FastEthernet0/0 is up, line protocol ibs up
  IPv6 is enabled, link-local address is FE80::21D:A1FF:FE8B:36D0
  No Virtual link-local address(es):
  No global unicast address is configured
  Joined group address(es):
    FF02::1
    FF02::1:FF8B:36D0

Above you can see that the router joined FF02::1:FF8B:36D0. The last 6 hexadecimal characters were copied from the link local address. Here’s a picture:

Above you can see the complete uncompressed solicited node multicast address.

I can configure multiple IPv6 addresses on the interface, if the last 6 hexadecimal characters are similar then there is no need to join another multicast address. For example, let’s configure an IPv6 unicast address:

R1(config)#interface FastEthernet 0/0
R1(config-if)#ipv6 address 2001:DB8:1212:1212::/64 eui-64

I’ll use EUI-64 to generate the last 64 bits. Take a look at the joined group addresses:

R1#show ipv6 interface FastEthernet 0/0
FastEthernet0/0 is up, line protocol is up
  IPv6 is enabled, link-local address is FE80::21D:A1FF:FE8B:36D0
  No Virtual link-local address(es):
  Global unicast address(es):
    2001:DB8:1212:1212:21D:A1FF:FE8B:36D0, subnet is 2001:DB8:1212:1212::/64 [EUI]
  Joined group address(es):
    FF02::1
    FF02::1:FF8B:36D0

The last 64 bits of the link local and unicast address are the same so the solicited node multicast group address remains the same. If we configure an IPv6 address where the last 6 hexadecimal characters are different then the router will join another multicast group. Let’s try that:

Forum Replies

If we are only taking last 6 hex then multiple hosts on the same LAN may join to the same multi-cast group. As many hosts may have the MAC addresses whose last 24 bits are same. Is generating unique “solicited multi-cast address” not our intention ?

Hi Harmeet,

6 hexadecimal characters are 24 bits. The last 24 bits of the MAC address is the “vendor assigned” part. If you have two network cards from the same vendor then the last 24 bits should be unique.

Theoretically, it could be possible that you have two network cards from different vendors that have the same 24 vendor assigned bits but the odds are extremely small

It is possible though to have two IPv6 addresses that use the same solicited node address. This is no problem though…both hosts will listen to the multicast group address and if needed, disca

... Continue reading in our forum

Hi
I have found these articles very helpful to understand this topic

http://www.networkcomputing.com/author/36436053

Hi Rene,

Thank you for the clarity of your work. Although I understand how the NSM works, I have some gray areas:
1 - When you activate ipv6 on an interface, the latter joins a multicast group. This multicast group is used by NDP to retrieve the MAC address of the neighbor or detect duplicate addresses. But in packet exchanges, I see that it is unicast and not multicast. When we talk about multicast, we are talking about broadcasting from a source to a group of recipients, which is quite confusing. Can you please bring me some clarification?
2 - How does a swit

... Continue reading in our forum

Hi Laz,
Thanks for your return …

3 more replies! Ask a question or join the discussion by visiting our Community Forum

We're Sorry, Full Content Access is for Members Only...

Forum Replies