Multicast addresses represent a group of interfaces, just like in IPv4 multicast. The format of a multicast address is:
| 8 bits | 4b | 4b | 112 bits |
|11111111|0RPT|SCOP| Group ID |
The R Flag is used for addresses with an embedded RP.
The P Flag indicates a multicast address that is assigned based on the network prefix.
The T Flag is set to 0 if the address is a well-known multicast (assigned by IANA) or 1 if the address is an administratively assigned address
The SCOP field (4 bits, 16 values) contains the scope of the multicast group. Available values are:
1: Interface Local – spans only on a single interface and is used for loopback multicast
2: Link Local – spans on a single link (point-to-point or multi-access)
4: Admin Local – administratively defined
5: Site Local – spans a single site
8: Organizational Local – multiple sites belonging to a single organization
Other Addresses, like NTP are assigned by IANA but have a variable scope. The address assigned is FF0X::101, where X can be any of the scopes seen earlier.
IPv6 Layer 2 Addresses
The frames that carry multicast traffic have the MAC address of the sender as the source and a special address derived from the Layer 3 destination IPv6 as the destination.
L2 Multicast addresses start with 33:33 followed by the last 32 bits in the IPv6 address.
MLD (Multicast Listener Discovery Protocol) replaces IGMP for IPv6 multicast routing. MLDv1 is similar to IGMPv2, while MLDv2 is similar to IGMPv3 (supports Source Specific Multicast).
To see the interfaces that run MLD, use:
R# show ipv6 mld interface
The default version of PIM used on Cisco routers, PIMv2 also supports IPv6. To enable IPv6 multicast routing, use:
R(config)# ipv6 multicast-routing
This command also enables PIM on all IPv6 enabled interfaces.
Only PIM-SM is supported for IPv6 and it works similar to IPv4 PIM.
Every router creates a tunnel to the RP that it uses to unicast PIM Register messages. These tunnels are automatically created. You can see them using:
R# show ipv6 pim tunnels
RP can be configured statically or using BSR. Auto-RP is not supported for IPv6.
To see a list of configured RPs, use: