IPv6 Multicast - Multicast Listener Discovery (MLD)

IPv6 multicast renames IGMP to the Multicast Listener Discovery Protocol (MLP). Version 1 of MLD is similar to IGMP Version 2, while Version 2 of MLD is similar to Version 3 IGMP. As such, MLD Version 2 supports Source Specific Multicast (SSM) for IPv6 environments.

Using MLD, hosts can indicate they want to receive multicast transmissions for select groups. Routers (queriers) can control the flow of multicast in the network through the use of MLD.

MLD uses the Internet Control Message Protocol (ICMP) to carry its messages. All such messages are link-local in scope, and they all have the router alert option set.

MLD uses three types of messages - Query, Report, and Done. The Done message is like the Leave message in IGMP version 2. It indicates a host no longer wants to receive the multicast transmission. This triggers a Query to check for any more receivers on the segment.

Configuration options for MLD will be very similar to configuration tasks we needed to master for IGMP. You can limit the number of receivers with the ipv6 mld limit command. If you want the interface to "permanently" subscribe, you can use the ipv6 mld join-group command. Also, like in IGMP, there are several timers you may manipulate for the protocol's mechanics.

Configuring IPv6 multicast-routing with the global configuration command ipv6 multicast-routing, automatically configures Protocol Independent Multicast (PIM) an all active interfaces. This also includes the automatic configuration of MLD. Here are verifications:

R0#show ipv6 pim interface

Interface          PIM  Nbr   Hello  DR

 Count Intvl  Prior
Tunnel0            off  0     30     1     

 Address: FE80::C000:2FF:FE97:0

 DR     : not elected

VoIP-Null0         off  0     30     1     

 Address: ::

 DR     : not elected

FastEthernet0/0    on   0     30     1     
 Address: FE80::C000:2FF:FE97:0
 DR     : this system

FastEthernet0/1    off  0     30     1     

 Address: ::

 DR     : not elected

Notice the PIM is indeed enabled on the Fa0/0 we have configured in this scenario. Now for the verification of MLD:

R0#show ipv6 mld interface

Tunnel0 is up, line protocol is up

 Internet address is FE80::C000:2FF:FE97:0/10

 MLD is disabled on interface

VoIP-Null0 is up, line protocol is up

 Internet address is ::/0

 MLD is disabled on interface

FastEthernet0/0 is up, line protocol is up

 Internet address is FE80::C000:2FF:FE97:0/10

 MLD is enabled on interface

 Current MLD version is 2

 MLD query interval is 125 seconds

 MLD querier timeout is 255 seconds

 MLD max query response time is 10 seconds

 Last member query response interval is 1 seconds

 MLD activity: 5 joins, 0 leaves

 MLD querying router is FE80::C000:2FF:FE97:0 (this system)

FastEthernet0/1 is administratively down, line protocol is down

 Internet address is ::/0

 MLD is disabled on interface

Notice the similarities to IGMP are obviously striking.

Thanks for reading, and I hope to "see you" again soon here at the INE blog.