What happens when a client wants to stop receiving
traffic from a specific source,
but wants to continue receiving traffic from other sources on the same group?
The client uses a report message, just like leaving ASM groups,
but the contents differ.
The source and destination addresses are exactly the
same for all MLDv2 report messages, so let's move on.
Likewise,
the type remains 143 because there doesn't need to be a special
message type to service this client request.
Instead,
a single group record contained in this report message has a type of
block old sources as it relates to group FF38::3.
Just like IGMPv3, this is a way of saying I still want traffic for FF38::3,
but not from source FC00:10:2:5::5.
There is one more step the router has to take before cutting
off this multicast flow onto the LAN segment.
The router must send a source and group-specific query onto the LAN
segment to see if anyone else wants traffic on this group.
The source is the router's link local address,
and the destination is the SSM multicast group of FF38::3.
I don't want to be overly repetitive,
since we have seen many query messages so far.
Let's skip to the bottom of the packet to see the interesting contents.
The query message contains a source address,
which asks the question,
who else wants multicast for group FF38::3
specifically from source FC00:10:2:5::5?
Silence means that no one wants it,
granting the routing permission to stop sending multicast
to that specific source group pair.
MLD has only two versions, so we can compare both on a single slide.
I'll post the RFC specification numbers here,
in case any viewers want to reference them in the future.
MLDv1 is operationally similar to IGMPv2,
and MLDv2 is operationally similar to IGMPv3.
In IPv6 multicast, inter-domain multicast is particularly challenging.
In lieu of MSDP, SSM is a huge improvement that MLDv2 brings.
MLDv1 renamed the IGMPv2 leave message to a done message,
but like IGMPv3, MLDv2 dumped this new message to use group modes instead.
This wraps up our deep dive into the core functions of IGMP and MLD. Let's explore some powerful troubleshooting and design techniques next.