Administrator Guide

Multicast Listener Discovery Protocol
Dell Networking OS Supports Multicast Listener Discovery (MLD) protocol.
Multicast Listener Discovery (MLD) is a Layer 3 protocol that IPv6 routers use to learn of the multicast receivers that are directly
connected to them and the groups in which the receivers are interested. Multicast routing protocols (like PIM) use the information learned
from MLD to route multicast traffic to all interested receivers. MLD is analogous to IGMP, which tracks IPv4 multicast receivers.
MLD version 1 is analogous to IGMP version 2. MLD version 3 adds the ability to include and exclude sources and is analogous to IGMP
version 3.
MLD Version 1
Routers use MLD to learn which multicast addresses have listeners on each of their attached links. For each link, the router keeps a list of
which multicast addresses have listeners and a timer associated with each of those addresses.
There are three types of MLD messages:
• Multicast Listener Query — a message sent by the Querier to learn which multicast groups have listeners.
• General Query — a query to which all listeners should respond.
• Multicast-Address-Specific Query — a query to which only listeners for the specified group should respond.
• Multicast Listener Report — a message sent by listeners declaring their multicast group subscriptions.
• Multicast Listener Done — a message sent by a listener declaring that it is leaving a multicast group.
• Maximum Response Delay—the maximum amount of time that the Querier waits to receive a response to a General or Multicast-
Address-Specific Query. The value is zero in reports and Done messages.
• Multicast Address — set to zero in General Queries, and set to the relevant multicast address in multicast-address-specific queries
and done messages.
MLD Version 1 packets have the following structure:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Code | Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Maximum Response Delay | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ +
| |
+ Multicast Address +
| |
+ +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
MLD Querier Router
MLD routers periodically ask connected hosts in which, if any, multicasts groups they are interested. For any subnet, only on router solicit
hosts for this information; this router is called the Querier, and all the other routers on the subnet are non-queriers. Initially, each router
assumes that it is the Querier, and transmits queries. If a router receives a query with a source IP address lower than its own, it stops
transmitting queries, and so the router with the lowest IP address is ultimately elected the Querier for the subnet.
Joining a Multicast Group
The Querier periodically sends a General Query to the all-nodes multicast address FF02::1. A host that wants to join a multicast group
responds to the general query with a report that contains the group address; the report is also addressed to the group (in the IPv6
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Multicast Listener Discovery Protocol 509