IPv6 Configuration Guide K/KA/KB.15.15

Forward
The switch forwards all IPv6 multicast packets through the port. This includes IPv6 multicast
data and MLD protocol packets.
Block
The switch drops all MLD packets received by the port and blocks all outgoing IPv6 multicast
packets through the port, except those packets destined for well-known IPv6 multicast addresses.
This has the effect of preventing IPv6 multicast traffic from moving through the port.
The switch floods all packets with "well-known" IPv6 multicast destination addresses through all
ports. Well-known addresses are permanent addresses defined by the Internet Assigned Numbers
Authority (www.iana.org). IPv6 standards define any address beginning with FF0x/12 (binary
1111 1111 0000) as a well-known address.
Listeners and joins
The "snooping" part of MLD snooping arises because a switch must keep track of which ports have
network nodes that are MLD hosts for any given multicast address. It does this by keeping track of
"joins" on a per-port basis.
A network node establishes itself as an MLD host by issuing a multicast "join" request (also called
a multicast "report") for a specific multicast address when it starts an application that listens to
multicast traffic. The switch to which the node is connected sees the join request and forwards
traffic for that multicast address to the node's port.
Queries
The querier is a multicast router or a switch that periodically asks MLD hosts on the network to
verify their multicast join requests. There is one querier for each VLAN, and all switches on the
VLAN listen to the responses of MLD hosts to multicast queries and forward or block multicast traffic
accordingly.
All of the HP switches have the querier function enabled by default. If there is another device on
the VLAN that is already acting as querier, the switch defers to that querier. If there is no device
acting as querier, the switch enters an election state and negotiates with other devices on the
network (if any) to determine which one will act as the querier.
The querier periodically sends general queries to MLD hosts on each multicast address that is active
on the VLAN. The time that the querier waits between sending general queries is known as the
query interval; the MLD standard sets the default query interval to 125 seconds.
Network nodes that wish to remain active as MLD hosts respond to the queries with join requests;
in this way they continue to assert their presence as MLD hosts. The switch through which any given
MLD host connects to the VLAN sees the join requests and continues forwarding traffic for that
multicast address to the MLD host's port.
Leaves
A node acting as an MLD host can be disconnected from a multicast address in two ways:
• It can stop sending join requests to the querier.
This might happen if the multicast application quits or the node is removed from the network.
If the switch goes for slightly more than two query intervals without seeing a join request from
the MLD host, it stops sending multicast traffic for that multicast address to the MLD host's port.
• It can issue a "leave" request.
This is done by the application software running on the MLD host. If the MLD host is the only
node connected to its switch port, the switch sees the leave request and stops sending multicast
packets for that multicast address to that port. (If there is more than one node connected to
the port the situation is somewhat more complicated, as explained under “Fast leaves and
forced fast leaves” (page 83).)
82 Multicast Listener Discovery (MLD) Snooping