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Patch SB251-13 For SwitchBlade 4000 Series Switches and AT-9800 Series Switches 15
Patch SB251-13 for Software Release 2.5.1
C613-10361-00 REV M
Multicast Listener Discovery Snooping
Multicast Listener Discovery (MLD) snooping enables the switch to forward IPv6
multicast traffic intelligently, instead of flooding it out all ports in the VLAN.
Without MLD snooping, multicast group membership for VLAN aware
devices is on a per-VLAN basis, because MLD is an IPv6-based protocol. If at
least one port in the VLAN is a member of a multicast group, and MLD
snooping is not used, IPv6 multicast packets will be flooded onto all ports in
the VLAN. With MLD snooping, the switch passively listens to MLD joins /
reports and leaves / done messages, to identify the switch ports that have
received joins and/or leaves from devices attached to them. Multicast traffic
will only be forwarded to those ports. MLD snooping will also identify ports
that are connected to another router or switch and forward messages out those
ports appropriately.
MLD snooping is performed at Layer 2 on VLAN interfaces automatically. By
default, the switch will only forward traffic out those ports with routers or IPv6
multicast listeners, therefore it will not act as a simple hub and flood all IPv6
multicast traffic out all ports. MLD snooping is independent of the MLD and
Layer 3 configuration, so an IPv6 interface does not have to be attached to the
VLAN, and MLD does not have to be enabled or configured. MLD is described
in the “IPv6 Multicasting” chapter of the AT-9800 Series Switch Software
Reference.
MLD snooping will not generate MLD query messages, but will relay MLD
queries from other routers or switches attached to one of its ports to other ports
in the same VLAN.
MLDv2 supports Multicast Address and Source Specific messages. These
messages enable a host to listen to traffic from a particular source to a
particular multicast address, instead of all traffic for the group. MLD snooping
cannot snoop these messages, because the address information is contained
within the packet’s IPv6 Layer 3 header. These messages will be flooded to all
ports in the VLAN. The switch will snoop MLDv1 message types 130 (Query),
131 (Listener Report) and 132 (Listener Done), as specified in RFC 2710,
Multicast Listener Discovery (MLD) for IPv6, October 1999.
Multicast group membership registration entries on the switch will time out
after no data or messages have been received for that group on that port for 270
seconds.
The following multicast addresses are used by IPv6 for special purposes, and
will always be flooded:
FF02::1 (All nodes)
FF02::2 (All routers)
FF02::4 (DVMRP)
FF02::5 (OSPFIGP)
FF02::6 (OSPFIGP Designated routers)
FF02::9 (RIPv2)
FF02::d (PIM)
FF02::f (CBT)
FF02::12 (VRRP)