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Catalyst 3750-X and 3560-X Switch Software Configuration Guide
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Chapter 27 Configuring IPv6 MLD Snooping
Understanding MLD Snooping
Multicast Router Discovery
Like IGMP snooping, MLD snooping performs multicast router discovery, with these characteristics:
• Ports configured by a user never age out.
• Dynamic port learning results from MLDv1 snooping queries and IPv6 PIMv2 packets.
• If there are multiple routers on the same Layer 2 interface, MLD snooping tracks a single multicast
router on the port (the router that most recently sent a router control packet).
• Dynamic multicast router port aging is based on a default timer of 5 minutes; the multicast router is
deleted from the router port list if no control packet is received on the port for 5 minutes.
• IPv6 multicast router discovery only takes place when MLD snooping is enabled on the switch.
• Received IPv6 multicast router control packets are always flooded to the ingress VLAN, whether or
not MLD snooping is enabled on the switch.
• After the discovery of the first IPv6 multicast router port, unknown IPv6 multicast data is forwarded
only to the discovered router ports (before that time, all IPv6 multicast data is flooded to the ingress
VLAN).
MLD Reports
The processing of MLDv1 join messages is essentially the same as with IGMPv2. When no IPv6
multicast routers are detected in a VLAN, reports are not processed or forwarded from the switch. When
IPv6 multicast routers are detected and an MLDv1 report is received, an IPv6 multicast group address
and an IPv6 multicast MAC address are entered in the VLAN MLD database. Then all IPv6 multicast
traffic to the group within the VLAN is forwarded using this address. When MLD snooping is disabled,
reports are flooded in the ingress VLAN.
When MLD snooping is enabled, MLD report suppression, called listener message suppression, is
automatically enabled. With report suppression, the switch forwards the first MLDv1 report received by
a group to IPv6 multicast routers; subsequent reports for the group are not sent to the routers. When
MLD snooping is disabled, report suppression is disabled, and all MLDv1 reports are flooded to the
ingress VLAN.
The switch also supports MLDv1 proxy reporting. When an MLDv1 MASQ is received, the switch
responds with MLDv1 reports for the address on which the query arrived if the group exists in the switch
on another port and if the port on which the query arrived is not the last member port for the address.
MLD Done Messages and Immediate-Leave
When the Immediate-Leave feature is enabled and a host sends an MLDv1 Done message (equivalent to
an IGMP leave message), the port on which the Done message was received is immediately deleted from
the group.You enable Immediate-Leave on VLANs and (as with IGMP snooping), you should only use
the feature on VLANs where a single host is connected to the port. If the port was the last member of a
group, the group is also deleted, and the leave information is forwarded to the detected IPv6 multicast
routers.
When Immediate Leave is not enabled in a VLAN (which would be the case when there are multiple
clients for a group on the same port) and a Done message is received on a port, an MASQ is generated
on that port. The user can control when a port membership is removed for an existing address in terms
of the number of MASQs. A port is removed from membership to an address when there are no MLDv1
reports to the address on the port for the configured number of queries.