Cisco Catalyst 3560-X Series, Software Configuration Manual

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Catalyst 3750-X and 3560-X Switch Software Configuration Guide
OL-25303-03
Chapter 1 Implementing IPv6 Multicast
Information About Implementing IPv6 Multicast
A set of queriers and hosts that receive multicast data streams from the same source is called a multicast
group. Queriers and hosts use MLD reports to join and leave multicast groups and to begin receiving
group traffic.
MLD uses the Internet Control Message Protocol (ICMP) to carry its messages. All MLD messages are
link-local with a hop limit of 1, and they all have the switch alert option set. The switch alert option
implies an implementation of the hop-by-hop option header.
MLD has three types of messages:
• Query--General, group-specific, and multicast-address-specific. The multicast address field is set to
0 when MLD sends a general query. The general query learns which multicast addresses have
listeners on an attached link.
Group-specific and multicast address-specific queries both set the multicast address field to the address
being queried. A group address is a multicast address.
• Report--In a report message, the multicast address field is that of the specific IPv6 multicast address
to which the sender is listening.
•
Done--In a done message, the multicast address field is that of the specific IPv6 multicast address
to which the source of the MLD message is no longer listening.
Note
There is no done message in MLD version 2.
An MLD report must be sent with a valid IPv6 link-local source address, or the unspecified address (::),
if the sending interface has not yet acquired a valid link-local address. Sending reports with the
unspecified address is allowed to support the use of IPv6 multicast in the Neighbor Discovery Protocol.
For stateless autoconfiguration, a node is required to join several IPv6 multicast groups in order to
perform duplicate address detection (DAD). Prior to DAD, the only address the reporting node has for
the sending interface is a tentative one, which cannot be used for communication. Therefore, the
unspecified address must be used.
MLD states that result from MLD version 2 or MLD version 1 membership reports can be limited
globally or by interface. The MLD group limits feature provides protection against denial of service
(DoS) attacks caused by MLD packets. Membership reports in excess of the configured limits will not
be entered in the MLD cache, and traffic for those excess membership reports will not be forwarded.
MLD provides support for source filtering. Source filtering allows a node to report interest in listening
to packets only from specific source addresses (as required to support SSM), or from all addresses except
specific source addresses sent to a particular multicast address.
When a host using MLD sends a leave message, the switch needs to send query messages to reconfirm
that this host was the last host joined to the group before it can stop forwarding traffic. This function
takes about 2 seconds. This “leave latency” is also present in IGMP version 2 for IPv4 multicast.
MLD Access Group
The MLD access group provides receiver access control in Cisco IOS IPv6 multicast switches. This
feature limits the list of groups a receiver can join, and it allows or denies sources used to join SSM
channels.
Explicit Tracking of Receivers
The explicit tracking feature allows a switch to track the behavior of the hosts within its IPv6 network.
This feature also enables the fast leave mechanism to be used with MLD version 2 host reports.