Cisco Catalyst 3560-X Software Configuration Manual page 1328

Information About Implementing IPv6 Multicast
SSM Mapping for IPv6
SSM mapping for IPv6 supports both static and dynamic Domain Name System (DNS) mapping for
MLD version 1 receivers. This feature allows deployment of IPv6 SSM with hosts that are incapable of
providing MLD version 2 support in their TCP/IP host stack and their IP multicast receiving application.
SSM mapping allows the switch to look up the source of a multicast MLD version 1 report either in the
running configuration of the switch or from a DNS server. The switch can then initiate an (S, G) join
toward the source.
PIM Shared Tree and Source Tree (Shortest-Path Tree)
By default, members of a group receive data from senders to the group across a single data distribution
tree rooted at the RP. This type of distribution tree is called shared tree or rendezvous point tree (RPT),
as illustrated in the figure below. Data from senders is delivered to the RP for distribution to group
members joined to the shared tree.
If the data threshold warrants, leaf switches on the shared tree may initiate a switch to the data
distribution tree rooted at the source. This type of distribution tree is called a shortest path tree or source
tree. By default, the Cisco IOS software switches to a source tree upon receiving the first data packet
from a source.
The following process details the move from shared tree to source tree:
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Join and prune messages are sent for sources and RPs. They are sent hop-by-hop and are processed by
each PIM switch along the path to the source or RP. Register and register-stop messages are not sent
hop-by-hop. They are sent by the designated switch that is directly connected to a source and are
received by the RP for the group.
Reverse Path Forwarding
Reverse-path forwarding is used for forwarding multicast datagrams. It functions as follows:
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Catalyst 3750-X and 3560-X Switch Software Configuration Guide
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Receiver joins a group; leaf Switch C sends a join message toward the RP.
RP puts the link to Switch C in its outgoing interface list.
Source sends the data; Switch A encapsulates the data in the register and sends it to the RP.
RP forwards the data down the shared tree to Switch C and sends a join message toward the source.
At this point, data may arrive twice at Switch C, once encapsulated and once natively.
When data arrives natively (unencapsulated) at the RP, the RP sends a register-stop message to
Switch A.
By default, receipt of the first data packet prompts Switch C to send a join message toward the
source.
When Switch C receives data on (S, G), it sends a prune message for the source up the shared tree.
RP deletes the link to Switch C from the outgoing interface of (S, G).
RP triggers a prune message toward the source.
If a switch receives a datagram on an interface it uses to send unicast packets to the source, the
packet has arrived on the RPF interface.
If the packet arrives on the RPF interface, a switch forwards the packet out the interfaces present in
the outgoing interface list of a multicast routing table entry.
If the packet does not arrive on the RPF interface, the packet is silently discarded to prevent loops.
Chapter 1 Implementing IPv6 Multicast
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