H3C Technologies H3C S12500-X Series Switches - Manual (Page 155)

145
Configuring IPv6 PIM
PIM overview
Protocol Independent Multicast for IPv6 (IPv6 PIM) provides IPv6 multicast forwarding by leveraging IPv6
unicast static routes or IPv6 unicast routing tables generated by any IPv6 unicast routing protocol, such
as RIPng, OSPFv3, IPv6 IS-IS, or IPv6 BGP. IPv6 PIM is not dependent on any particular IPv6 unicast
routing protocol, and it uses the underlying IPv6 unicast routing to generate a routing table with routes.
IPv6 PIM uses the RPF mechanism to implement multicast forwarding. When an IPv6 multicast packet
arrives on an interface of the device, the packet undergoes an RPF check. If the RPF check succeeds, the
device creates an IPv6 multicast routing entry and forwards the packet. If the RPF check fails, the device
discards the packet. For more information about RPF, see " Configuring IPv6 multicast routing and
forwarding ."
Based on the implementation mechanism, IPv6 PIM includes the following categories:
• Protocol Independent Multicast–Dense Mode for IPv6 (IPv6 PIM-DM)
• Protocol Independent Multicast–Sparse Mode for IPv6 (IPv6 PIM-SM)
• Protocol Independent Multicast Source-Specific Multicast for IPv6 (IPv6 PIM-SSM)
The term "IPv6 PIM domain" in this chapter refers to a network composed of IPv6 PIM routers.
The term "interface" in this chapter collectively refers to VLAN interfaces and Layer 3 Ethernet interfaces.
You can set an Ethernet port as a Layer 3 interface by using the port link-mode route command (see
Layer 2—LAN Switching Configuration Guide).
IPv6 PIM-DM overview
IPv6 PIM-DM uses the push mode for multicast forwarding and is suitable for small networks with densely
distributed IPv6 multicast members.
The following describes the basic implementation of IPv6 PIM-DM:
• IPv6 PIM-DM assumes that all downstream nodes want to receive IPv6 multicast data when a source
starts sending, so IPv6 multicast data is flooded to all downstream nodes on the network.
• Branches without downstream receivers are pruned from the forwarding trees, leaving only those
branches that contain receivers.
• The pruned state of a branch has a finite holdtime timer. When the timer expires, IPv6 multicast data
is again forwarded to the pruned branch. This flood-and-prune cycle takes place periodically to
maintain the forwarding branches.
• To reduce join latency when a new receiver on a previously pruned branch joins an IPv6 multicast
group, IPv6 PIM-DM uses a graft mechanism to turn the pruned branch into a forwarding branch.
In IPv6 PIM-DM, the multicast forwarding paths for an IPv6 multicast group constitute a source tree, which
is rooted at the IPv6 multicast source and has multicast group members as its "leaves." Because the
source tree consists of the shortest paths from the IPv6 multicast source to the receivers, it is also called
a "shortest path tree (SPT)."
The operating mechanism of IPv6 PIM-DM is summarized as follows: