IPv6 PIM-DM

IPv6 PIM-DM uses the push mode for multicast forwarding and is suitable for small networks with densely distributed IPv6 multicast members.

IPv6 PIM-DM assumes that all downstream nodes want to receive IPv6 multicast data from a source, 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. When the pruned branch has new receivers, the graft mechanism turns the pruned branch into a forwarding branch.

In IPv6 PIM-DM, the multicast forwarding paths for an IPv6 multicast group constitute a forwarding tree. The forwarding tree is rooted at the IPv6 multicast source and has multicast group members as its "leaves." Because the forwarding tree consists of the shortest paths from the IPv6 multicast source to the receivers, it is also called a "shortest path tree (SPT)."

IPv6 PIM-DM mechanisms include neighbor discovery, SPT building, graft, and assert.

In an IPv6 PIM domain, each IPv6 PIM interface periodically multicasts IPv6 PIM hello messages to all other IPv6 PIM devices on the local subnet. Through the exchanging of hello messages, all IPv6 PIM devices determine their IPv6 PIM neighbors, maintain IPv6 PIM neighboring relationship with other devices, and build and maintain SPTs.

The process of building an SPT is the flood-and-prune process:

1. In an IPv6 PIM-DM domain, the IPv6 multicast data from the IPv6 multicast source S to the IPv6 multicast group G is flooded throughout the domain. A device performs an RPF check on the IPv6 multicast data. If the check succeeds, the device creates an (S, G) entry and forwards the data to all downstream nodes in the network. In the flooding process, all the devices in the IPv6 PIM-DM domain create the (S, G) entry.

2. The nodes without downstream receivers are pruned. A device that has no downstream receivers multicasts a prune message to all IPv6 PIM devices on the subnet. When the upstream node receives the prune message, it removes the receiving interface from the (S, G) entry. In this way, the upstream stream node stops forwarding subsequent packets addressed to that IPv6 multicast group down to this node.

   	NOTE: An (S, G) entry contains an IPv6 multicast source address S, an IPv6 multicast group address G, an outgoing interface list, and an incoming interface.

A prune process is initiated by a leaf device. As shown in Figure 106, the device interface that does not have any downstream receivers initiates a prune process by sending a prune message toward the IPv6 multicast source. This prune process goes on until only necessary branches are left in the IPv6 PIM-DM domain, and these necessary branches constitute an SPT.

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. The flood-and-prune cycle takes place periodically to maintain the forwarding branches.

A previously pruned branch might have new downstream receivers. To reduce the latency for resuming the forwarding capability of this branch, a graft mechanism is used as follows:

1. The node that needs to receive the IPv6 multicast data sends a graft message to its upstream node, telling it to rejoin the SPT.

2. After receiving this graft message on an interface, the upstream node adds the receiving interface to the outgoing interface list of the (S, G) entry. It also sends a graft-ack message to the graft sender.

3. If the graft sender receives a graft-ack message, the graft process finishes. Otherwise, the graft sender continues to send graft messages at a graft retry interval until it receives an acknowledgment from its upstream node.

On a subnet with more than one multicast device, the assert mechanism shuts off duplicate multicast flows to the network. It does this by electing a unique multicast forwarder for the subnet.

As shown in Figure 107, after Device A and Device B receive an (S, G) packet from the upstream node, they both forward the packet to the local subnet. As a result, the downstream node Device C receives two identical multicast packets. In addition, both Device A and Device B, on their downstream interfaces, receive a duplicate packet forwarded by the other. After detecting this condition, both devices send an assert message to all IPv6 PIM devices on the local subnet through the interface that received the packet. The assert message contains the IPv6 multicast source address (S), the IPv6 multicast group address (G), and the metric preference and metric of the IPv6 unicast route/MBGP route/static multicast route to the IPv6 multicast source. By comparing these parameters, either Device A or Device B becomes the unique forwarder of the subsequent (S, G) packets on the subnet. The comparison process is as follows:

1. The device with a higher metric preference to the IPv6 multicast source wins.

2. If both devices have the same metric preference to the IPv6 multicast source, the device with a smaller metric to the IPv6 multicast source wins.

3. If both devices have the same metric, the device with a higher IPv6 link-local address on the downstream interface wins.