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ME60 V800R010C10SPC500 Feature Description - IP Multicast 01

This is ME60 V800R010C10SPC500 Feature Description - IP Multicast

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Multicast protocols are required to implement data forwarding on a multicast network. Protocol Independent Multicast (PIM) is the most widely used multicast protocol that forwards data between devices in the same domain. Protocol Independent Multicast-Dense Mode (PIM-DM) is one type of PIM.

PIM-DM mainly uses the flood-prune mechanism to implement multicast data forwarding. Specifically, PIM-DM floods a multicast flow to all network segments and then prunes the network segments on which no receivers want the flow. PIM-DM periodically performs flood-prune operations to build up and maintain a shortest path tree (SPT) that connects a multicast source and multicast receivers. Then, PIM-DM forwards multicast data along this unidirectional loop-free SPT. PIM-DM applies to small-scale networks on which multicast receivers are densely located. PIM-DM is not a good choice for large-scale networks because the flood-prune period will be long on such a network. PIM-DM neither suits for networks with sparsely located receivers because excessive Prune messages will be generated on such a network.

Related Concepts

This section provides basic PIM-DM concepts. See Figure 4-1.

Figure 4-1 PIM-DM networking
  • PIM device

    A multicast router that supports PIM is called a PIM device. A PIM-enabled interface on a PIM device is called a PIM interface.

  • SPT

    A shortest path tree (SPT) is a multicast distribution tree (MDT) with the multicast source at the root and group members at leaves. SPTs can be used in PIM-DM, Protocol Independent Multicast-Sparse Mode (PIM-SM), and Protocol Independent Multicast-Source-Specific Multicast (PIM-SSM) scenarios.


The multicast data forwarding process in a PIM-DM domain is as follows:
  1. Neighbor Discovery

    Each PIM device in a PIM-DM domain periodically sends Hello messages to all other PIM devices to discover PIM neighbors and maintain PIM neighbor relationships.

    By default, a PIM device permits other PIM control messages or multicast messages from a neighbor, irrespective of whether the PIM device has received Hello messages from the neighbor. However, if a PIM device has the neighbor check function enabled, the PIM device permits other PIM control messages or multicast messages from a neighbor only after the PIM device has received Hello messages from the neighbor.

  2. Flooding

    PIM-DM assumes that at least one multicast group member exists on each network segment, and floods multicast data to all routers on the network. Therefore, all PIM devices on the network can receive multicast data.

  3. Prune

    After flooding multicast data, PIM-DM prunes network segments that have no multicast data receiver and retains only the network segments that have multicast data receivers. Only PIM devices that require multicast data can receive multicast data.

  4. State Refresh

    If a downstream device is in the prune state, the upstream device maintains a prune timer for this device. When the prune timer expires, the upstream device resumes data forwarding to the downstream device, which wastes network resources. To prevent this problem, the state-refresh function can be enabled on the upstream router. This function enables the upstream router to periodically send State-Refresh messages to refresh the status of the prune timers of downstream devices. Downstream devices that do not require multicast data remain in the prune state.

  5. Graft

    If a node on a pruned network segment has new group members, PIM-DM uses the graft mechanism to enable the node to immediately forward multicast data.

  6. Assert

    If there are multiple PIM devices on a network segment, the same multicast packets are sent repeatedly across the network segment. The Assert mechanism can be used to select a unique multicast data forwarder, preventing redundant multicast data forwarding.

The detailed PIM-DM implementation process is as follows:

Neighbor Discovery

This mechanism is the same as that in PIM-SM. For details about this mechanism, see PIM-SM.


The following example uses the network shown in Figure 4-2 to describe the flooding function. The source sends a data packet to Device A. Then Device A floods the packet to all its neighbors. Device B and Device C also exchange data packets with each other. To prevent data duplication, PIM-DM capable Device B uses the reverse path forwarding (RPF) mechanism to ensure that it only permits data packets from one neighbor, Device A or Device C. (For details about RPF check, see RPF Check.) Finally, data is flooded to Device B with receivers, as well as Device C without receivers. This process is called flooding.

Figure 4-2 PIM-DM flooding


The following example uses the network shown in Figure 4-3 to describe the prune function. Device C has no receivers, so it sends a Prune message upstream to Device A to instruct Device A to stop forwarding data to the interface connected to Device C. After receiving the Prune message, Device A stops forwarding data to the downstream interface connected to Device C. This process is called pruning.

Because a downstream interface on Device A is connected to Device B that has a receiver, Device A forwards multicast data to the downstream interface connected to Device B. In this manner, a unidirectional and loop-free SPT is set up from the source to User A.

Figure 4-3 PIM-DM prune

State Refresh

The following example uses the network shown in Figure 4-3 to describe the state refresh function. After Device A prunes the network segment of Device C, Device A maintains a prune timer for Device C. When the prune timer expires, Device A resumes data forwarding to Device C. This results in a waste of network resources.

The state refresh function can prevent this problem and works as follows: Device A periodically floods State-Refresh messages to all its downstream interfaces to reset the prune timers of all the downstream devices.


The following example uses the network shown in Figure 4-4 to describe the graft function. After Device C in the pruned state receives an IGMP Report message from user B, Device C uses the graft function to implement fast data forwarding, without waiting a flood-prune period. The graft function works as follows:

Device C sends a Graft messages upstream to require Device A to restore the forwarding status of the downstream interface connected to Device C. After restoring the forwarding the status, Device A sends multicast data to Device C. Therefore, the graft function implements rapid data forwarding for devices in the pruned state.

Figure 4-4 PIM-DM graft


The following example uses the network shown in Figure 4-5 to describe the assert function. Device B and Device C can receive multicast packets from the multicast source and the multicast packets that pass the RPF check. (S, G) entries can be created on Device B and Device C. Because the downstream interfaces of Device B and Device C are connected to the same network segment, Device A and Device C can both send multicast data to the network segment. The assert function is used to ensure that only one multicast data forwarder exists on the network segment. The assert process is as follows:

  1. Device B receives a multicast packet from Device C through a downstream interface, but this packet fails the RPF check and is discarded by Device B. At the same time, Device B sends an Assert message to the network segment.

  2. Device C compares its routing information with that carried in the Assert message sent by Device B. Device C is denied because the route cost from Device B to the source is lower. The downstream interface of Device C is prohibited from forwarding multicast packets and deleted from the downstream interface list of the (S, G) entry.

  3. Device C receives a multicast packet from Device B through the network segment, but the packet fails the RPF check and therefore is discarded.

    Figure 4-5 PIM-DM assert
Updated: 2019-01-04

Document ID: EDOC1100059456

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