way: SPT state entries use more router memory because there is an entry for each sender and group pair, but the traffic is sent over the optimal path to each receiver, thus minimizing the delay in packet delivery. Shared distribution tree state entries consume less router memory, but you may get suboptimal paths from a source to receivers, thus introducing extra delay in packet delivery.
Content 7.3 Multicast Routing Protocols 7.3.4 IP Multicast Routing In unicast routing, when the router receives the packet, the decision about where to forward the packet depends on the destination address of the packet. In multicast routing, the decision about where to forward the multicast packet depends on where the packet came from. Multicast routers must know the origin of the packet rather than its destination. With multicast origination, the IP address denotes the known source, and the destination IP address denotes a group of unknown receivers. Multicast routing uses a mechanism called Reverse Path Forwarding (RPF) to prevent forwarding loops and to ensure the shortest path from the source to the receivers.
Content 7.3 Multicast Routing Protocols 7.3.5 Protocol-Independent Multicast: Describing PIM-DM PIM dense mode (PIM-DM) initially floods traffic out of all non-RPF interfaces where there is another PIM-DM neighbor or a directly connected member of the group. In Figure , multicast traffic being sent by the source is flooded throughout the entire network. As each router receives the multicast traffic via its RPF interface (the interface in the direction of the source), it forwards the multicast traffic to all of its PIM-DM neighbors. This results in some traffic arriving via a non-RPF interface, as with the two routers in the center and far right of the figure. Packets arriving via the non-RPF interface are discarded. These non-RPF flows are normal for the initial flooding of data and are corrected by the normal PIM-DM pruning mechanism. In Figure , PIM-DM prune messages are sent (denoted by dashed arrows) to stop the unwanted traffic. Prune messages are also sent on non-RPF interfaces to shut off the flow of multicast traffic, because it is arriving via an interface that is not on the shortest path to the source. The example of prune messages sent on a non-RPF interface can be seen on the routers in the middle and far right of the figure. Prune messages are sent on an RPF interface only when the router does not have any downstream receivers for multicast traffic. Figure shows the SPT resulting from pruning the unwanted multicast traffic in the network. Although the flow of multicast traffic is no longer reaching most of the routers in the network, the (S, G) state remains for all of them and will remain until the source stops sending. In PIM-DM, all prune messages expire in 3 minutes. After that, the multicast traffic is flooded again to all of the routers. This periodic flood-and-prune behavior is normal and must be taken into account when the network is designed to use PIM-DM.
Content 7.3 Multicast Routing Protocols 7.3.6 Protocol-Independent Multicast: Describing PIM-SM PIM sparse mode (PIM-SM) is described in RFC 2362. As with PIM-DM, PIM-SM is also independent of underlying unicast protocols. PIM-SM uses shared distribution trees, but it can also switch to the SPT. PIM-SM is based on an explicit pull model. Therefore, traffic is forwarded only to the parts of the network that need it. PIM-SM uses an RP to coordinate forwarding of multicast traffic from a source to receivers. Senders register with the RP and send a single copy of multicast data through it to the registered receivers. Group members are joined to the shared tree by their local designated router (DR). A shared tree that is built this way is always rooted at the RP. PIM-SM is appropriate for wide-scale deployment for both densely and sparsely populated groups in the enterprise network. It is the optimal choice for all production networks, regardless of size and membership density. There are many optimizations and enhancements to PIM, including the following: In Figure , an active receiver (attached to a leaf router at the bottom of the figure) has joined multicast group G. The last-hop router knows the IP address of the RP router for group G, and it sends a (*, G) join for this group toward the RP. This (*, G) join travels hop-by-hop toward the RP, building a branch of the shared tree that extends from the RP to the last-hop router directly connected to the receiver. At this point, group G traffic flows down the shared tree to the receiver.
Content 7.3 Multicast Routing Protocols 7.3.7 PIM Sparse-Dense-Mode Figure depicts two multicast sources. For maximum efficiency, multiple RPs can be implemented, with each RP in an optimum location. This design is difficult to configure, manage, and troubleshoot with manual configurations of RPs.PIM sparse-dense mode supports automatic selection of RPs for each multicast. Router A in the figure could be the RP for source 1, and router F could be the RP for source 2. PIM sparse-dense mode is the recommended solution from Cisco for IP multicast, because PIM-DM does not scale well and requires heavy router resources, and PIM-SM offers limited RP configuration options. If no RP is discovered for the multicast group or none is manually configured, PIM sparse-dense mode operates in dense mode. Therefore, you should implement automatic RP discovery with PIM sparse-dense mode.
Content 7.4 Multicast Configuration and Verification 7.4.1 Enabling PIM Sparse Mode and Sparse-Dense Mode The commands needed for simple PIM-SM and PIM sparse-dense mode deployment are the following: Note
The recommended method for configuring an interface for PIM-SM operation is to use the ip pim sparse-dense-mode interface command. This method permits auto RP, bootstrap router (BSR), or statically defined RPs to be used with the least configuration effort.
Content 7.4 Multicast Configuration and Verification 7.4.2 Inspecting the Multicast