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Increase employee productivity and loyalty. Interactive Hand-on Labs allowing you to practice what you are learning in each course. Another benefit of sparse-dense mode is that Auto-RP information can be distributed in a dense mode; yet, multicast groups for user groups can be used in a sparse mode manner. Therefore there is no need to configure a default RP at the leaf routers. When an interface is treated in dense mode, it is populated in the outgoing interface list of a multicast routing table when either of the following conditions is true:.
When an interface is treated in sparse mode, it is populated in the outgoing interface list of a multicast routing table when either of the following conditions is true:. An explicit Join message has been received by a PIM neighbor on the interface. Multicast groups in bidirectional mode can scale to an arbitrary number of sources with only a minimal amount of additional overhead. The shared trees that are created in PIM sparse mode are unidirectional. This means that a source tree must be created to bring the data stream to the RP the root of the shared tree and then it can be forwarded down the branches to the receivers.
Source data cannot flow up the shared tree toward the RP--this would be considered a bidirectional shared tree. In bidirectional mode, traffic is routed only along a bidirectional shared tree that is rooted at the RP for the group. This IP address need not be a router address, but can be any unassigned IP address on a network that is reachable throughout the PIM domain.
This feature eliminates any source-specific state and allows scaling capability to an arbitrary number of sources. In PIM, packet traffic for a multicast group is routed according to the rules of the mode configured for that multicast group. The Cisco implementation of PIM supports four modes for a multicast group:. A router can simultaneously support all four modes or any combination of them for different multicast groups.
In bidirectional mode, traffic is routed only along a bidirectional shared tree that is rooted at the rendezvous point RP for the group. This technique is the preferred configuration method for establishing a redundant RP configuration for bidir-PIM. Membership to a bidirectional group is signalled via explicit Join messages. Traffic from sources is unconditionally sent up the shared tree toward the RP and passed down the tree toward the receivers on each branch of the tree.
Sparse mode operation centers around a single unidirectional shared tree whose root node is called the rendezvous point RP. Sources must register with the RP to get their multicast traffic to flow down the shared tree by way of the RP. This registration process actually triggers a shortest path tree SPT Join by the RP toward the source when there are active receivers for the group in the network.
A sparse mode group uses the explicit join model of interaction. Receiver hosts join a group at a rendezvous point RP. Different groups can have different RPs. Multicast traffic packets flow down the shared tree to only those receivers that have explicitly asked to receive the traffic. In populating the multicast routing table, dense mode interfaces are always added to the table.
Multicast traffic is forwarded out all interfaces in the outgoing interface list to all receivers. Interfaces are removed from the outgoing interface list in a process called pruning. In dense mode, interfaces are pruned for various reasons including that there are no directly connected receivers. A pruned interface can be reestablished, that is, grafted back so that restarting the flow of multicast traffic can be accomplished with minimal delay.
In the PIM-SM model, only network segments with active receivers that have explicitly requested multicast data will be forwarded the traffic. Routers that have no downstream neighbors or directly connected receivers prune back the unwanted traffic. An RP acts as the meeting place for sources and receivers of multicast data.
This traffic is then forwarded to receivers down a shared distribution tree. By default, when the first hop device of the receiver learns about the source, it will send a Join message directly to the source, creating a source-based distribution tree from the source to the receiver. This source tree does not include the RP unless the RP is located within the shortest path between the source and receiver. In most cases, the placement of the RP in the network is not a complex decision. By default, the RP is needed only to start new sessions with sources and receivers. Consequently, the RP experiences little overhead from traffic flow or processing.
In the first version of PIM-SM, all leaf routers routers directly connected to sources or receivers were required to be manually configured with the IP address of the RP. This type of configuration is also known as static RP configuration. Configuring static RPs is relatively easy in a small network, but it can be laborious in a large, complex network.
Auto-RP has the following benefits:. Configuring the use of multiple RPs within a network to serve different groups is easy. Auto-RP allows load splitting among different RPs and arrangement of RPs according to the location of group participants. Auto-RP avoids inconsistent, manual RP configurations that can cause connectivity problems. Multiple RPs can be used to serve different group ranges or serve as backups to each other.
The RP-mapping agent then sends the consistent group-to-RP mappings to all other routers. Thus, all routers automatically discover which RP to use for the groups they support. If router interfaces are configured in sparse mode, Auto-RP can still be used if all routers are configured with a static RP address for the Auto-RP groups. The RP mapping agent then sends the consistent group-to-RP mappings to all other routers by dense mode flooding.
One advantage of Auto-RP is that any change to the RP designation must be configured only on the routers that are RPs and not on the leaf routers. Each method for configuring an RP has its own strengths, weaknesses, and level of complexity. In conventional IP multicast network scenarios, we recommend using Auto-RP to configure RPs because it is easy to configure, well-tested, and stable. A prerequisite of Auto-RP is that all interfaces must be configured in sparse-dense mode using the ip pim sparse-dense-mode interface configuration command. An interface configured in sparse-dense mode is treated in either sparse mode or dense mode of operation, depending on which mode the multicast group operates.
If a multicast group has a known RP, the interface is treated in sparse mode. If a group has no known RP, by default the interface is treated in dense mode and data will be flooded over this interface. To successfully implement Auto-RP and prevent any groups other than A sink RP is a statically configured RP that may or may not actually exist in the network. We recommend configuring a sink RP for all possible multicast groups in your network, because it is possible for an unknown or unexpected source to become active.
If no RP is configured to limit source registration, the group may revert to dense mode operation and be flooded with data. A BSR performs similarly as an RP, except that it does not run the risk of reverting to dense mode operation, and it does not offer the ability to scope within a domain. In the PIM sparse mode model, multicast sources and receivers must register with their local rendezvous point RP. RPs in other domains have no way of knowing about sources that are located in other domains.
RPs know about the receivers in their local domain. When RPs in remote domains hear about the active sources, they can pass on that information to their local receivers.
Multicast data can then be forwarded between the domains. A useful feature of MSDP is that it allows each domain to maintain an independent RP that does not rely on other domains, but it does enable RPs to forward traffic between domains. PIM-SM is used to forward the traffic between the multicast domains. The encapsulated data is decapsulated and forwarded down the shared tree of that RP. When the last hop router the router closest to the receiver receives the multicast packet, it may join the shortest path tree to the source.
Originally developed for interdomain multicast applications, MSDP used for Anycast RP is an intradomain feature that provides redundancy and load-sharing capabilities. The Anycast RP loopback address should be configured with a bit mask, making it a host address. IP routing automatically will select the topologically closest RP for each source and receiver.
Assuming that the sources are evenly spaced around the network, an equal number of sources will register with each RP. That is, the process of registering the sources will be shared equally by all the RPs in the network. Because a source may register with one RP and receivers may join to a different RP, a method is needed for the RPs to exchange information about active sources.
This information exchange is done with MSDP.
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When a source registers with one RP, an SA message will be sent to the other RPs informing them that there is an active source for a particular multicast group. The result is that each RP will know about the active sources in the area of the other RPs. New sources would register with the backup RP. Receivers would join toward the new RP and connectivity would be maintained. The RP is normally needed only to start new sessions with sources and receivers. The RP facilitates the shared tree so that sources and receivers can directly establish a multicast data flow.
If a multicast data flow is already directly established between a source and the receiver, then an RP failure will not affect that session. Anycast RP ensures that new sessions with sources and receivers can begin at any time. Forwarding of multicast traffic is accomplished by multicast-capable routers. These routers create distribution trees that control the path that IP multicast traffic takes through the network in order to deliver traffic to all receivers.
Multicast traffic flows from the source to the multicast group over a distribution tree that connects all of the sources to all of the receivers in the group. This tree may be shared by all sources a shared tree or a separate distribution tree can be built for each source a source tree. The shared tree may be one-way or bidirectional.
Before describing the structure of source and shared trees, it is helpful to explain the notations that are used in multicast routing tables. These notations include the following:. The simplest form of a multicast distribution tree is a source tree. A source tree has its root at the source host and has branches forming a spanning tree through the network to the receivers. Because this tree uses the shortest path through the network, it is also referred to as a shortest path tree SPT. The figure shows an example of an SPT for group Using standard notation, the SPT for the example shown in the figure would be The S,G notation implies that a separate SPT exists for each individual source sending to each group--which is correct.
Unlike source trees that have their root at the source, shared trees use a single common root placed at some chosen point in the network. This shared root is called a rendezvous point RP. Figure 5 shows a shared tree for the group This shared tree is unidirectional. Source traffic is sent towards the RP on a source tree. The traffic is then forwarded down the shared tree from the RP to reach all of the receivers unless the receiver is located between the source and the RP, in which case it will be serviced directly.
In this example, multicast traffic from the sources, Hosts A and D, travels to the root Router D and then down the shared tree to the two receivers, Hosts B and C. Both source trees and shared trees are loop-free. Messages are replicated only where the tree branches. Members of multicast groups can join or leave at any time; therefore the distribution trees must be dynamically updated. When all the active receivers on a particular branch stop requesting the traffic for a particular multicast group, the routers prune that branch from the distribution tree and stop forwarding traffic down that branch.
If one receiver on that branch becomes active and requests the multicast traffic, the router will dynamically modify the distribution tree and start forwarding traffic again. Source trees have the advantage of creating the optimal path between the source and the receivers. This advantage guarantees the minimum amount of network latency for forwarding multicast traffic.
However, this optimization comes at a cost. The routers must maintain path information for each source. In a network that has thousands of sources and thousands of groups, this overhead can quickly become a resource issue on the routers. Memory consumption from the size of the multicast routing table is a factor that network designers must take into consideration. Shared trees have the advantage of requiring the minimum amount of state in each router. This advantage lowers the overall memory requirements for a network that only allows shared trees.
The disadvantage of shared trees is that under certain circumstances the paths between the source and receivers might not be the optimal paths, which might introduce some latency in packet delivery. Network designers must carefully consider the placement of the rendezvous point RP when implementing a shared tree-only environment.
In unicast routing, traffic is routed through the network along a single path from the source to the destination host. A unicast router does not consider the source address; it considers only the destination address and how to forward the traffic toward that destination. The router scans through its routing table for the destination address and then forwards a single copy of the unicast packet out the correct interface in the direction of the destination.
In multicast forwarding, the source is sending traffic to an arbitrary group of hosts that are represented by a multicast group address. The multicast router must determine which direction is the upstream direction toward the source and which one is the downstream direction or directions toward the receivers. If there are multiple downstream paths, the router replicates the packet and forwards it down the appropriate downstream paths best unicast route metric --which is not necessarily all paths.
Forwarding multicast traffic away from the source, rather than to the receiver, is called Reverse Path Forwarding RPF. RPF is described in the following section. The router scans through its routing table for the destination network and then forwards a single copy of the unicast packet out the correct interface in the direction of the destination.
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RPF is an algorithm used for forwarding multicast datagrams. Protocol Independent Multicast PIM uses the unicast routing information to create a distribution tree along the reverse path from the receivers towards the source. The multicast routers then forward packets along the distribution tree from the source to the receivers. RPF is a key concept in multicast forwarding. It enables routers to correctly forward multicast traffic down the distribution tree.
RPF makes use of the existing unicast routing table to determine the upstream and downstream neighbors. A router will forward a multicast packet only if it is received on the upstream interface.
This RPF check helps to guarantee that the distribution tree will be loop-free. When a multicast packet arrives at a router, the router performs an RPF check on the packet. If the RPF check succeeds, the packet is forwarded. Otherwise, it is dropped. The router looks up the source address in the unicast routing table to determine if the packet has arrived on the interface that is on the reverse path back to the source.
If the packet has arrived on the interface leading back to the source, the RPF check succeeds and the packet is forwarded out the interfaces present in the outgoing interface list of a multicast routing table entry. As the figure illustrates, a multicast packet from source A check of the unicast route table shows that S1 is the interface this router would use to forward unicast data to Because the packet has arrived on interface S0, the packet is discarded. In this example, the multicast packet has arrived on interface S1.
The router refers to the unicast routing table and finds that S1 is the correct interface. The RPF check passes, and the packet is forwarded. Dense mode fallback describes the event of the PIM mode changing falling back from sparse mode which requires an RP to dense mode which does not use an RP. Dense mode fallback occurs when RP information is lost. If all interfaces are configured with the ip pim sparse-mode command, there is no dense mode fallback because dense mode groups cannot be created over interfaces configured for sparse mode. Routers that lose RP information will fallback into dense mode and any new states that must be created for the failed group will be created in dense mode.
Prior to the introduction of PIM-DM fallback prevention, all multicast groups without a group-to-RP mapping would be treated as dense mode. By default, if all of the interfaces are configured to operate in PIM sparse mode using the ip pim sparse-mode command , there is no need to configure the no ip pim dm-fallback command that is, the PIM-DM fallback behavior is enabled by default.
If any interfaces are not configured using the ip pim sparse-mode command for example, using the ip pim sparse-dense-mode command , then the PIM-DM fallback behavior can be explicit disabled using the no ip pim dm-fallback command. When the no ip pim dm-fallback command is configured or when ip pim sparse-mode is configured on all interfaces, any existing groups running in sparse mode will continue to operate in sparse mode but will use an RP address set to 0. Multicast entries with an RP address set to 0. Before beginning the configuration process, you must decide which PIM mode needs to be used.
This determination is based on the applications you intend to support on your network. In general, if the application is one-to-many or many-to-many in nature, then PIM-SM can be used successfully. For optimal many-to-many application performance, bidirectional PIM is appropriate but hardware support is limited to Cisco devices and the Catalyst series switches with Sup No new or modified standards are supported, and support for existing standards has not been modified.
The Cisco Support and Documentation website provides online resources to download documentation, software, and tools. Use these resources to install and configure the software and to troubleshoot and resolve technical issues with Cisco products and technologies. Access to most tools on the Cisco Support and Documentation website requires a Cisco. The following table provides release information about the feature or features described in this module.
This table lists only the software release that introduced support for a given feature in a given software release train. Unless noted otherwise, subsequent releases of that software release train also support that feature. Preventing the use of dense mode is very important to multicast networks whose reliability is critical. This feature provides a mechanism to keep the multicast groups in sparse mode, thereby preventing dense mode flooding. Supports multicast applications within an enterprise campus.
Also provides an additional integrity in the network with the inclusion of a reliable multicast transport, PGM. It allows the switches to forward multicast traffic to only those ports that are interested in the traffic.