Tag Archives: 3.2b

3.2.b Implement and troubleshoot IPv4 protocol independent multicast

3.2.b [i] PIM dense mode, sparse mode, sparse-dense mode

PIM can operate in dense mode (DM), sparse mode (SM), or in sparse-dense mode (PIM DM-SM), which handles both sparse groups and dense groups at the same time.

PIM DM builds source-based multicast distribution trees. In dense mode, a PIM DM router or multilayer switch assumes that all other routers or multilayer switches forward multicast packets for a group. If a PIM DM device receives a multicast packet and has no directly connected members or PIM neighbors present, a prune message is sent back to the source to stop unwanted multicast traffic. Subsequent multicast packets are not flooded to this router or switch on this pruned branch because branches without receivers are pruned from the distribution tree, leaving only branches that contain receivers. When a new receiver on a previously pruned branch of the tree joins a multicast group, the PIM DM device detects the new receiver and immediately sends a graft message up the distribution tree toward the source. When the upstream PIM DM device receives the graft message, it immediately puts the interface on which the graft was received into the forwarding state so that the multicast traffic begins flowing to the receiver.

PIM-SM uses shared trees and shortest-path-trees (SPTs) to distribute multicast traffic to multicast receivers in the network. In PIM-SM, a router or multilayer switch assumes that other routers or switches do not forward multicast packets for a group, unless there is an explicit request for the traffic (join message). When a host joins a multicast group using IGMP, its directly connected PIM-SM device sends PIM join messages toward the root , also known as the RP. This join message travels router-by-router toward the root, constructing a branch of the shared tree as it goes. The RP keeps track of multicast receivers. It also registers sources through register messages received from the source’s first-hop router (designated router [DR]) to complete the shared tree path from the source to the receiver.

When using a shared tree, sources must send their traffic to the RP so that the traffic reaches all receivers.

Prune messages are sent up the distribution tree to prune multicast group traffic. This action permits branches of the shared tree or SPT that were created with explicit join messages to be torn down when they are no longer needed.

When the number of PIM-enabled interfaces exceeds the hardware capacity and PIM-SM is enabled with the SPT threshold is set to infinity, the switch does not create (S, G) entries in the multicast routing table for some directly connected interfaces if they are not already in the table. The switch might not correctly forward traffic from these interfaces.

Adam, Paul (2014-07-12). All-in-One CCIE V5 Written Exam Guide (Kindle Locations 2454-2475).  . Kindle Edition.


3.2.b Implement and troubleshoot IPv4 protocol independent multicast

3.2.b [ii] Static RP, auto-RP, BSR

A rendezvous point (RP) is required only in networks running PIM sparse mode (PIM-SM). An RP acts as the meeting place (rendezvous) for sources and receivers of multicast data. In a PIM-SM network, sources must send their traffic to the RP. This traffic is then forwarded to receivers down a shared distribution tree . By default, when the first hop router 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.

Following the introduction of PIM-SM v1, Cisco implemented a version of PIM-SM with the Auto-RP feature. Auto-RP automates the distribution of group-to-RP mappings in a PIM network. To make Auto-RP work, a router must be designated as an RP mapping agent, which receives the RP announcement messages from the RPs and arbitrates conflicts. The RP mapping agent then sends the consistent group -to-RP mappings to all other routers by dense mode flooding. Thus, all routers automatically discover which RP to use for the groups they support. The Internet Assigned Numbers Authority (IANA) has assigned two group addresses, and, for Auto-RP. 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. Another advantage of Auto-RP is that it offers the ability to scope the RP address within a domain. Scoping can be achieved by defining the time-to-live (TTL) value allowed for the Auto-RP advertisements.

Another RP selection model called bootstrap router (BSR) was introduced after Auto-RP in PIM-SM v2. BSR performs similarly to Auto-RP in that it uses candidate routers for the RP function and for relaying the RP information for a group. RP information is distributed through BSR messages, which are carried within PIM messages. PIM messages are link-local multicast messages that travel from PIM router to PIM router. Because of this single hop method of disseminating RP information, TTL scoping cannot be used with BSR. 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. Each method for configuring an RP has its own strengths , weaknesses, and level of complexity.

Adam, Paul (2014-07-12). All-in-One CCIE V5 Written Exam Guide (Kindle Locations 2492-2494).  . Kindle Edition.



3.2.b Implement and troubleshoot IPv4 protocol independent multicast

3.2.b [iii] BiDirectional PIM

Bidir-PIM is a variant of the Protocol Independent Multicast (PIM) suite of routing protocols for IP multicast and is an extension of the existing PIM sparse mode (PIM-SM) feature. Bidir-PIM resolves some limitations of PIM-SM for groups with a large number of sources.

Membership to a bidirectional group is signaled 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. Bidir-PIM is designed to be used for many-to-many applications within individual PIM  domains. Multicast groups in bidirectional mode can scale to an arbitrary number of sources without incurring overhead due to the number of sources.

Bidir-PIM is derived from the mechanisms of PIM-SM and shares many shortest-path tree (SPT) operations. Bidir-PIM also has unconditional forwarding of source traffic toward the RP upstream on the shared tree, but no registering process for sources as in PIM-SM. These modifications are necessary and sufficient to allow forwarding of traffic in all routers solely based on the (*, G) multicast routing entries. This feature eliminates any source-specific state and allows scaling capability to an arbitrary number of sources.

Adam, Paul (2014-07-12). All-in-One CCIE V5 Written Exam Guide (Kindle Locations 2502-2507).  . Kindle Edition.



3.2.b Implement and troubleshoot IPv4 protocol independent multicast

3.2.b [iv] Source-specific multicast

The Source Specific Multicast (SSM) feature is an extension of IP multicast where datagram traffic is forwarded to receivers from only those multicast sources to which the receivers have explicitly joined. For multicast groups configured for SSM, only source-specific multicast distribution trees (no shared trees) are created.

PIM-SSM is the routing protocol that supports the implementation of SSM and is derived from PIM sparse mode (PIM-SM). IGMPv3 supports source filtering, which is required for SSM . To run SSM with IGMPv3, SSM must be supported in the Cisco IOS router, the host where the application is running, and the application itself. IGMP v3lite and URD are two Cisco-developed transition solutions that enable the immediate development and deployment of SSM services, without the need to wait for the availability of full IGMPv3 support in host operating systems and SSM receiver applications. IGMP v3lite is a solution for application developers that allows immediate development of SSM receiver applications switching to IGMPv3 as soon as it becomes available . URD is a solution for content providers and content aggregators that enables them to deploy receiver applications that are not yet SSM enabled (through support for IGMPv3). IGMPv3, IGMP v3lite, and URD interoperate with each other , so that both IGMP v3lite and URD can easily be used as transitional solutions toward full IGMPv3 support in hosts.

Adam, Paul (2014-07-12). All-in-One CCIE V5 Written Exam Guide (Kindle Locations 2512-2521).  . Kindle Edition.




3.2.b Implement and troubleshoot IPv4 protocol independent multicast

3.2.b [v] Group to RP mapping

There are various mechanisms for disseminating RP information across a multicast network. Each mechanism has different pros and cons.

Adam, Paul (2014-07-12). All-in-One CCIE V5 Written Exam Guide (Kindle Locations 2524-2525).  . Kindle Edition.