Foundation Summary

The Foundation Summary provides a convenient review of many key concepts in this chapter. If you are already comfortable with the topics in this chapter, this summary might help you recall a few details. If you just read this chapter, this review should help solidify some key facts. If you are doing your final prep before the exam, the following lists and tables are a convenient way to review the day before the exam.

Multicast routing protocols include the following. Note that Cisco fully supports only PIM.

  • Multicast OSPF (MOSPF)

  • Distance Vector Multicast Routing Protocol (DVMRP)

  • Center-Based Trees

  • Core-Based Trees

  • Protocol Independent Multicast (PIM)

RPF checks to ensure traffic flows away from the root.

The two types of distribution trees are

  • Shared

  • Source-rooted

Multicast routing protocols are described as either sparse or dense:

  • A dense multicast routing protocol assumes that all hosts on all links are interested in the traffic and so floods multicasts out all paths, only to have some paths ask not to receive the traffic (also called "bush out, prune back").

  • Sparse multicast routing protocols assume that no one wants the traffic until they ask for it.

The PIM modes are as follows:

  • PIM dense mode— Assumes that a multicast group's recipients are located on every subnet.

  • PIM sparse mode— The multicast tree is not extended to a router unless a host there already has joined the group.

  • PIM sparse-dense mode— Cisco hybrid that allows a PIM router to use sparse or dense mode on a per-group basis. If a group has an RP defined, sparse mode is used; otherwise, dense mode is used.

The PIM versions are

  • PIMv1— Uses auto-RP

  • PIMv2— Uses bootstrap routers

The three steps to set up IP multicast routing on a Cisco router are as follows:

Step 1.
Enable multicast routing with the following command:
Router(config)#ip multicast-routing

Step 2.
Turn on PIM in appropriate mode on selected interfaces:

Router(config-if)#ip pim {dense-mode | sparse-dense-mode | sparse-mode}

Step 3.
Set up RPs:

Router(config)#ip pim send-rp-announce {interface type} {scope ttl} {group-list acl}
Router(config)#ip pim send-rp-discovery {interface type} scope {ttl}


The principal commands used to understand a multicast configuration are shown in Table 19-2.

Table 19-2. Multicast show Commands
show ip mroute [group-address] [summary] [count][active kpbs]Shows multicast routing tables
show ip pim interfaceShows settings on the interfaces
show ip pim neighborShows a list of neighbors
show ip pim rp [group-name |group-address | mapping]Allows you to inspect the configured RPs
show ip pim rpDisplays RPs for active groups
show ip pim rp mappingsMaps multicast IP addresses to RPs
show ip rpfAllows you to inspect reverse path forwarding information for your traffic