IP Multicast Overview
PIM uses the concept of a designated router (DR). The DR is responsible for sending Internet Group Management Protocol (IGMP) Host-Query messages, PIM Register messages on behalf of sender hosts, and Join messages on behalf of member hosts.
PIM Dense Mode
PIM Dense Mode (PIM-DM) is a protocol that floods multicast packets to every PIM enabled interface on every router in the network. Because it is difficult to scale and has a propensity to stress network performance, dense mode is not optimal for most multicast applications and, therefore, not recommended.
PIM Sparse Mode
PIM Sparse Mode (PIM-SM) Version 2 is a more effective multicasting protocol than PIM-DM. PIM-SM assumes that no one on the network wants a multicast stream unless they request it via IGMP. In a PIM-SM environment, RPs act as matchmakers, matching sources to receivers. With PIM-SM, the tree is rooted at the RP not the source. When a match is established, the receiver joins the multicast distribution tree. Packets are replicated and sent down the multicast distribution tree toward the receivers.
Sparse mode's ability to replicate information at each branching transit path eliminates the need to flood router interfaces with unnecessary traffic or to clog the network with multiple copies of the same data. As a result, sparse mode is highly scalable across an enterprise network and is the multicast routing protocol of choice in the enterprise.
In a many-to-many deployment (many sources to many receivers), Bidir PIM is the recommended forwarding mode. Bidir PIM is outside the scope of this document. For more information on Bidir PIM, see the IP Multicast Technology Overview white paper located at: http://www.cisco.com/en/US/tech/tk648/tk363/technologies_white_paper09186a00800d6b5e.shtml
The memory impact on the router occurs when the router has to carry (*,G) state, which is the indication that a receiver has signaled an IGMP join, and (S,G), which is the indication that the Source is sending to the Group. The RP and any other router between the RP and the source are required to carry both state entries.
The default behavior of PIM-SM is to perform a SPT-switchover. By default, all routers will carry both states. The spt-threshold infinity command, described in Chapter 2, “IP Multicast in a Campus Network”, can be used to control the state.
When deciding which routers should be used as RPs, use the following to determine the memory impact on the router:
Each (*,G) entry requires 380 bytes + outgoing interface list (OIL) overhead. Each (S,G) entry requires 220 bytes + outgoing interface list overhead. The outgoing interface list overhead is 150 bytes per OIL entry.
Cisco AVVID Network Infrastructure IP Multicast Design