PIM is a multicast routing protocol. We have had many discussion at GroupStudy about IGMP and CGMP. Those protocols allow routers and switches to learn which local ports should receive multicast streams. In an internetwork, there's more to the story, however. The routers must also learn the paths to multicast recipients. PIM is one of many protocols that handle that aspect of multicasting.
There's an excellent paper on multicasting at www.certificationzone.com. It's by Dave Wolfefer who has written many good papers for them. Here's what I had to say about the topic in my book, Top-Down Network Design. PIM works in tandem with IGMP; it also works with a unicast routing protocol, such as OSPF, RIP, Enhanced IGRP, and so on. PIM has two modes: dense mode and sparse mode. Dense-mode PIM is similar to an older dense-mode protocol, the Distance-Vector Multicast Routing Protocol (DVMRP), which is described in RFC 1075 and is a derivative of RIP. Both protocols use a reverse-path forwarding (RPF) mechanism to compute the shortest (reverse) path between a source and all possible recipients of a packet. Dense-mode PIM is simpler than DVMRP, however, because it does not require the computation of routing tables. If a router running dense-mode PIM receives a multicast packet from a source to a group, it first verifies in the standard unicast routing table that the incoming interface is the one that it uses for sending unicast packets toward the source. If this is not the case, it drops the packet and sends back a prune message. If it is the case, the router forwards a copy of the packet on all interfaces for which it has not received a prune message for the source/group destination pair. If there are no such interfaces, it sends back a prune message. The first packet for a group is flooded to all interfaces. Once this has occurred, however, routers listen to prune messages to help them develop a map of the network that lets them send multicast packets only to those networks that should receive the packets. The prune messages also let routers avoid loops that would cause more than one router to send a multicast packet to a segment. Dense-mode PIM works best in environments with large multicast groups and a high likelihood that any given LAN has a group member, which limits the router's need to send prune messages. Because of the flooding of the first packet for a group, dense-mode does not make sense in environments where a few sparsely-located users wish to participate in a multicast application. In this case, sparse-mode PIM, which is described in the next section, is a better solution. Sparse-mode PIM is quite different than dense-mode PIM. Rather than allowing traffic to be sent everywhere and then pruned back where it is not needed, sparse-mode PIM defines a rendezvous point. The rendezvous point provides a registration service for a multicast group. Sparse-mode PIM relies on IGMP, which lets a host join a group by sending a membership-report message, and detach from a group by sending a leave message. A designated router for a network segment tracks membership-report and leave messages on its segment, and periodically sends join and prune PIM messages to the rendezvous point. The join and prune messages are processed by all the routers between the designated router and the rendezvous point. The result is a distribution tree that reaches all group members and is centered at the rendezvous point. When a source initially sends data to a group, the designated router on the source's network unicasts register messages to the rendezvous point with the source's data packets encapsulated within. If the data rate is high, the rendezvous point can send join/prune messages back towards the source. This enables the source's data packets to follow a source-specific shortest-path tree, and eliminates the need for the packets to be encapsulated in register messages. Whether the packets arrive encapsulated or not, the rendezvous point forwards the source's decapsulated data packets down the distribution tree toward group members. Priscilla At 11:03 PM 2/17/02, kaushalender wrote: >Hello group, > >Can somebody tell me what is pim.how it works.Plz give the information > >Thanx in advance >kaushalender ________________________ Priscilla Oppenheimer http://www.priscilla.com Message Posted at: http://www.groupstudy.com/form/read.php?f=7&i=35749&t=35702 -------------------------------------------------- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
