|Email This Page
PIM-DM: Protocol Independent Multicast - Dense Mode
Protocol Independent Multicast (PIM) has two modes: Sparse Mode and Dense Mode. We focus on the Dense Mode in this document.
PIM-DM is mainly designed for multicast LAN applications, while the PIM-SM is for wide area, inter-domain network. PIM-DM implements the same flood-and-prune mechanism that Distance Vector Multicast Routing Protocol (DVMRP) and other dense mode routing protocols employ. The main difference between DVMRP and PIM-DM is that PIM-DM introduces the concept of protocol independence. PIM-DM can use the routing table populated by any underlying unicast routing protocol to perform reverse path forwarding (RPF) checks.
ISPs typically appreciate the ability to use any underlying unicast routing protocol with PIM-DM because they need not introduce and manage a separate routing protocol just for RPF checks. Unicast routing protocols extended as Multiprotocol Extensions to BGP (MBGP) and Multitopology Routing for IS-IS (M-ISIS) were later employed to build special tables to perform RPF checks, but PIM-DM does not require them.
PIM-DM can use the unicast routing table populated by OSPF, IS-IS, BGP, and so on, or PIM-DM can be configured to use a special multicast RPF table populated by MBGP or M-ISIS when performing RPF checks.
Protocol Structure - PIM-DM: Protocol Independent Multicast - Dense Mode
Reserved (Address length)
PIM version - The current PIM version is 2.
- Type -- Types for specific PIM messages.
- Address length -- Address length in bytes. The length of the address field throughout, in the specific message.
Reserved (the value of this field is set to 0, ignore on receipt)
- Checksum - The 16-bit one - complement, of the one's complement sum of the entire PIM message.
IP , PIM-SM, ICMP, OSPF , RIP , DVMRP, IS-IS, BGP , IGRP , EIGRP
PGM is circulated by IETF (http://www.ietf.org ) as an experimental protocol.