Cisco Systems 3130 Understanding PIM

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Cisco Catalyst Blade Switch 3130 for Dell Software Configuration Guide

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Chapter44 Configuring IP Multicast Routing

Understanding Cisco’s Implementation of IP Multicast Routing

Understanding PIM

PIM is called protocol-independent: regardless of the unicast routing protocols used to populate the

unicast routing table, PIM uses this information to perform multicast forwarding instead of maintainin g

a separate multicast routing table.

PIM is defined in RFC 2362, Protocol-Independent Multicast-Sparse Mode (PIM-SM): Protocol

Specification. PIM is defined in these Internet Engineering Task Force (IETF) Internet drafts:

•Protocol Independent Multicast (PIM): Motivation and Architecture

•Protocol Independent Multicast (PIM), Dense Mode Protocol Specification

•Protocol Independent Multicast (PIM), Sparse Mode Protocol Specification

•draft-ietf-idmr-igmp-v2-06.txt, Internet Group Management Protocol, Version 2

•draft-ietf-pim-v2-dm-03.txt, PIM Version 2 Dense Mode

PIMv2 includes these improvements over PIMv1:

•A single, active rendezvous point (RP) exists per multicast group, with multiple backup RPs. This

single RP compares to multiple active RPs for the same group in PIMv1.

•A bootstrap router (BSR) provides a fault-tolerant, automated RP discovery and distribution

mechanism that enables routers and multilayer switches to dynamically learn the group-to-RP

mappings.

•Sparse mode and dense mode are properties of a group, as opp ose d to a n in terfa ce . We strongly

recommend sparse-dense mode, as opposed to either sparse mod e or de nse m od e onl y.

•PIM join and prune messages have more flexible encoding for multiple address families.

•A more flexible hello packet format replaces the query packet to encode current and future

capability options.

•Register messages to an RP specify whether they are s ent by a b ord er ro ut er or a d es ig na ted r oute r.

•PIM packets are no longer inside IGMP packets; they are standalone packets.

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.