Cisco Catalyst 3560-X Series, Software Configuration Manual

Page: 1260 / 1538

1-4
Catalyst 3750-X and 3560-X Switch Software Configuration Guide
OL-25303-03
Chapter 1 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 maintaining
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
PIM Versions
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 opposed to an interface. We strongly
recommend sparse-dense mode, as opposed to either sparse mode or dense mode only.
•
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 sent by a border router or a designated router.
• PIM packets are no longer inside IGMP packets; they are standalone packets.
PIM Modes
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
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.