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Catalyst 3750-E and 3560-E Switch Software Configuration Guide
OL-9775-02
Chapter 42 Configuring IP Multicast Routing
Understanding Cisco’s Implementation of IP Multicast Routing
Figure 42-3 RPF Check
PIM uses both source trees and RP-rooted shared trees to forward datagrams (described in the “PIM
DM” section on page 42-4 and the “PIM-SM” section on page 42-5). The RPF check is performed
differently for each:
• If a PIM router or multilayer switch has a source-tree state (that is, an (S,G) entry is present in the
multicast routing table), it performs the RPF check against the IP address of the source of the
multicast packet.
• If a PIM router or multilayer switch has a shared-tree state (and no explicit source-tree state), it
performs the RPF check on the RP address (which is known when members join the group).
Sparse-mode PIM uses the RPF lookup function to decide where it needs to send joins and prunes:
• (S,G) joins (which are source-tree states) are sent toward the source.
• (*,G) joins (which are shared-tree states) are sent toward the RP.
DVMRP and dense-mode PIM use only source trees and use RPF as previously described.
Understanding DVMRP
DVMRP is implemented in the equipment of many vendors and is based on the public-domain mrouted
program. This protocol has been deployed in the MBONE and in other intradomain multicast networks.
Cisco routers and multilayer switches run PIM and can forward multicast packets to and receive from a
DVMRP neighbor. It is also possible to propagate DVMRP routes into and through a PIM cloud. The
software propagates DVMRP routes and builds a separate database for these routes on each router and
multilayer switch, but PIM uses this routing information to make the packet-forwarding decision. The
software does not implement the complete DVMRP. However, it supports dynamic discovery of
DVMRP routers and can interoperate with them over traditional media (such as Ethernet and FDDI) or
over DVMRP-specific tunnels.
Table 42-1 Routing Table Example for an RPF Check
Network Port
151.10.0.0/16 Gigabit Ethernet 1/0/1
198.14.32.0/32 Gigabit Ethernet 1/0/3
204.1.16.0/24 Gigabit Ethernet 1/0/4
Multicast
packet from
source 151.10.3.21
is forwarded.
Multicast
packet from
source 151.10.3.21
packet is discarded.
Gigabit Ethernet 0/1
Gigabit Ethernet 0/3
Gigabit Ethernet 0/4
Gigabit Ethernet 0/2
101242
Layer 3 switch