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Cisco ME 3400 Ethernet Access Switch Software Configuration Guide
78-17058-01
Chapter 34 Configuring IP Multicast Routing
Understanding Cisco’s Implementation of IP Multicast Routing
Some multicast routing protocols maintain a separate multicast routing table and use it for the RPF
check. However, PIM uses the unicast routing table to perform the RPF check.
Figure 34-1 shows port 2 receiving a multicast packet from source 151.10.3.21. Table 34-1 shows that
the port on the reverse path to the source is port 1, not port 2. Because the RPF check fails, the multilayer
switch discards the packet. Another multicast packet from source 151.10.3.21 is received on port 1, and
the routing table shows this port is on the reverse path to the source. Because the RPF check passes, the
switch forwards the packet to all port in the outgoing port list.
Figure 34-1 RPF Check
PIM uses both source trees and RP-rooted shared trees to forward datagrams (described in the “PIM
DM” section on page 34-4 and the “PIM SM” section on page 34-4). 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.
Dense-mode PIM uses only source trees and use RPF as previously described.
Table 34-1 Routing Table Example for an RPF Check
Network Port
151.10.0.0/16 Gigabit Ethernet 0/1
198.14.32.0/32 Fast Ethernet 0/1
204.1.16.0/24 Fast Ethernet 0/2
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
Fast Ethernet 0/1
Fast Ethernet 0/2
Gigabit Ethernet 0/2
141002
Layer 3 switch