Cisco Catalyst 3120 Software Manual page 1002

Understanding Cisco's Implementation of IP Multicast Routing
With multicasting, the source is sending traffic to an arbitrary group of hosts represented by a multicast
group address in the destination address field of the IP packet. To decide whether to forward or drop an
incoming multicast packet, the router or multilayer switch uses a reverse path forwarding (RPF) check
on the packet as follows and shown in
The router or multilayer switch examines the source address of the arriving multicast packet to
1.
decide whether the packet arrived on an interface that is on the reverse path back to the source.
If the packet arrives on the interface leading back to the source, the RPF check is successful and the
2.
packet is forwarded to all interfaces in the outgoing interface list (which might not be all interfaces
on the router).
If the RPF check fails, the packet is discarded.
3.
Some multicast routing protocols, such as DVMRP, 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 44-3
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 44-3
Multicast
packet from
source 151.10.3.21
is forwarded.
Gigabit Ethernet 0/1
Gigabit Ethernet 0/3
Table 44-1
Network
151.10.0.0/16
198.14.32.0/32
204.1.16.0/24
PIM uses both source trees and RP-rooted shared trees to forward datagrams (described in the
DM" section on page 44-4
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).
Cisco Catalyst Blade Switch 3120 for HP Software Configuration Guide
44-8
shows port 2 receiving a multicast packet from source 151.10.3.21.
RPF Check
Gigabit Ethernet 0/2
Layer 3 switch
Gigabit Ethernet 0/4
Routing Table Example for an RPF Check
Port
Gigabit Ethernet 1/0/1
Gigabit Ethernet 1/0/3
Gigabit Ethernet 1/0/4
and the
Figure 44-3:
Multicast
packet from
source 151.10.3.21
packet is discarded.
"PIM-SM" section on page
Chapter 44 Configuring IP Multicast Routing
Table 44-1
44-5). The RPF check is performed
shows that
"PIM
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