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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
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 Figure44-3:
1. The router or multilayer switch examines the source address of the arriving multicast packet to
decide whether the packet arrived on an interface that is on the reverse path back to the source.
2. If the packet arrives on the interface leading back to the source, the RPF chec k is su ccessfu l and the
packet is forwarded to all interfaces in the outgoing interface list (which might not be all interfaces
on the router).
3. If the RPF check fails, the packet is discarded.
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 shows port 2 receiving a multicast packet from source 151.10.3.21. Table 44-1 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 r eceived 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 RPF Check
PIM uses both source trees and RP-rooted shared trees to forward datagrams (described in the “PIM
DM” section on page 44-4 and the “PIM-SM” section on page 44-5). The RPF chec k i s p erfor med
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).
Table44-1 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
s
ource 151.10.3.21
is forwarded.
Multicast
packet from
source 151.10.3.2
1
packet is discarded
.
Gigabit Ethernet 0/1
Gigabit Ethernet 0/3 Gigabit Ethernet 0/4
Gigabit Ethernet 0/2
1
01242
Layer 3 switch