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Cisco ME 3800X and 3600X Switch Software Configuration Guide
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Chapter 33 Configuring IP Multicast Routing
Understanding Cisco’s Implementation of IP Multicast Routing
domain. These messages travel hop-by-hop through the network to all routers and switches, which store
the RP information in the BSR message in their local RP cache. The routers and switches select the same
RP for a given group because they all use a common RP hashing algorithm.
Multicast Forwarding and Reverse Path Check
With unicast routing, routers and multilayer switches forward traffic through the network along a single
path from the source to the destination host whose IP address appears in the destination address field of
the IP packet. Each router and switch along the way makes a unicast forwarding decision, using the
destination IP address in the packet, by looking up the destination address in the unicast routing table
and forwarding the packet through the specified interface to the next hop toward the destination.
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
Figure 33-2
:
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 check is successful 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 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 33-2
shows port 2 receiving a multicast packet from source 151.10.3.21.
Table 33-1
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 33-2
RPF Check
Table 33-1
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
