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Software Configuration Guide—Release IOS XE 3.3.0SG and IOS 15.1(1)SG
OL-25340-01
Chapter 35 Configuring IP Multicast
About IP Multicast
Replication is a particular type of forwarding where, instead of sending out one copy of the packet, the
packet is replicated and multiple copies of the packet are sent out. At Layer 3, replication occurs only
for multicast packets; unicast packets are never replicated to multiple Layer 3 interfaces. In IP
multicasting, for each incoming IP multicast packet that is received, many replicas of the packet are sent
out.
IP multicast packets can be transmitted on the following types of routes:
• Hardware routes
• Software routes
• Partial routes
Hardware routes occur when the integrated switching engine hardware forwards all replicas of a packet.
Software routes occur when the CPU subsystem software forwards all replicas of a packet. Partial routes
occur when the integrated switching engine forwards some of the replicas in hardware and the CPU
subsystem forwards some of the replicas in software.
Partial Routes
Note The conditions listed below cause the replicas to be forwarded by the CPU subsystem software, but the
performance of the replicas that are forwarded in hardware is not affected.
The following conditions cause some replicas of a packet for a route to be forwarded by the CPU
subsystem:
• The switch is configured with the ip igmp join-group command as a member of the IP multicast
group on the RPF interface of the multicast source.
• The switch is the first-hop to the source in PIM sparse mode. The switch must send PIM-register
messages to the RP.
Software Routes
Note If any one of the following conditions is configured on the RPF interface or the output interface, all
replication of the output is performed in software.
The following conditions cause all replicas of a packet for a route to be forwarded by the CPU subsystem
software:
• The interface is configured with multicast helper.
• The interface is a generic routing encapsulation (GRE) or Distance Vector Multicast Routing
Protocol (DVMRP) tunnel.
• The interface uses non-Advanced Research Products Agency (ARPA) encapsulation.
The following packets are always forwarded in software:
• Packets sent to multicast groups that fall into the range 224.0.0.* (where * is in the range from 0 to
255). This range is used by routing protocols. Layer 3 switching supports all other multicast group
addresses.
• Packets with IP options.
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