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Software Configuration Guide—Release 12.2(25)SG
OL-7659-03
Chapter24 Understanding and Conf iguring IP Multicast
Overview of IP Multicast
Hardware routes occur when the Integrated Switching Engine hardware forwards all replicas of a packet.
Software routes occur when the CPU subsystem softwa re 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. In this case, 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 for warded 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.
Non-Reverse Path Forwarding Traffic
Traffic that fails an Reverse Path Forwarding (RPF) check is called non-RPF traffic. Non-RPF traffic is
forwarded by the Integrated Switching Engine by filtering (persistently dropping) or rate limiting the
non-RPF traffic.
In a redundant configuration where multiple Layer 3 switches o r routers connect to the same LAN
segment, only one device forwards the multicast traffic from the source to the receivers on the outgoing
interfaces. Figure2 4-6 shows how Non-RPF traffic can occur in a common network configuration.