Cisco Catalyst 4500 Series, Software Configuration Manual

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Catalyst 4500 Series Switch, Cisco IOS Software Configuration Guide - Cisco IOS XE 3.9.xE and IOS 15.2(5)Ex
Chapter 38 Configuring IP Multicast
About IP Multicast
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.
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 or routers connect to the same LAN
segment, only one device forwards the multicast traffic from the source to the receivers on the outgoing
interfaces.
Figure 38-6 shows how non-RPF traffic can occur in a common network configuration.