Cisco Catalyst 4500 Series - IP Multicast Tables

38-7
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 route (*,224.1.2.3) is loaded in the hardware FIB table and the list of output interfaces is loaded into
the MET. A pointer to the list of output interfaces, the MET index, and the RPF interface are also loaded
in the hardware FIB with the (*,224.1.2.3) route. With this information loaded in hardware, merging of
the Layer 2 information can begin. For the output interfaces on VLAN1, the Integrated Switching Engine
must send the packet to all switch ports in VLAN1 that are in the spanning tree forwarding state. The
same process applies to VLAN 2. To determine the set of switch ports in VLAN 2, the Layer 2
Forwarding Table is used.
When the hardware routes a packet, in addition to sending it to all of the switch ports on all output
interfaces, the hardware also sends the packet to all switch ports (other than the one it arrived on) in the
input VLAN. For example, assume that VLAN 3 has two switch ports in it, Gig 3/1 and Gig 3/2. If a host
on Gig 3/1 sends a multicast packet, the host on Gig 3/2 might also need to receive the packet. To send
a multicast packet to the host on Gig 3/2, all of the switch ports in the ingress VLAN must be added to
the port set that is loaded in the MET.
If VLAN 1 contains 1/1 and 1/2, VLAN 2 contains 2/1 and 2/2, and VLAN 3 contains 3/1 and 3/2, the
MET chain for this route would contain these switch ports: (1/1,1/2,2/1,2/2,3/1, and 3/2).
If IGMP snooping is on, the packet should not be forwarded to all output switch ports on VLAN 2. The
packet should be forwarded only to switch ports where IGMP snooping has determined that there is
either a group member or router. For example, if VLAN 1 had IGMP snooping enabled, and IGMP
snooping determined that only port 1/2 had a group member on it, then the MET chain would contain
these switch ports: (1/1,1/2, 2/1, 2/2, 3/1, and 3/2).
IP Multicast Tables
Figure 38-4 shows some key data structures that the Catalyst 4500 series switch uses to forward IP
multicast packets in hardware.