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24-7
Software Configuration Guide—Release 12.2(25)SG
OL-7659-03
Chapter 24 Understanding and Configuring IP Multicast
Overview of IP Multicast
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 switchports: (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 switchports on VLAN 2. The
packet should be forwarded only to switchports 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
switchports: (1/1,1/2, 2/1, 2/2, 3/1, and 3/2).
IP Multicast Tables
Figure 24-4 shows some key data structures that the Catalyst 4500 series switch uses to forward IP
multicast packets in hardware.
Figure 24-4 IP Multicast Tables and Protocols
The Integrated Switching Engine maintains the hardware FIB table to identify individual IP multicast
routes. Each entry consists of a destination group IP address and an optional source IP address. Multicast
traffic flows on primarily two types of routes: (S,G) and (*,G). The (S,G) routes flow from a source to a
group based on the IP address of the multicast source and the IP address of the multicast group
destination. Traffic on a (*,G) route flows from the PIM RP to all receivers of group G. Only
sparse-mode groups use (*,G) routes. The Integrated Switching Engine hardware contains space for a
total of 128,000 routes, which are shared by unicast routes, multicast routes, and multicast fast-drop
entries.
68135
Integrated Switching Engine CPU Subsystem
Routing Protocols
L3 Protocols
•PIM
•IGMP
L2 Protocols
•
•
IGMP
snooping
Spanning tree
Hardware FIB Table
(S,G), RPF Vlan, MET Index
(*,G), Interfaces, MET Index
Hardware Tables
Hardware MET Table
(Replication)
Index, set of {Vlan, switchport}
Software Tables
Multicast Routing
Table
• (S,G), RPF
output interfaces
L2 Forwarding
Table
Vlan, MAC address
S,G rpf interface met index
S1, G1 vlan 3 1219
S2, G2 vlan 7 1241
*, G4 vlan 99 1356
index set of {vlan, switchport}
0
..
..
..
..
..
1279
{vlan3, Fa 3/1} {vlan5, Fa 3/2}, …
switchports
interface, set of