Cisco 4500M Software Manual page 332

Overview of IP Multicast
The Catalyst 4500 series switch performs Layer 3 routing and Layer 2 bridging at the same time. There
can be multiple Layer 2 switchports on any VLAN interface. To determine the set of output switchports
on which to forward a multicast packet, the Supervisor Engine III combines Layer 3 MFIB information
with Layer 2 forwarding information and stores it in the hardware MET for packet replication.
Figure 25-3
routing, multicast routing, and Layer 2 bridging information to forward in hardware.
Figure 25-3 Combining CEF, MFIB, and Layer 2 Forwarding Information in Hardware
EIGRP / OSPF
Protocols
Unicast
Software
Unicast Routing
Table
Tables
CEF
Hardware
H/W Adjacency
Table
Tables
Like the CEF unicast routes, the MFIB routes are Layer 3 and must be merged with the appropriate
Layer 2 information. The following example shows an MFIB route:
(*,224.1.2.3)
RPF interface is Vlan3
Output Interfaces are:
Vlan 1
Vlan 2
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 switchports in VLAN1 that are in the spanning tree forwarding state. The
same process applies to VLAN 2. To determine the set of switchports 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 switchports on all output
interfaces, the hardware also sends the packet to all switchports (other than the one it arrived on) in the
input VLAN. For example, assume that VLAN 3 has two switchports 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 switchports in the ingress VLAN must be added to
the portset that is loaded in the MET.
Software Configuration Guide—Release 12.2(25)EW
25-6
shows a functional overview of how the Catalyst 4500 series switch combines unicast
PIM / IGMP
Multicast
Multicast Routing
Table
MFIB
H/W FIB
Table
Chapter 25 Understanding and Configuring IP Multicast
IGMP Snooping
Spanning Tree
L2 Multicast
Layer 2 Forwarding
Table
CEF – MFIB Subsystem
MET Replication
Table
CPU
Subsystem
Software
Integrated
Switching
Engine
OL-6696-01