Cisco Catalyst 4500 Series - Page 776

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Software Configuration Guide—Release IOS XE 3.3.0SG and IOS 15.1(1)SG
OL-25340-01
Chapter 35 Configuring IP Multicast
About IP Multicast
protocol-specific information and leaves only the essential forwarding information. Each entry in the
MFIB table consists of an (S,G) or (*,G) route, an input RPF VLAN, and a list of Layer 3 output
interfaces. The MFIB subsystem, together with platform-dependent management software, loads this
multicast routing information into the hardware FIB and Replica Expansion Table (RET).
The Catalyst 4500 series switch performs Layer 3 routing and Layer 2 bridging at the same time. There
can be multiple Layer 2 switch ports on any VLAN interface. To determine the set of output switch ports
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 35-3 shows a functional overview of how the Catalyst 4500 series switch combines unicast
routing, multicast routing, and Layer 2 bridging information to forward in hardware.
Figure 35-3 Combining CEF, MFIB, and Layer 2 Forwarding Information in Hardware
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 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
Unicast Routing
Table
Multicast Routing
Table
Layer 2 Forwarding
Table
H/W FIB
Table
H/W Adjacency
Table
MET Replication
Table
Protocols
Hardware
Tables
Software
Tables
Unicast Multicast
CEF – MFIB Subsystem
CPU
Subsystem
Software
Integrated
Switching
Engine
L2 Multicast
OSPF PIM / IGMP
IGMP Snooping
Spanning Tree
CEF MFIB
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