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Understanding How IP Multicast Layer 3 Switching Works
The PFC rewrites the packet as follows:
The result is a rewritten IP multicast packet that appears to have been routed. The PFC replicates the
rewritten packet onto the appropriate destination VLANs, where it is forwarded to members of IP
multicast group G1.
After the PFC performs the packet rewrite, the packet is formatted (conceptually) as follows:
Frame Header
Destination
Source
Group G1 MAC
MSFC MAC

Partially and Completely Switched Flows

When at least one outgoing Layer 3 interface for a given flow is multilayer switched and at least one
outgoing interface is not multilayer switched, that flow is considered partially switched. When a partially
switched flow is created, all multicast traffic belonging to that flow still reaches the MSFC and is
software forwarded on those outgoing interfaces that are not multilayer switched.
These sections describe partially and completely switched flow:
Partially Switched Flows with PFC1 or PFC2
If your system has a PFC1 or PFC2 installed, a flow might be partially switched instead of completely
switched in these situations:
Catalyst 6500 Series Switch Cisco IOS Software Configuration Guide—Release 12.1 E
18-4
Changes the source MAC address in the Layer 2 frame header from the MAC address of the host to
the MAC address of the MSFC (this MAC address is stored in the multicast Layer 3 switching cache
entry for the flow)
Decrements the IP header Time to Live (TTL) by one and recalculates the IP header checksum
IP Header
Destination
Group G1 IP
Partially Switched Flows with PFC1 or PFC2, page 18-4
Partially Switched Flows with PFC2, page 18-5
Completely Switched Flows, page 18-5
The switch is configured as a member of the IP multicast group (using the ip igmp join-group
command) on the RPF interface of the multicast source.
During the registering state if the switch is the first-hop router to the source in PIM sparse mode (in
this case, the switch must send PIM-register messages to the rendezvous point [RP]).
The multicast TTL threshold is configured on an outgoing interface for the flow (using the ip
multicast ttl-threshold command).
The multicast helper is configured on the RPF interface for the flow, and multicast to broadcast
translation is required.
The outgoing interface is a generic routing encapsulation (GRE) Distance Vector Multicast Routing
Protocol (DVMRP) tunnel interface.
The maximum transmission unit (MTU) of the RPF interface is greater than the MTU of any outgoing
interface.
If Network Address Translation (NAT) is configured on an interface, and source address translation
is required for the outgoing interface.
Chapter 18
Configuring IP Multicast Layer 3 Switching
Source
TTL
Source A IP
n–1
Data
FCS
Checksum
calculation2
78-14099-04

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