Cisco WS-X6066-SLB-APC - Content Switching Module Software Manual page 256

Understanding How Layer 3 Switching Works
Partially and Completely Switched Multicast Flows
Some flows might be partially Layer 3 switched instead of completely Layer 3 switched in these
situations:
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For partially switched flows, all multicast traffic belonging to the flow reaches the MSFC and is software
switched for any interface that is not Layer 3 switched.
The PFC prevents multicast traffic in flows that are completely Layer 3 switched from reaching the
MSFC, reducing the load on the MSFC. The show ip mroute and show mls ip multicast commands
identify completely Layer 3-switched flows with the text string RPF-MFD (Multicast Fast Drop [MFD]
indicates that from the perspective of the MSFC, the multicast packet is dropped, because it is switched
by the PFC).
For all completely Layer 3-switched flows, the PFC periodically sends multicast packet and byte count
statistics to the MSFC, because the MSFC cannot record multicast statistics for completely switched
flows, which it never sees. The MSFC uses the statistics to update the corresponding multicast routing
table entries and reset the appropriate expiration timers.
MLS Examples
Figure 14-1
(IP subnet 171.59.1.0), Host B is on the Marketing VLAN (IP subnet 171.59.3.0), and Host C is on the
Engineering VLAN (IP subnet 171.59.2.0).
When Host A initiates an HTTP file transfer to Host C, an MLS entry for this flow is created (this entry
is the second item in the MLS cache shown in
MSFC and Host C in the MLS entry when the MSFC forwards the first packet from Host A through the
switch to Host C. The PFC uses this information to rewrite subsequent packets from Host A to Host C.
Catalyst 6000 Family Software Configuration Guide—Releases 6.3 and 6.4
14-8
The MSFC 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.
The MSFC is the first-hop router to the source in PIM sparse mode (in this case, the MSFC must
send PIM-register messages to the rendezvous point).
The multicast TTL threshold is configured on an egress interface for the flow.
The extended access list deny condition on the RPF interface specifies anything other than the
Layer 3 source, Layer 3 destination, or IP protocol (an example is Layer 4 port numbers).
The multicast helper is configured on the RPF interface for the flow, and multicast to broadcast
translation is required.
Multicast tag switching is configured on an egress interface.
Network address translation (NAT) is configured on an interface, and source address translation is
required for the outgoing interface.
shows a simple IP MLS network topology. In this example, Host A is on the Sales VLAN
Figure 14-1). The PFC stores the MAC addresses of the
Chapter 14 Configuring MLS
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