Cisco WS-C6506 Software Manual page 367

Chapter 14 Configuring MLS
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 the multicast packet and byte
count statistics to the MSFC, because the MSFC cannot record the multicast statistics for completely
switched flows, which it never sees. The MSFC uses the statistics to update the corresponding multicast
routing table entries and to 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 the subsequent packets from Host A to
Host C.
Figure 14-1
Source IP
Address
171.59.1.2
171.59.1.2
171.59.2.2
MAC = Aa
Host A
171.59.1.2
Figure 14-2
(IPX address 01.Aa), Host B is on the Marketing VLAN (IPX address 03.Bb), and Host C is on the
Engineering VLAN (IPX address 02.Cc).
OL-8978-04
shows a simple IP MLS network topology. In this example, Host A is on the Sales VLAN
IP MLS Example Topology
Destination
IP Address
171.59.3.1
171.59.2.2
171.59.1.2
Subnet 1/Sales
Data 171.59.1.2: 2000 Aa:Dd
shows a simple IPX MLS network topology. In this example, Host A is on the Sales VLAN
Figure 14-1). The PFC stores the MAC addresses of the
Rewrite Src/Dst
Application
MAC Address
FTP Dd:Bb
HTTP Dd:Cc
HTTP Dd:Aa
MAC = Dd
MSFC
Data
Catalyst 6500 Series Switch Software Configuration Guide—Release 8.7
Understanding How Layer 3 Switching Works
Destination
VLAN
Marketing
Engineering
Sales
MAC = Bb
Host B
171.59.3.1
MAC = Cc
Host C
171.59.2.2
171.59.1.2: 2000 Dd:Cc
14-11