Cisco Systems 6500, EA6500 IP MLS Operation

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Catalyst 6500 Series Switch Cisco IOS Software Configuration Guide—Release 12.1 E
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Chapter 19 Configuring IP Unicast Layer 3 Switching on Supervisor Engine 1
Understanding How IP MLS Works
After the PFC performs the packet rewrite, the packet is formatted (conceptually) as follows:
IP MLS Operation
Figure 19-1 shows a simple IP MLS network topology. In this example, Host A is on the Sales VLAN
(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 Figure 19-1). The PFC stores the MAC addresses of the
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.
Figure 19-1 IP MLS Example Topology
Frame Header IP Header Payload
Destination Source Destination Source TTL Checksum Data Checksum
Host B MAC MSFC MAC Host B IP Host A IP n-1 calculation2
Source IP
Address
171.59.1.2
171.59.1.2
Host A
171.59.1.2
Host B
171.59.3.1
Host C
171.59.2.2
171.59.2.2
171.59.1.2: 2000Data
171.59.3.1
171.59.2.2
171.59.1.2
FTP
HTTP
HTTP
Dd:Bb
Dd:Cc
Dd:Aa
Marketing
Engineering
Sales
Destination
IP Address Application Rewrite Src/Dst
MAC Address
Destination
VLAN
MSFC
Subnet 1/Sales
MAC = Aa
MAC = Dd
MAC = Bb
MAC = Cc
Subnet 3/Marketing
Subnet 2/Engineering
Aa:Dd
171.59.1.2: 2000Data Dd:Cc
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