Chapter 19
Configuring IP Unicast Layer 3 Switching on Supervisor Engine 1
After the PFC performs the packet rewrite, the packet is formatted (conceptually) as follows:
Frame Header
Destination
Host B MAC
IP MLS Operation
Figure 19-1
(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.
Figure 19-1 IP MLS Example Topology
Source IP
Address
171.59.1.2
171.59.1.2
171.59.2.2
MAC = Aa
Host A
171.59.1.2
78-14064-04
Source
MSFC MAC
shows a simple IP MLS network topology. In this example, Host A is on the Sales VLAN
Destination
Application
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
IP Header
Destination Source
TTL Checksum
Host B IP
Host A IP n-1
Figure
19-1). The PFC stores the MAC addresses of the
Rewrite Src/Dst
MAC Address
FTP
Dd:Bb
HTTP
Dd:Cc
HTTP
Dd:Aa
MAC = Dd
MSFC
Data
Cisco 7600 Series Router Cisco IOS Software Configuration Guide—12.1E
Understanding How IP MLS Works
Payload
Data
calculation2
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
Checksum
19-5