Cisco 7609 Configuration Manual page 253
Cisco ios software configuration guide—12.1e
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Chapter 17
Configuring IP Unicast Layer 3 Switching on Supervisor Engine 2
A received IP unicast packet is formatted (conceptually) as follows:
Layer 2 Frame Header
Destination
MSFC2 MAC Source A MAC
After the router rewrites an IP unicast packet, it is formatted (conceptually) as follows:
Layer 2 Frame Header
Destination
Destination B MAC MSFC2 MAC Destination B IP Source A IP n-1
Hardware Layer 3 Switching Examples
Figure 17-1 on page 17-3
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, Hardware Layer 3 switching uses the information
in the local forwarding information base (FIB) and adjacency table to forward packets from Host A to
Host C.
Figure 17-1 Hardware Layer 3 Switching Example Topology
MAC = Aa
Host A
171.59.1.2
Data
78-14064-04
Layer 3 IP Header
Source
Destination
Destination B IP
Source
shows a simple network topology. In this example, Host A is on the Sales
Source IP
Destination
Address
IP Address
171.59.1.2
171.59.3.1
171.59.1.2
171.59.2.2
171.59.2.2
171.59.1.2
MAC = Dd
Subnet 1/Sales
171.59.1.2:171.59.2.2
Aa:Dd
Cisco 7600 Series Router Cisco IOS Software Configuration Guide—12.1E
Source
Source A IP
Layer 3 IP Header
Destination
Source
Rewrite Src/Dst
Destination
MAC Address
VLAN
Dd:Bb
Marketing
Dd:Cc
Engineering
Dd:Aa
Sales
MSFC
Data
171.59.1.2:171.59.2.2
Understanding How Layer 3 Switching Works
TTL Checksum
n
calculation1
TTL Checksum
calculation2
MAC = Bb
Host B
171.59.3.1
MAC = Cc
Host C
171.59.2.2
Dd:Cc
Data FCS
Data FCS
17-3
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