Cisco WS-SUP32-GE-3B - Supervisor Engine 32 Software Configuration Manual page 359

Chapter 23 Configuring IP Unicast Layer 3 Switching
In IP unicast and IP multicast traffic, the switch decrements the Layer 3 TTL value by 1 and recomputes
the Layer 3 packet checksum. The switch recomputes the Layer 2 frame checksum and forwards (or, for
multicast packets, replicates as necessary) the rewritten packet to Destination B's subnet.
A received IP unicast packet is formatted (conceptually) as follows:
Layer 2 Frame Header
Destination
PISA MAC
After the switch rewrites an IP unicast packet, it is formatted (conceptually) as follows:
Layer 2 Frame Header
Destination
Destination B MAC PISA MAC
Hardware Layer 3 Switching Examples
Figure 23-1 on page 23-3
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 23-1
MAC = Aa
Host A
171.59.1.2
Data
OL-11439-03
Layer 3 IP Header
Source Destination
Source A MAC Destination B IP
Source
shows a simple network topology. In this example, Host A is on the Sales
Hardware Layer 3 Switching Example Topology
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
Catalyst Supervisor Engine 32 PISA Cisco IOS Software Configuration Guide, Release 12.2ZY
Source
Source A IP
Layer 3 IP Header
Destination Source
Destination B IP Source A IP n-1
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
23-3