13-3
Catalyst 6000 Family Software Configuration Guide—Releases 6.3 and 6.4
78-13315-02
Chapter 13 Configuring CEF for PFC2
Understanding How Layer 3 Switching Works
In IP unicast and IP multicast traffic, the switch decrements the Layer 3 TTL value by 1 and recomputes
the Layer 3 packet checksum. In IPX traffic, the switch increments the Layer 3 Transport Control 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
VLAN.
These sections describe how the packets are rewritten:
•
Understanding IP Unicast Rewrite, page 13-3
•
Understanding IPX Unicast Rewrite, page 13-3
•
Understanding IP Multicast Rewrite, page 13-4
Understanding IP Unicast Rewrite
Received IP unicast packets are (conceptually) formatted as follows:
After the switch rewrites an IP unicast packet, it is (conceptually) formatted as follows:
Understanding IPX Unicast Rewrite
Received IPX packets are (conceptually) formatted as follows:
After the switch rewrites an IPX packet, it is (conceptually) formatted as follows:
Layer 2 Frame Header
Layer 3 IP Header
Data
FCS
Destination
Source
Destination
Source
TTL
Checksum
MSFC2 MAC Source A MAC
Destination B IP
Source A IP
n
calculation1
Layer 2 Frame Header
Layer 3 IP Header
Data FCS
Destination
Source
Destination
Source
TTL Checksum
Destination B MAC
MSFC2
MAC
Destination B IP
Source A IP n-1
calculation2
Layer 2 Frame Header
Layer 3 IPX Header
Data FCS
Destination
Source
Checksum/
IPX Length/
Transport Control
Destination Net/
Node/
Socket
Source Net/
Node/
Socket
MSFC2 MAC
Source A MAC
n
Destination B IPX
Source A IPX
Layer 2 Frame Header
Layer 3 IPX Header
Data FCS
Destination
Source
Checksum/
IPX Length/
Transport Control
Destination Net/
Node/
Socket
Source Net/
Node/
Socket
Destination B
MAC
MSFC2 MAC
n+1
Destination B IPX
Source A IPX