Understanding Layer 3-Switched Packet Rewrite - Cisco WS-X6066-SLB-APC - Content Switching Module Software Manual

Understanding How Layer 3 Switching Works
These sections describe Layer 3 switching and MLS on the Catalyst 6000 family switches:
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Understanding Layer 3-Switched Packet Rewrite

When a packet is Layer 3 switched from a source in one VLAN to a destination in another VLAN, the
switch performs a packet rewrite at the egress port based on information learned from the MSFC so that
the packets appear to have been routed by the MSFC.
Rather than just forwarding multicast packets, the switch replicates them as necessary on the
Note
appropriate VLANs.
Packet rewrite alters five fields:
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If Source A and Destination B are on different VLANs and Source A sends a packet to the MSFC to be
routed to Destination B, the switch recognizes that the packet was sent to the Layer 2 (MAC) address of
the MSFC.
To perform Layer 3 switching, the switch rewrites the Layer 2 frame header, changing the Layer 2
destination address to the Layer 2 address of Destination B and the Layer 2 source address to the Layer 2
address of the MSFC. The Layer 3 addresses remain the same.
In IP unicast and IP multicast traffic, the switch decrements the Layer 3 Time to Live (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:
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Understanding IP Unicast Rewrite
Received IP unicast packets are (conceptually) formatted as follows:
Layer 2 Frame Header
Destination
MSFC MAC
Catalyst 6000 Family Software Configuration Guide—Releases 6.3 and 6.4
14-2
Understanding Layer 3-Switched Packet Rewrite, page 14-2
Understanding MLS, page 14-4
Layer 2 (MAC) destination address
Layer 2 (MAC) source address
Layer 3 IP Time to Live (TTL) or IPX Transport Control
Layer 3 checksum
Layer 2 (MAC) checksum (also called the frame checksum or FCS)
Understanding IP Unicast Rewrite, page 14-2
Understanding IPX Unicast Rewrite, page 14-3
Understanding IP Multicast Rewrite, page 14-3
Source
Source A MAC
Layer 3 IP Header
Destination Source
Destination B IP Source A IP
Chapter 14 Configuring MLS
Data FCS
TTL Checksum
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