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Catalyst 6000 Family Software Configuration Guide—Releases 6.3 and 6.4
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Chapter 13 Configuring CEF for PFC2
Understanding How Layer 3 Switching Works
Layer 3 Switching Overview
Layer 3 switching allows the switch, instead of a router, to forward IP and IPX unicast traffic and IP
multicast traffic between VLANs. Layer 3 switching is implemented in hardware and provides
wire-speed interVLAN forwarding on the switch, rather than on the MSFC2. Layer 3 switching requires
minimal support from the MSFC2. The MSFC2 routes any traffic that cannot be Layer 3 switched.
Note
Layer 3 switching supports the routing protocols configured on the MSFC2. Layer 3 switching does
not replace the routing protocols configured on the MSFC2. Layer 3 switching uses Protocol
Independent Multicast (PIM) for multicast route determination.
Layer 3 switching on Catalyst 6000 family switches provides flow statistics that you can use to identify
traffic characteristics for administration, planning, and troubleshooting. Layer 3 switching uses NetFlow
Data Export (NDE) to export flow statistics (for more information about NDE, see
Chapter 15,
“Configuring NDE”
)
.
Note
Traffic is Layer 3 switched after being processed by the VLAN access control list (VACL) feature
and the quality of service (QoS) feature.
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 MSFC2 so that
the packets appear to have been routed by the MSFC2.
Note
Rather than just forwarding IP multicast packets, the PFC2 replicates them as necessary on the
appropriate VLANs.
Packet rewrite alters five fields:
•
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)
Note
Packets are rewritten with the encapsulation appropriate for the next-hop subnet.
If Source A and Destination B are on different VLANs and Source A sends a packet to the MSFC2 to be
routed to Destination B, the switch recognizes that the packet was sent to the Layer 2 (MAC) address of
the MSFC2.
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 MSFC2. The Layer 3 addresses remain the same.
