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Cisco 7600 Series Router Cisco IOS Software Configuration Guide, Release 12.2SX
OL-4266-08
Chapter 26 Configuring IP Unicast Layer 3 Switching
Understanding How Layer 3 Switching Works
Understanding How Layer 3 Switching Works
These sections describe Layer 3 switching:
• Understanding Hardware Layer 3 Switching, page 26-2
• Understanding Layer 3-Switched Packet Rewrite, page 26-2
Understanding Hardware Layer 3 Switching
Hardware Layer 3 switching allows the PFC and DFCs, instead of the MSFC, to forward IP unicast
traffic between subnets. Hardware Layer 3 switching provides wire-speed forwarding on the PFC and
DFCs, instead of in software on the MSFC. Hardware Layer 3 switching requires minimal support from
the MSFC. The MSFC routes any traffic that cannot be hardware Layer 3 switched.
Hardware Layer 3 switching supports the routing protocols configured on the MSFC. Hardware Layer 3
switching does not replace the routing protocols configured on the MSFC.
Hardware Layer 3 switching runs equally on the PF3 and DFCs to provide IP unicast Layer 3 switching
locally on each module. Hardware Layer 3 switching provides the following functions:
• Hardware access control list (ACL) switching for policy-based routing (PBR)
• Hardware NetFlow switching for TCP intercept, reflexive ACL forwarding decisions
• Hardware Cisco Express Forwarding (CEF) switching for all other IP unicast traffic
Hardware Layer 3 switching on the PFC supports modules that do not have a DFC. The MSFC forwards
traffic that cannot be Layer 3 switched.
Traffic is hardware Layer 3 switched after being processed by access lists and quality of service (QoS).
Hardware Layer 3 switching makes a forwarding decision locally on the ingress-port module for each
packet and sends the rewrite information for each packet to the egress port, where the rewrite occurs
when the packet is transmitted from the Cisco 7600 series router.
Hardware Layer 3 switching generates flow statistics for Layer 3-switched traffic
. Hardware Layer 3
flow statistics can be used for NetFlow Data Export (NDE). (See Chapter 51, “Configuring NDE”.)
Understanding Layer 3-Switched Packet Rewrite
When a packet is Layer 3 switched from a source in one subnet to a destination in another subnet, the
Cisco 7600 series router 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.
Packet rewrite alters five fields:
• Layer 2 (MAC) destination address
• Layer 2 (MAC) source address
• Layer 3 IP Time to Live (TTL)
• 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.
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