CEF-Based MLS Operation
These are the steps that would occur when
using CEF to forward frames between host A and host B on different VLANs:

Step 1 Host A sends a packet to host B. The switch
recognizes the frame as a Layer 3 packet because the destination MAC (MAC-M)
matches the Layer 3 engine MAC.
Step 2 The
switch performs a CEF lookup based on the destination IP address (IP-B). The
packet hits the CEF entry for the connected (VLAN20) network and is redirected
to the Layer 3 engine using a "glean" adjacency.
Step 3 The Layer 3 engine installs an ARP throttling
adjacency in the switch for the host B IP address.
Step 4 The Layer 3 engine sends ARP requests for host B on
VLAN20.
Step 5 Host B sends an ARP response to
the Layer 3 engine.
Step 6 The Layer 3 engine
installs the resolved adjacency in the switch (removing ARP throttling
adjacency).
Step 7 The switch forwards the
packet to host B.
Step 8 The switch receives a
subsequent packet for host B (IP-B).
Step 9 The
switch performs a Layer 3 lookup and finds a CEF entry for host B. The entry
points to the adjacency with rewrite information for host B.
The
switch rewrites packets per the adjacency information and forwards the packet
to host B on VLAN20.
Frame Rewrite Using CEF
Figure
shows
how the frame and packet header would be altered when CEF is used to forward
frames. IP unicast packets are rewritten on the output interface as
follows:
- The source MAC address changes from the sender MAC address to the router
MAC address.
- The destination MAC address changes from the router MAC to the next-hop MAC
address.
- The Time to Live (TTL) is decremented by one.
The IP header and frame checksums are recalculated.