Deploying Cisco Express Forwarding – CEF-Based Multilayer Switching
CEF-Based MLS operation and frame rewrite

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.


Lab Activity

e-Lab Activity: Implementing Multilayer Switching in the Network

In this lab, the student will verify that Cisco Express Forwarding (CEF) is operating correctly.