Deploying Cisco Express Forwarding – CEF-Based Multilayer Switching
Identifying the multilayer switch packet forwarding process

CEF separates the control plane hardware from the data plane hardware and switching. ASICs in switches are used to separate the control plane and data plane, thereby achieving higher data throughput. The control plane is responsible for building the Forwarding Information Base or FIB table and adjacency tables in software. The data plane is responsible for forwarding IP unicast traffic using hardware.

When traffic cannot be processed in hardware, it must receive processing in software by the Layer 3 Engine, thereby not receiving the benefit of expedited hardware-based forwarding. There are a number of different packet types that may force the Layer 3 engine to process them. Some examples of IP exception packets are:

  • IP Packets that use IP header options (Packets that use TCP header options are switched in hardware because they do not affect the forwarding decision.)
  • Packets that have an expiring IP TTL counter
  • Packets that are forwarded to a tunnel interface
  • Packets that arrive with nonsupported encapsulation types
  • Packets that are routed to an interface with nonsupported encapsulation types
  • Packets that exceed the maximum transmission unit (MTU) of an output interface and must be fragmented