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