About this interactive
How a router forwards a packet, slowest to fastest:
- **Process switching**: the data plane sends every packet up to the CPU, which looks up the routing table each time.
- **Fast switching**: the first packet to a destination goes to the CPU, and the result is cached; later packets that hit the cache bypass the CPU.
- **Cisco Express Forwarding (CEF)**: the forwarding information base (FIB) and adjacency table are built ahead of time from the routing table and the Layer 2 details, so packets are forwarded in the data plane without the CPU.
The video calls process switching "the standard". On current Cisco routers, CEF is enabled by default.
What you'll learn
- Distinguish process switching, fast switching and Cisco Express Forwarding (CEF) by how each uses the CPU and the data plane
- Explain why CEF is fastest: its forwarding information base and adjacency table are built before traffic arrives
Aligned to
Cisco CCNA
3.2
Determine how a router makes a forwarding decision by default
1.1
Explain the role and function of network components
Key terms
- Process Switching
- A packet-forwarding method in which every packet is sent to the router's CPU for a routing table lookup before being forwarded.
- Fast Switching
- A packet-forwarding method that caches the first CPU-resolved routing decision in a fast-forward cache so subsequent packets to the same destination bypass the CPU.
- Cisco Express Forwarding
CEF
- A packet-forwarding method that pre-populates the data plane with a Forwarding Information Base and adjacency tables, allowing packets to be forwarded without CPU involvement.
- Forwarding Information Base
FIB
- A data structure used by CEF that contains pre-built Layer 3 forwarding entries, enabling the data plane to make forwarding decisions without consulting the CPU.
- Data Plane
- The part of a network device responsible for actually forwarding packets toward their destinations based on rules and tables built by the control plane. In a router, the data plane looks up each packet's destination in the forwarding table and sends it out the correct interface at high speed.
Topics
Routing
Router Processing
Process Switching
Fast Switching
Cef
Interactive
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