Implementing and Operating Cisco Enterprise Network Core Technologies (ENCOR) v1.2InfrastructureHard
A network engineer is configuring a new Layer 3 switch to support inter-VLAN routing. The switch has several VLANs configured, and hosts in each VLAN need to communicate with hosts in other VLANs. The engineer wants to ensure that the routing process on the switch is efficient and uses the most appropriate Layer 3 forwarding mechanism for directly connected networks. Which mechanism is used by Cisco switches to perform high-speed Layer 3 forwarding between directly connected VLANs?
- AFast Switching
- BProcess Switching
- CSoftware Switching
- DCisco Express Forwarding (CEF)
Show answer & explanationAnswer & explanation
Correct answer: D. Cisco Express Forwarding (CEF)
Cisco Express Forwarding (CEF) is the most advanced and efficient Layer 3 IP forwarding mechanism used by Cisco routers and multilayer switches. It uses a Forwarding Information Base (FIB) and an adjacency table to make forwarding decisions, enabling hardware-based, high-speed routing without CPU involvement for every packet, which is crucial for inter-VLAN routing performance.
Why the other options are wrong
- A. Fast Switching caches routing information after the first packet of a flow, but still involves the CPU for the first packet and cache lookups, less efficient than CEF.
- B. Process Switching is the slowest forwarding method, where every packet is processed by the CPU, making it unsuitable for high-speed inter-VLAN routing.
- C. Software Switching is a generic term that encompasses process and fast switching, but CEF is the specific, hardware-accelerated mechanism.
Cisco Express Forwarding (CEF)
A highly optimized, hardware-accelerated Layer 3 IP forwarding mechanism used by Cisco routers and multilayer switches to achieve high throughput and low latency.
- Default and most efficient forwarding method.
- Uses a FIB (Forwarding Information Base) and Adjacency Table.
- Hardware-based forwarding (ASIC-driven) on many platforms.
- Eliminates CPU involvement for most packet forwarding decisions.
Memory trick: Forwarding: Different speeds for different needs.