350-401 Question 70
Single answerA network engineer is troubleshooting an issue with packet forwarding in a Cisco enterprise network. The engineer discovers that some packets are dropped because certain Layer 3 routes are missing in the hardware-based forwarding tables. Which table is responsible for storing Layer 3 forwarding information used by Cisco Express Forwarding (CEF)?
- A
Content Addressable Memory (CAM) table
- B
Forwarding Information Base (FIB)
- C
Routing Information Base (RIB)
- D
Adjacency Table
Show answer and explanation
Correct answer: B
Explanation
Cisco Express Forwarding (CEF) uses two key data structures for high-speed packet forwarding: the Forwarding Information Base (FIB) and the adjacency table. The FIB stores precomputed Layer 3 routes and is optimized for fast lookups during packet forwarding, while the adjacency table stores Layer 2 next-hop information. In this scenario, the missing Layer 3 routes indicate an issue with the FIB, as it is responsible for hardware-based Layer 3 forwarding.
- A. Incorrect.
The CAM table is used to store MAC address-to-port mappings in a switch for Layer 2 forwarding, not Layer 3 routing information.
- B. Correct.
The FIB is a hardware-based forwarding table that stores precomputed Layer 3 routes for high-speed packet forwarding in Cisco Express Forwarding (CEF).
- C. Incorrect.
The RIB is a software-based routing table that stores routing information learned from routing protocols and static entries, but it does not handle hardware-based forwarding.
- D. Incorrect.
The adjacency table is used to store Layer 2 next-hop information, such as MAC addresses and ARP data, but it does not store Layer 3 forwarding information.