What is CEF and How Does it Work on Cisco Router?

What is CEF?

Cisco Express Forwarding (CEF) is a Layer 3 switching technology that offers high performance, low latency, and Scalability. It is the successor to Cisco’s Distributed Forwarding technology. CEF is used in Campus Networks and Data Centers to provide optimal forwarding of traffic.

CEF uses a central Adjacency Table which contains next-hop information for all destinations reachable by the router. The FIB (Forwarding Information Base) is populated from the Routing Table and is used to make forwarding decisions. The Adjacency Table and FIB are both stored in memory which allows for fast lookup and retrieval of data.

CEF uses a hardware-based switching architecture which means that it can take advantage of specialized hardware to further improve forwarding performance. This includes the use of ASICs (Application-Specific Integrated Circuits) and TCAMs (Ternary Content Addressable Memories).

Components of CEF

There are Four main components to CEF:

• Adjacency Table
• FIB
• Data Plane
• Control plane

The Adjacency Table contains next-hop information for all destinations reachable by the router. This information is used to populate the FIB. The Adjacency Table is populated by ARP entries and Layer 2 header information.

The FIB is used to make forwarding decisions. It is populated from the Adjacency Table and the Routing Table. The FIB contains information on how to reach all destinations reachable by the router.

The Data Plane is the hardware that CEF uses to forward traffic. This includes ASICs and TCAMs. ASICs are used to perform high-speed packet processing. TCAMs are used to store and lookup forward to information.

The control plane is responsible for managing the data plane. It does this by maintaining the FIB and Adjacency Table. The control plane is also responsible for routing protocols and updating the Routing Table.

Difference between Routing Table and FIB Table

The Routing Table is used to store information on how to reach all destinations. This includes the next-hop information and the metric. The FIB Table is used to store information on how to reach all destinations reachable by the router. The FIB Table is populated from the Routing Table and Adjacency Table.

How does CEF work on a Cisco Ruter?

Here are the steps:

1. The router receives a packet from a host.

2. The router looks up the destination address in the Adjacency Table.

3. The router populates the FIB with the next-hop information for the destination.

4. The router uses the FIB to make forwarding decisions.

5. The router forwards the packet to the next hop.

6. The router updates the Adjacency Table with information from the packet headers.

7. The router uses the Adjacency Table to populate the FIB.

8. The router repeats steps 4-7 for each packet received.

Importance of CEF in Routing Protocols (EIGRP, OSPF, BGP & MPLS)

CEF is important in routing protocols because it offers a number of benefits. CEF is scalable, efficient, and offers high performance. It is also easy to troubleshoot and provides better security.

CEF is scalable: The Adjacency Table and FIB can be easily expanded to accommodate more routes. This makes CEF scalable and able to support a large number of routes.

CEF is efficient: The use of hardware-based switching means that CEF can take advantage of specialized hardware to further improve forwarding performance. This includes the use of ASICs and TCAMs.

CEF offers high performance: The Adjacency Table and FIB are stored in memory which allows for fast lookup and retrieval of data. CEF also uses a hardware-based switching architecture which means that it can take advantage of specialized hardware to further improve forwarding performance.

CEF is easy to troubleshoot: The use of hardware-based switching makes CEF easy to troubleshoot. This is because the control plane and data plane are separate. The control plane is responsible for managing the data plane. This separation of duties makes it easy to troubleshoot problems.

CEF provides better security: The use of hardware-based switching means that CEF can take advantage of specialized hardware to further improve forwarding performance. This includes the use of ASICs and TCAMs. This hardware can be used to provide better security by implementing access control lists (ACLs).