what pre-populates the fib on cisco devices that use cef to process packets?

Ultimate Guide to Cisco Express Forwarding(CEF)

Ultimate Guide to Cisco Express Forwarding(CEF)

What is CEF (Cisco Limited Forwarding)?

Cisco Limited Forwarding (CEF) is a package-switching technique that is the default for virtually of Cisco routing platforms.

CEF Cisco provides the power to switch packets (through advanced, Layer iii IP switching engineering) via a device in a style that minimises the load on router's processor.

This fashion the routing process can be tasked with dealing with other responsibilities that crave larger amounts of processor time (QOS, Encryption, etc.).

It's Noteworthy that CEF is a Cisco proprietary protocol, however other OEM and vendors of Routing and Switching platforms too offer CEF comparable features set which is performed in hardware-based ASICs instead of CPU software itself.

CEF Cisco machinery works by creating two types of tables in Data plane –

1. Adjacency table –

   This table is responsible for maintaining the layer 2 forwarding information for each FIB entry. Adjacency tabular array takes its input from ARP Enshroud and hence populates itself with layer two information to be used for packet forwarding. This offloads the need for the router to send out ARP requests.

2. The FIB (Forwarding Information Base) –

   FIB is known past the name of  CEF table. FIB table learns the routing data from the IP Routing Table and tracks the next-hop for all routes. So while the Adjacency table manages layer 2 data, the CEF table manages the layer 3 forwarding information. The adjacency table maintains Layer 2 data for next hops listed in the FIB.

   

Below is the Pace by Stride procedure of how Cisco Express Forwarding handles packets that enter the Router –

1. Once the packets make it at the router, its layer 2 data is stripped off. This is a normal procedure that happens whenever a frame is accustomed to a layer 3 device)
2. The router looks up the destination using the CEF table. (Annotation that fifty-fifty ane^st packet router does not attain the Router CPU. CEF handles all the packets as they enter the interface.
3. Router finds the corresponding adjacency table entry.
4. The router then adds the corresponding layer 2 data (institute in the adjacency table) back to the bundle and forwards the packets.

Related – Cisco Datasheets

Cisco Express Forwarding (CEF) can be enabled in one of 2 modes described in the following sections:

• Cardinal CEF Mode
• Distributed CEF Style

Cardinal CEF MODE

When CEF mode is enabled, the CEF FIB and adjacency tables reside on the RP, and the RP performs the limited forwarding.

You can use CEF way when line cards are not bachelor for CEF switching or when you need to use features not uniform with dCEF switching.

The figure below shows the relationship between the routing table, FIB, and adjacency table during CEF way.

The Goad switches forrad traffic from workgroup LANs to a Cisco 7500 series router on the enterprise backbone running CEF. The RP performs the limited forwarding.

Distributed CEF Mode

When dCEF is enabled, line cards, such as VIP line cards or GSR line cards, maintain an identical copy of the FIB and adjacency tables. The line cards perform the limited forwarding between port adapters, relieving the RSP of involvement in the switching operation.

dCEF uses an Inter-Process Communication (IPC) mechanism to ensure synchronization of FIB tables and adjacency tables on the RP and line cards.

The figure below shows the human relationship betwixt the RP and line cards when dCEF mode is active.

Related – Primal CEF vs Distributed CEF

Cisco Limited Forwarding offers the following benefits –

• Meliorate functioning— CEF is less CPU-intensive than fast switching route caching and Procedure Switching. CEF offloads CPU process for other services like QoS and encryption.
• Scalability— when dCEF is working, CEF offers full switching capacity to each Line card.
• Resilience—CEF offers an unparalleled level of switching consistency and stability in large dynamic networks. In dynamic networks, other methods like fast-switched entries are frequently invalidated due to routing changes which puts more load on process switching hence high CPU load. Since the Forwarding Data Base (FIB) lookup table contains all known routes that exist in the routing tabular array, hence we can aptly say that CEF can switch traffic more efficiently than typical demand caching schemes.

Related- Procedure vs Fast Switching

In some Cisco platforms, Cisco Limited Forwarding is enabled by default. However, other platforms may require to enable CEF globally with the below command syntax –

Additionally, we tin you tin can enable information technology on that interface with interface configuration command equally below –

R1(config-if)# ip road-enshroud cef

Cisco Express Forwarding (CEF) status verification commands –

Verify interface statistics, including data about an interface's bundle-switching mode –

R1# evidence ip interface fa0/0

Verify the router's FIB contents –

Verify information available in adjacency table of the router –

For more information on Cisco Express forwarding sentry this video –

Reference –

http://www.cisco.com/c/en/us/td/docs/ios/12_2/switch/configuration/guide/fswtch_c/xcfcef.html

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Nearly The Author

Rashmi Bhardwaj

I am here to share my knowledge and experience in the field of networking with the goal being - "The more you share, the more yous larn." I am a biotechnologist by qualification and a Network Enthusiast by interest. I developed interest in networking beingness in the company of a passionate Network Professional, my husband. I am a stiff laic of the fact that "learning is a abiding process of discovering yourself." - Rashmi Bhardwaj (Writer/Editor)

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