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Bidirectional Forwarding Detection (BFD) in the CCIE R&S Written

Sometimes Cisco written exams can throw a bit of a curve ball at students when it comes to the questions that can creep into the overall pool of queries. One way in which this can happen is when Cisco asks about a technology or two that is not supported on the current equipment that Cisco is using in the practical lab exam for the same track. Currently we know that for the Version 4.0 of the CCIE R&S exam, Cisco features the following:

  • 1841 series routers – IOS 12.4(T) – Advanced Enterprise Services
  • 3825 series routers – IOS 12.4(T) – Advanced Enterprise Services
  • Catalyst 3560 Series switches running IOS version 12.2 – Advanced IP Services

Correctly so, students tend to study for the written exam against the backdrop of these Cisco devices and Operating Systems. That is a fine idea, but let us be sure to be ready for some features here and there there are not in the scope of our lab equipment.

In this post, I want to discuss Bidirectional Forwarding Detection (BFD) as this feature could very well find its way into a Cisco written exam on the subject of Routing and Switching. This technology is found on Cisco 6500/7600 series routers, as well as 12000 series and Carrier Routing System (CRS-1 Routers).

What is the point of this technology? Like so many protocols and techniques in modern Cisco networking, Bidirectional Forwarding Detection seeks to speed up routing protocol convergence. How does BFD seek to accomplish this? BFD focuses on the ultra-fast detection of node or link failures in the routing infrastructure.

What is the typical approach for the detection of a link or node failure? It is for some type of Hello or Keepalive mechanism to be used at Layer 3. In fact, we have seen these typical mechanisms attempt to evolve by permitting the use of sub-second intervals, as opposed to the long standing multi-second intervals that worked just fine in non mission critical data networks.

BFD operates in a similar fashion, except the fast link hellos are accomplished at Layer 2 instead of the traditional Layer 3. Why is this so exciting? It is not just the fact that there can be blindingly speedy detection of a failure, but also the fact that the CPU impact of the BFD process ends up being much less compared to other Layer 3 fast hello approaches. In testing accomplished by Cisco Systems, network devices running 100 concurrent BFD sessions experienced an increase in CPU utilization of a mere 2%.

For Bidirectional Forwarding Detection to operate correctly in your network environment, you obviously need to use the Cisco Feature Navigator (cisco.com/go/fn) and ensure that you are running the correct hardware and software version for full feature support. This feature does indeed require the cooperation of your Layer 3 protocols, and the great news is that Cisco implementations of EIGRP, IS-IS, and BGP, as well as such important support protocols like HSRP all support Bidirectional Forwarding Detection now.

Thanks for reading this blog post here at blog.ipexpert.com, and should you encounter this Cisco technology in your written, you are now MORE than ready for it! Continue to enjoy those studies!

Should you want more information on BFD at this time – here is a Cisco Documentation Link:

http://www.cisco.com/en/US/docs/ios-xml/ios/iproute_bfd/configuration/12-4t/Bidirectional_Forwarding_Detection.html

Anthony Sequeira CCIE, CCSI
Twitter: @compsolv
Facebook: http://www.facebook.com/compsolv

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