Fast Convergence and the Fast Reroute – Definitions/Design Considerations in IP and MPLS

Fast Convergence and the Fast Reroute Network reliability is an important design aspect for deployability of time and loss sensitive applications. When a link, node or SRLG failure occurs in a routed network, there is inevitably a period of disruption to the delivery of traffic until the network reconverges on the new topology. Fast reaction is essential for the failed element for some applications.

There are two approaches for the fast reaction in case of failure: Fast convergence and fast reroute. Although people use these terms interchangeably, they are not the same thing. In this post I will explain the definitions and high level design considerations for fast convergence and the fast reroute.

Fast Reroute mechanisms in IP and MPLS , design considerations and pros and cons of each one of them will be explained in a separate post. When a local failure occur four steps are necessary for the convergence. These steps are completed before traffic continues on the backup/alternate link.

1. Failure detection (Protocol Hello Timers , Carrier Delay and Debounce Timers, BFD and so on)

2. Failure propagation (LSA and LSP Throttling timers)

3. New information process (Backup/Alternate path calculation) (SPF Wait and Run times)

4. Update new route into RIB/FIB (After this step, traffic can continue to flow through backup link) For fast convergence, these steps are tuned. Tuning the timers mean generally lowering them as most vendors use higher timers to be on the safe side. Because as you will see later in this post, lowering these timers can create stability issue in the network.

When you tune the timers for failure detection, propagation and the new path calculation, it is called fast convergence. Because traffic can continue towards alternate link faster than regular convergence since you use lower timers. (Instead of 30seconds hello timer, you can use 1 second hello , or instead of 5 seconds SPF wait time, you can make it 10 ms and so on.) Although the RIB/FIB update is hardware dependent, the network operator can configure all the other steps. One thing always needs to be kept in mind; Fast convergence and fast reroute can affect network stability. If you configure the timers very low, you might see false-positives. Unlike fast convergence, for the fast reroute, backup path is pre-computed and pre-programmed into the router RIB/FIB.

This increases the memory utilization on the devices. There are many Fast Reroute mechanisms available today. Most known ones are; Loop Free Alternate (LFA), Remote Loop Free Alternate (rLFA), MPLS Traffic Engineering Fast Reroute and Segment Routing Fast Reroute. Loop Free Alternate and the Remote Loop Free Alternate if also known as IP or IGP Fast Reroute Mechanisms. Main difference between MPLS Traffic Engineering Fast Reroute and the IP Fast Reroute mechanisms are the coverage. MPLS TE FRR can protect the any traffic in any topology. IP FRR mechanisms need the physical topology of the networks to be highly connected. Ring and square topologies are hard for the IP FRR topologies but not a problem for MPLS TE FRR at all. In other words, finding a backup path is not always possible with IP FRR mechanisms if the physical topology is ring or square. Best physical topologies from this aspect is full mesh. If MPLS is not enabled on the network, adding MPLS and RSVP-TE for just MPLS TE FRR functionality is considered as complicated. In that case network designers may want to evaluate their existing physical structure and try to alternate/backup path by adding or removing some circuit in the network.

IGP metric tuning also helps router to find alternate loop free paths. Fast reroute mechanisms can be considered as subset of fast convergence. But as you can understand from this post, all the above steps are taken after the failure in fast convergence and all of them are already ready in fast reroute. In fast reroute, traffic can flow through the backup path as soon as failure is detected. For the fast failure detection, best thing is to rely on physical detection mechanisms such as carrier delay , debounce timers, Automatic Protection Switching and so on.

Sometimes it is impossible to use these mechanisms (If there is Layer 1 or 2 device between the routers for example) then the best mechanism for the fast failure detection is BFD. Last but not least, convergence time is always faster with fast reroute mechanisms (50 ms is not a magic with them) compare to fast convergence (Generally less than a second but after that stability is a problem) mechanisms.

Created by
Orhan Ergun

Orhan Ergun, CCIE/CCDE Trainer, Author of Many Networking Books, Network Design Advisor, and Cisco Champion 2019/2020/2021

He created OrhanErgun.Net 10 years ago and has been serving the IT industry with his renowned and awarded training.

Wrote many books, mostly on Network Design, joined many IETF RFCs, gave Public talks at many Forums, and mentored thousands of his students.  

Today, with his carefully selected instructors, OrhanErgun.Net is providing IT courses to tens of thousands of IT engineers. 

View profile

Daniel Lardeux
Daniel Lardeux Senior Network Consultant at Post Telecom

I passed the CCDE Practical exam and Orhan’s CCDE course was very important contributor to my success. I attended the CCDE course of Orhan Ergun in July and it was exactly what I needed, Orhan is taking the pain to break down the different technologies.

Roy Lexmond
Roy Lexmond Senior Network Designer at Routz CCDE #20150017 & CCIE R&S; #26557

After I attended Orhan Ergun’s CCDE course I passed the CCDE practical exam.I really enjoyed the course a lot ...

Nicholas Russo
Nicholas Russo Network Consulting Engineer (CCDE/CCIEx2), Cisc

I signed up for Orhan’s CCDE training. This training is very technically detailed and the use-cases, quizzes, scenarios, and mind maps are all great resources in the overall training program. Orhan teaches his students to think like a network designer ...

Slide Heading
Slide Heading Network Systems Engineer at Conscia A/S CCIE #42544 (SP) & CCDE #20160015

Orhan is forcing you to take off the implementation hat that most of us have been wearing for many years, instead he is providing a new fancy design hat, which makes you see and deal with the issues presented ...

Kim Pedersen
Kim Pedersen CCIE in RS and SP (#29189) CCDE#20170021

I’ve used Orhan’s self-paced CCDE training material. If you are interested in knowing how all the technologies go together in a coherent design i can highly recommend it.I also enjoyed the Quizzes which helped pick out my weak spots in selecting ...

Laurent Metzger
Laurent Metzger 3xCCIE/CCDE Senior Network Architect

Hi Orhan. I passed the CCDE exam on February 22. I read everything that you put on your Self Paced CCDE Training course and it was very helpful in my success. Thank you very much.

Martin J. Duggan
Martin J. Duggan Network Architect at AT&T;, Ciscopress Author CCDE #20160006 & CCIE#7942

I attended Orhan’s April 201610 days CCDE Bootcamp. I am CCDE now !

You can tell Orhan has a great deal of experience, it really comes through when he presents his design case studies and the CCDE Practical scenarios.

Muhammad Abubakar
Muhammad Abubakar Lead Network Architect – CCDE #20160016 2xCCIE #26693 2xJNCIE VCIX

Your excellent CCDE materials and amazing Bootcamp helped me tremendously through my learning journey.Also thank you very much for being available whenever I have a design question or a complex design topics. I can’t compare your design skills ...

Jennifer Pai
Jennifer Pai Network/Security Engineer at KNET Technology

Thanks Orhan very much for this course. It helped strengthen my “Network design mind”.

Ruslan Silyayev
Ruslan Silyayev Solution Architect at R.I.S.K Company

Training by Orhan is not a CCDE preperation training only. It will be useful for engineers which are dealing with design. You want to pass CCDE exam or learn network design, then don’t look at anywhere else!

Sameer Meher
Sameer Meher Solutions Architect at 23 Wards/Japan

Orhan Ergun’s CCDE course was really very good. CCDE Level Intelligence was delivered very well and with very useful case studies and the scenarios, I am thankful to Orhan for all his help!

Ken Young
Ken Young Senior Technical Architect Province of Nova Scotia, 2xCCIE #41597 | CCDE #20170047

If anyone wants to understand network design and architecture, also pass CCDE exam , I recommend you to attend Orhan’s online courses! I am a CCDE now but learning is a journey, we will be together in your other courses too Orhan!

Matt Cross
Matt Cross Technical Architect at Heartland – CCDE #2019::7

Orhan did an excellent job of filling in the gaps of knowledge that I had that took me to the finish line of the practical exam CCDE. The community of people that Orhan facilitates are both engaging and supportive of the journey to CCDE. Orhan ...

Shiling Ding
Shiling Ding Sentinel Technologies – CCDE #2019::12

Just passed the CCDE Practical exam! I attended Orhan Ergun’s CCDE training program , used Orhan’s Instructor Led and Self-Paced CCDE training and Online CCDE Practical Scenarios during my CCDE journey. Orhan’s CCDE In Depth book is an excellent summary ...

Abelardo Basurto
Abelardo Basurto Solutions Architect at Cisco Systems – CCDE 2018::6

Hi everyone, I’ve just passed the CCDE Exam. My Number is CCDE 2018::6 I attended to Online CCDE Bootcamp of Orhan. I want to thank Orhan not only for the great book and bootcamp, but also for his commitment, availability and willingness to assist the ...

Hady Mohamed Abdellah
Hady Mohamed Abdellah Network Architect Hamad International Airport Qatar – CCDE 2018::1

Hi guys, I’m so happy that I passed the exam. I’ve already got my number CCDE 2018::1. Thanks to Orhan for being the best CCDE instructor in the world. I highly recomend Orhan’s CCDE Training and In-Depth-CCDE ...

Bryan Bartik
Bryan Bartik Sr. Systems Engineer at CompuNet – CCDE 20170059

Hi Orhan I passed CCDE Practical exam on November 2017 ! I really enjoyed your materials and quizzes and use cases. They were definitely helpful in my preparation. Thanks a lot !

Giedrius Trapkauskas
Giedrius Trapkauskas Network Solutions Architect at Liberty Global – CCDE 20180004

I attended Orhan’s CCDE Training in Istanbul and it was very helpful in my preparation. I passed the exam recently and I want to say Thank you Orhan! For those who want to pass the CCDE exam, definitely start with ...

Alaa Issa
Alaa Issa Sr.Solutions Architect – CCDE#20180033 3xCCIE ( Collab|DC|Security )#27146

I registered to Orhan’s training in Feb 2017. From that time, I attended Orhan’s training several times. The depth of knowledge which Orhan has is amazing, and how to present such consistent knowledge to the ...

Mazin Ahsan Design Lead Engineer | Solutions Engineer | CCDE License # 20160030 | CCIE Licence # 23892

I passed the CCDE Practical Lab exam on November 17,2016 from supplications of elders and dedication from my Sensei Mr. Orhan Ergun I took different CCDE bootcamps in the past. Orhan has the most depth and expertise ...

Jeff Patterson CCDE# 2018::11

Hi Orhan I wanted to pass along my appreciation for the outstanding training material. I used the online CCDE training provided by Orhan as well as the In-Depth-CCDE book and passed the exam in February 2018. Thank you Orhan!

Mehdi Sfar
Mehdi Sfar Network and Security Architect / CCDE #20210003 | CCIE R&S; #51583

I signed up for Orhan’s CCDE Self paced Course. This course, along with the CCDE In Depth book, helped me for my CCDE Practical as well as Written exams. It pushed me to ask the "WHY" questions and allowed ...

Related courses

Network Design Fundamentals Course

02:28:37 Hours
17 Lectures


Network Convergence Training

04:36:21 Hours
12 Lectures


Network Assurance and Management Course

05:20:32 Hours
18 Lectures