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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.


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What is MPLS tunnel label and why it is used ?

In networking we use many times different terms to define the same thing. MPLS tunnel label or transport label are just two of those.

Not only transport and tunnel labels but also other terms are used to define the same thing which these labels provide.

Let me explain first why and where MPLS tunnel label is used. Read more

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What is flow-based load balancing ?

Flow-based load balancing is used mostly in layer 2 networks, although in Layer 3 routing, packets can be load balanced per packets or per flow, flow-based load balancing is commonly used with the Local area network, datacenter and datacenter interconnect technologies.

There are two important load balancing mechanisms in layer 2. Vlan-based load balancing and Flow-based load balancing. Understanding the differences of these two is important for network engineers, thus please read Vlan-based load balancing post from here as well.

Load-balancing is probably a wrong term though and load-sharing should be used for stateless devices and I explained the differences between load-balancing and load-sharing here.

Let’s look at below figure to understand flow-based load balancing.

flow-based load balancing

Figure -1 Flow based load balancing with basic switch topology

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BGP Best External

BGP Best External is used in Active Standby BGP Topologies generally but not limited with that.BGP Best External feature helps BGP to converge much faster by sending external BGP prefixes which wouldn’t normally be sent if they are not overall BGP best path.

There are BGP best internal, BGP best external and BGP Overall best path.

BGP Best external in an active-standby scenarios can be used in MPLS VPN, Internet Business Customers, EBGP Peering Scenarios, Hierarchical large scale Service Provider backbone and many others.

But,How active-standby scenario connection with BGP is created ? In which situation people use active-standby instead of active-active connection ?

Let’s start with the below scenario.

bgp best external

Figure -1 BGP Active-Standby Path Selection Example

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EIGRP Feasible Successor

One of the advantages of EIGRP Feasible Successor is that it speeds up the EIGRP. In fact, if there is a Feasible Successor in the EIGRP network, such network converges faster than OSPF or IS-IS.

But what is EIGRP Feasible Successor and how can we find EIGRP Feasible Successor?

Or, if there is EIGRP Feasible Successor, how does EIGRP converges faster than OSPF or ISIS?

In this post, I will explain the answers to the above questions.

EIGRP Feasible Successor is a backup node that can satisfy the EIGRP feasibility condition.

Feasibility condition simply means that the backup router should be loop-free.

Let’s examine the topology shown below (Figure-1) to understand how EIGRP finds loop-free alternate/backup node.
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PIM Sparse Mode Vs PIM SSM

One of my students asked me this question a month ago. “What is the difference between PIM Sparse Mode and PIM SSM (Source Specific Multicast)?”

But, since I had two CCDE bootcamps in one month, I didn’t have time to answer the question on this platform.

By the way I have seen this mis understanding in the recent popular design books as well, so writing about it was necessary anymore.

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What is VLAN Load Balancing?

In layer 2 switching, the Ethernet frames should be received from the same port where it was sent, otherwise layer 2 switching or switching loop occurs.

Let me explain this concept with the topology depicted below (Figure -1).


vlan load balancingFigure -1 VLAN load balancing

In the above figure, either Port 1 or Port2 is used to send the traffic, and the same port should be used to receive the traffic. The switches use MAC addresses to process the Ethernet frames. Read more

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What is DFZ (Default Free Zone)?

In the context of the Internet and BGP routing, DFZ – commonly known as Default Free Zone – refers to the collections of all the public IPv4 BGP prefixes without default route on the global Internet.

Most of the time, you hear full-route or full-Internet-route terms which are the same with Default Free Zone term. Having all BGP routes, which are announced by all the available AS (Autonomous System), on the Internet.

Currently, there are more than 600,000 IPv4 BGP routes and about 30,000 IPv6 routes in the DFZ (Default Free Zone).  These numbers, however, could easily be reduced to 300,000 for IPv4 and less than 20,000 for IPv6 based on the CIDR reports (IPv4 CIDR report and  IPv6 CIDR report). Read more

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What does BGP free core mean ?

What is the meaning of BGP free core?

BGP refers to an Internet protocol used between different Autonomous System on the Internet. The purpose of this post is not to explain the fundamentals of BGP, as I believe that readers are already familiar with the basic of BGP and IP routing operation. To understand the rudimentary aspect of BGP, click here to peruse articles on BGP.

Let’s look at the topology shown below to understand the BGP operation and IP destination-based lookup. Read more

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What is RSVP-TE ?

What is RSVP-TE (RSVP Traffic Engineering)? 

RSVP-TE refers to a resource reservation protocol that is invented in order to allocate a bandwidth for the individual flows on the network devices.

To say it another way, RSVP-TE are extensions to the RSVP protocol specified in the RFC 3209.

Although, RSVP-TE has been initially invented as a Quality Service Mechanism in an Intserv QoS architecture, it has never been used in modern networks due to scalability problems.

This is also because allocating tremendous number of flows on the networking nodes creates scalability problems. Read more

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What is a Micro Loop in Routing?

Micro loop can be found in fast-rerouted networks. Fast reroute, as a proactive convergence mechanism, provides sub-second data plane convergence.

If any link fails, upstream node sends the traffic through the repaired path to the downstream device. For the downstream node to be used as a backup/repair node, it should be loop-free.

What do all these mean? The picture below provides further explanations.


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