Fast Convergence and the Fast Reroute Network reliability is considered to be 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 re-converges on the new topology. Before we move on to discuss them further, if you wish to have more information, you could obtain by joining the SPOTO.
Fast reaction is very much 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.
When a local failure occur four steps are necessary to be taken for the convergence. These steps should be 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 (Alternate/Backup path calculation) (SPF Wait and Run times)
4. Update new route into RIB/FIB (After this step, traffic can continue to flow through the 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 could be continued 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 the candidates should need to be kept in mind always; 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, the 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 popular ones are; Loop-Free Alternate (LFA), Remote Loop-Free Alternate (rLFA), MPLS Traffic Engineering Fast Reroute and Segment Routing Fast Reroute. The main difference between MPLS Traffic Engineering Fast Reroute and the IP Fast Reroute mechanisms are the coverage.
MPLS TE FRR can protect any traffic in any topology. IP FRR mechanisms are needed for 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 are full mesh.
If MPLS wouldn’t be 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 the router to find alternate loop-free paths.
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